Monsoon Convection in the Himalayan Region As Seen by the TRMM Precipitation Radar

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) Published online 3 August 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.106

Monsoon convection in the Himalayan region as seen by the TRMM Precipitation Radar

Robert A. Houze Jr.,* Darren C. Wilton and Bradley F. Smull University of Washington, Seattle, USA

ABSTRACT: Three-dimensional structure of summer monsoon convection in the Himalayan region and its overall variability are examined by analyzing data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar over the June–September seasons of 2002 and 2003. Statistics are compiled for both convective and stratiform components of the observed radar echoes. Deep intense convective echoes (40 dBZ echo reaching heights >10 km) occur primarily just upstream (south) of and over the lower elevations of the Himalayan barrier, especially in the northwestern concave indentation of the barrier. The deep intense convective echoes are vertically erect, consistent with the relatively weak environmental shear. They sometimes extend above 17 km, indicating that exceptionally strong updraughts loft graupel to high altitudes. Occasionally, scattered isolated deep intense convective echoes occur over the Tibetan Plateau. Wide intense convective echoes (40 dBZ echo >1000 km2 in horizontal dimension) also occur preferentially just upstream of and over the lower elevations of the Himalayas, most frequently in the northwestern indentation of the barrier. The wide intense echoes have an additional tendency to occur along the central portion of the Himalayas, and they seldom if ever occur over the Tibetan Plateau. The wide intense echoes exhibit three mesoscale structures: amorphous areas, lines parallel to the mountain barrier, and arc-shaped squall lines perpendicular to and propagating parallel to the steep Himalayan barrier. The latter are rare, generally weaker than those seen in other parts of the world, and occur when a midlevel jet is aligned with the Himalayan escarpment. Deep and wide intense convective echoes over the northwestern subcontinent tend to occur where the low-level moist layer of monsoon air from the Arabian Sea meets dry downslope ﬂow, in a manner reminiscent of severe convection leeward of the Rocky Mountains in the central USA. As the low-level layer of moist air from the sea moves over the hot arid northwestern subcontinent, it is capped by an elevated layer of dry air advected off the Afghan or Tibetan Plateau. The capped low-level monsoonal airﬂow accumulates instability via surface heating until this instability is released by orographically induced lifting immediately adjacent to or directly over the foothills of the Himalayas. Broad (>50 000 km2 in area) stratiform echoes occur in the eastern and central portions of the Himalayan region in connection with Bay of Bengal depressions. Their centroids are most frequent just upstream of the Himalayas, in the region of the concave indentation of the barrier at the eastern end of the range. The steep topography apparently enhances the formation and longevity of the broad stratiform echoes. Monsoonal depressions provide a moist maritime environment for the convection, evidently allowing mesoscale systems to develop larger stratiform echoes than in the western Himalayan region. Copyright  2007 Royal Meteorological Society

KEY WORDS convective echoes; deep convection; orographic precipitation; Bay of Bengal; Arabian Sea Received 3 July 2006; Revised 26 January 2007; Accepted 2 March 2007

1. Introduction largest mountain barrier (Figure 2(a); Murakami, 1987). The South Asian monsoon provides an ideal setting During the monsoon, large amounts of rain fall in to observe orographically influenced precipitation under this mountain-influenced regime over the northern Asian very warm and highly unstable conditions. During the subcontinent, where the upper-level winds are weak, boreal summer, warm moisture-laden low-level winds between strong easterlies to the south and westerlies to bring air from the Bay of Bengal and the Arabian the north (Figure 1(b)). Most of the rain in this region Sea over the hot South Asian continent (Figure 1(a); falls during the monsoon season of June–September Rao, 1976; Krishnamurti, 1985; Webster, 1987a,b; John- (Nayava, 1974; Shrestha, 2000). The physical interac- son and Houze, 1987; Das, 1995; Pant and Kumar, tion between the southwesterly monsoon flow and the 1997; Webster et al., 1998). Upon making landfall, the South Asian topography affects the climatological dis- low-level winds encounter the Himalayas, the world’s tribution of precipitation in the region (Figure 2(b)). Studies dating back to the colonial period in India (Hill, 1881) have examined the distribution of surface * Correspondence to: Robert A. Houze, University of Washington, 604 ATG Bldg., Box 351640, Seattle, Washington 98195, USA. rainfall in South Asia and its relation to orographic E-mail: [email protected] features.

(a) 40°N

20 elev. 6.0 15 5.5 10 5.0 4.5 5 4.0 3.5 0 3.0 10 ms-1 (b) 2.5 40°N 2.0 1.5 35 1.0 0.5 30 0.1 0.0 25

0 40°E 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 10 ms-1

Figure 1. Mean wind at (a) 1000 mb and (b) 200 mb superimposed on topography. Wind data were obtained from the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado (http://www.cdc.noaa.gov/) and plotted in MountainZebra (James et al., 2000). The colour scale shows topography elevation in kilometres.

Rather than surface rain mapping, the goal of our has accumulated a climatological sample of radar data research is to gain insight into the physical mechanisms over the monsoon region. Our objective is to use the by which the heavy monsoon precipitation is produced. TRMM PR data to assess the three-dimensional struc- To gain this insight, we examine the three-dimensional tures of the radar echoes in the Himalayan region in structure of the storms producing intense monsoon pre- relation to the details of the topography and proximity cipitation. For this we turn to meteorological radar. to surrounding oceans. Narayanan (1967) foresaw radars as providing ‘better insight into the intricacies of the hitherto hidden phenom- ena of monsoon clouds...’ Specifically, we analyze data 2. Data acquisition, processing, and analysis from the satellite-borne Precipitation Radar (PR) on the 2.1. Acquisition of the PR dataset Tropical Rainfall Measuring Mission (TRMM) satellite (Kummerow et al., 1998; Kummerow et al., 2000). The The TRMM data products obtained for this study were satellite-borne radar is especially well suited for deter- the qualitative rain characteristics field (TRMM prod- mining storm structure in mountainous regions since it uct 2A23) and the gridded, attenuation-corrected three- points downwards from high altitude. It can detect pre- dimensional reflectivity field (product 2A25). Version cipitation (with a horizontal resolution of ∼5 km) both 5 of the 2A23 and 2A25 products (Awaka et al., over peaks of terrain and within valleys, which would be 1997; Iguchi et al., 2000a,b) was used. The rain- blocked from the view of a ground-based radar (Joss and type field includes the categorization of the echo as Waldvogel, 1990). The TRMM satellite’s orbit (which convective or stratiform (see Section 2.3 below). We is confined between 36 °N and 36 °S) results in multiple corrected the 2A23 version 5 data to eliminate the daily samples of the South Asian region, including the potential misclassification of shallow isolated precipita- Himalayas. Since its launch in late 1997, the TRMM PR tion, in a manner consistent with one of the updates

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1391

(a) elev. 35°N 6.5 6.0 5.5 5.0 30 4.5 4.0 3.5

25 3.0 2.5 2.0 1.5 20 1.0 0.5 0.0

(b) mm hr-1 ° 35 N 2.0

1.8

1.6 30 1.4

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0.6

0.4 20 0.2

0.0

Figure 2. (a) South Asian topography in kilometres of elevation. (b) Rain rate in millimetres per hour averaged over all time, including non-raining periods, for 0.1° × 0.1° grid squares derived from TRMM overpasses from 1998 to 2004. employed in the current version 6 of this product, 2.2. Remapping the PR data described at the Japanese Space Agency (NASDA) To analyze the precipitation processes in the monsoon TRMM website (http://www.eorc.nasda.go.jp/TRMM/ convection documented in this study, we used software document/pr manual/pr manual v6.pdf). Since we designed to easily display and assess the TRMM PR applied this correction, updating to version 6 would have data. The primary visualization tool was MountainZebra had no effect on the results of this study. The 2A23 (James et al., 2000), a specialized version of the NCAR and 2A25 data contain the orbital time and geoloca- Zebra software (Corbet et al., 1994), which permits tion data (latitude and longitude coordinates) used in this visualization of the data overlaid on detailed topography. study. For further discussion of these TRMM products, MountainZebra requires a Cartesian grid structure as see Schumacher and Houze (2003a,b) and Houze et al. input, thus the non-Cartesian native PR data (Kum- (2004). merow et al., 1998) required remapping to a regular The TRMM PR dataset is obtainable via the NASA latitude–longitude grid. The data were remapped to Distributed Active Archive Center (DAAC) website Cartesian grid points separated by 250 m in the verti- (http://disc.gsfc.nasa.gov/). However, in order to reduce cal and 0.05° (approximately 5 km) in both latitude and the volume of data, we obtained reprocessed subsets of longitude. Prior to remapping, small corrections were the 2A23 and 2A25 data directly from NASA (Erich applied to the geolocation of upper-level data as fol- Stocker, personal communication) that included partial lows. The PR has a beamwidth of 0.71° and a scan angle orbit ﬁles containing only the data from our region of of ±17°, resulting in a swath width of 247.25 km and study. a horizontal ground resolution of 5 km at the orbital

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1392 R. A. HOUZE JR, ET AL. height of 402.5 km. (The TRMM altitude was boosted a given height incorporates the data within a horizontal from 350 km to 402.5 km in August 2001.) The PR radius of 4.25 km for height bins both at and immediately data were collected in 49 angle bins separated by 0.71° above and below the level in question. That is, data in (Figure 3(a)). The observable vertical range of the PR a 0.75 km-deep layer centred on the height of the grid extends from the surface (with surface clutter removed point are considered to determine the remapped value by the rain profiling algorithm) to a height of 19.75 km of reflectivity at the grid point. The echo features we above the earth ellipsoid (surface of an idealized ocean- examine in this study are of a sufficiently large scale that covered earth). Both the 2A23 and 2A25 algorithms any slight smoothing that results from this remapping assign the geolocation for each ray to the position of procedure has no effect on the conclusions we draw from the range bin at the earth ellipsoid for that ray. Thus, all the TRMM PR data. Details of our remapping method are range bins of the reflectivity data in each angle ray are in the Appendix. referenced to the same coordinate pair at the earth ellipsoid, despite the angular offset (Toshio Iguchi, personal 2.3. Methods of analyzing the radar data communication). At nadir, there is no offset; however, the 2.3.1. Using radar echoes to indicate precipitation offset increases as the angle from nadir increases, with processes those bins at the highest altitude having the most sig- nificant offset. Our data processing involved shifting the Houze (1997) has described the microphysical and nominal latitude and longitude coordinates of the bins dynamical differences between the convective and strat- above the earth ellipsoid to allow for this offset, such iform components of a precipitating tropical cloud sys- that the coordinates in space correspond to the surface tem, and methodology exists for objectively separating coordinates vertically below the bin (Figure 3(b)). These radar echoes into convective and stratiform components corrected locations were the first step in remapping the (Churchill and Houze, 1984; Steiner et al., 1995; Awaka data into a regularly spaced three-dimensional Cartesian et al., 1997). The TRMM PR rain-type fields in product grid for viewing in Zebra and for other calculations. 2A23 have been subjected to a separation algorithm based To complete the remapping, we emulated the on these methods, and we use the separation provided by three-dimensional interpolation method employed by this product as our first-order subdivision of the data. We REORDER, a method often used with airborne radar further identify certain extreme degrees of convective and systems (Oye and Case, 1995). However, we also take stratiform structure to select the best defined examples of advantage of the regularity in the retrieved data in these echo types in Himalayan monsoon convection. terms of the distance from the satellite and the small 2.3.2. Analysis of convective echoes: definition of along-beam sample depth (range bin). Our approach is intense convection an adaptation of the Cressman (1959) method, which employs a radius of influence N. However, we use a To analyze the characteristics of convective radar echoes, small radius of influence (N = 4.25 km), which is small we examine only those echoes identified by TRMM enough to retain most of the detail of the recorded data, product 2A23 as convective. An intense convective echo while minimizing gaps in the Cartesian-gridded field. The is defined as a subvolume of a convective echo consisting remapped value for radar reflectivity at a grid point at of an array of contiguous Cartesian bins with reflectivity

(a) (b) 400 20

350

300 15

250 10 200 Altitude (km)

150 Bin height (km) 5 100

50 0 0 -100 0 100 -125 -120 -115 -110 -105 Distance from nadir (km)

Figure 3. Approximate geometry of TRMM overpass. (a) TRMM satellite orbit altitude of approximately 400 km with 17° scan width (0.71° beam width) results in 49 angle bins and 80 height bins in the lowest 19.75 km, indicated by near-vertical lines at the bottom of the ﬁgure. (b) Exploded view of the inset square in (a), showing off-nadir bins. Arrows indicate the corrected reference position for the three outer beams at three different heights. Note that no correction is needed for the nadir position, and the amount of correction increases with angle from nadir and altitude.

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1393

≥40 dBZ. Defined in this way, intense convective echoes from the Lightning Imaging Sensor (LIS; Christian et al., may comprise either single or multiple convective ‘cells’, 1999) on board the TRMM satellite were obtained as identified by discrete, vertically oriented reflectivity via the Global Hydrology Resource Center (GHRC; maxima occurring on the scale of individual buoyant http://ghrc.msfc.nasa.gov/ghrc.html). convective updraughts. Often, the intense convective echo consisted of 40+ dBZ echo conjoined only at some level, usually at lower levels. Since stratiform echo was eliminated by the 2A23 algorithm, stratiform bright- 3. Locations of extreme precipitation events relative band returns did not affect the identification of intense to the Himalayas convective echoes. For purposes of analysis and discussion, two special Figure 4 shows the locations of deep and wide intense categories of intense convective echoes are defined: convective echoes and broad stratiform echoes over the deep and wide. Deep intense convective echoes are two monsoon seasons of this study. The data are from all those for which the top heights of the 40+ dBZ echo TRMM overpasses for which the PR swath included at volume exceed 10 km. Wide intense convective echoes least a portion of the three rectangles shown in the figure are defined as those with horizontally contiguous regions and referred to here as the western, central,andeastern of 40 dBZ echo exceeding 1000 km2 in area at some subregions. altitude. The altitude of the areal maxima ranged from 1.0 km to 4.5 km amsl. Echoes with intense convective components classified (a) as deep or wide were further examined with the aid 35°N of MountainZebra. The 117 deepest and 121 widest intense convective echo cases were subjected to detailed 30 analysis.

2.3.3. Analysis of stratiform echoes: deﬁnition of a 25 broad stratiform area

The locations and sizes of stratiform areas were deter- 20 mined by finding contiguous grid areas that were classified as stratiform precipitation by the 2A23 algo- (b) rithm. We applied no reflectivity thresholds to the 35°N data. However, the PR has a rather low sensitivity and can detect only reflectivities in excess of approximately 17 dBZ (Kummerow et al., 1998). A broad 30 stratiform area in the PR data was defined as a stratiform echo region exceeding 50 000 km2 in horizontal 25 extent.

20 2.4. Ancillary datasets Both intense convective echoes and broad stratiform (c) ° areas were analyzed visually with the aid of Moun- 35 N tainZebra to asses their vertical structures in relation to ancillary synoptic and satellite data. Climatological 30 wind plots (Figure 1) were obtained though the National Centers for Environmental Prediction (NCEP)–National 25 Center for Atmospheric Research (NCAR) global reanalysis dataset (Kalnay et al., 1996). Individual synoptic maps were generated for us by the Indian National 20 Centre for Medium Range Weather Forecasting (NCM- RWF) from their global T80 model. These reanaly- 67°E 78 89 100 ses incorporated regional data exclusive to the NCM- Ocean Lowland Foothills Mountain RWF (Rajagopal et al. 2001; Goswami and Rajagopal 2003). Sounding data were obtained through the Uni- Figure 4. Regional distribution of (a) deep intense convective echoes versity of Wyoming (http://weather.uwyo.edu/upperair/). (40 dBZ echo >10 km in height), (b) wide intense convective echoes 2 Visible and infrared satellite imagery from Meteosat- (40 dBZ echo >1000 km in area), and (c) large stratiform areas (>50 000 km2 in area). The rectangles indicate locations of west- 5 were obtained from the European Organization for ern, central, and eastern Himalayan subregions. The terrain height the Exploitation of Meteorological Satellites (EUMET- categories are: lowland 0–300 m, foothills 300–3000 m, mountain SAT, http://www.eumetsat.int/). Lightning ﬂash data >3000 m.

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1394 R. A. HOUZE JR, ET AL.

3.1. Occurrence of intense convection as a function of interacting with the larger-scale flow, or as a result of distance from the barrier blocking effects extending well upstream of the barrier (Grossman and Durran, 1984). The southwest monsoon undergoes intraseasonal variations known as active and break periods (Webster, 1987b; 3.2. West–east variability in the occurrence of intense Webster et al., 1998; Lawrence and Webster, 2002; Wang convection et al., 2005; Webster, 2006). It also undergoes day-to-day fluctuations. However, the daily wind patterns throughout Figure 4(a) shows the locations of deep intense convec- the season retain a persistent gross similarity to the clima- tive echoes. They occur occasionally over the extremely tological mean (Figure 1). When deep convection occurs high terrain of the Tibetan Plateau, in the eastern and during active periods in the regions shown in Figure 4, central subregions. However, both deep and wide intense the low-level moist flow (Figure 1(a)) tends to underrun convective echoes are most frequent in the western sub- dry flow coming off the Afghan or Tibetan Plateau. region, especially within the concave indentation at the Carlson et al. (1983) described how ‘a particular con- northwestern end of the Himalayan range (Figures 4(a) figuration of topography and air flow can produce a and (b)). The occurrence of the most intense convec- low-level restraining inversion or “lid” which focuses tion in the northwestern indentation of the Himalayas is the location and even enhances the intensity of severe consistent with the results of Barros et al. (2004), who local storms’. While capping inversions had ‘often showed a strong maximum of lightning frequency in that been attributed to subsidence’, the Carlson et al. model region (Figure 6). The concentration of the deep and wide ‘demonstrated that the lid may originate from differen- intense convective echoes in the northwestern indentation tial advection of a hot, dry mixed layer from an elevated of the Himalayan barrier suggests further that the com- plateau over a cooler, moister layer’. Sawyer (1947) pre- plex shape of the mountain barrier may play a role in the sented a similar interpretation of the environmental fac- distribution of intense convection. The concave indenta- tors affecting monsoon convection occurring over north- tion in the western subregion may be the natural preferred western India. He analyzed soundings from ten days (23 end point of trajectories of the Arabian Sea air heated August–1 September) of the 1945 monsoon. and inferred by passage over the desert. Alternatively, the concave that the low-level moist layer of air entering India from indentation in the barrier may affect the flow in a way the southwest (Figure 1(a)) underran dry continental air that concentrates low-level convergence in the region of advected off the Afghan or Tibetan Plateau (Figure 5(b)). the indentation, as in the case of precipitation maxima According to this idea, the dry air flowing off the high in concave indentations on the Mediterranean side of the plateau caps the moist layer and provides a lid, which European Alps (Frei and Schar,¨ 1998). allows the low-level southwesterly flow with high mois- Figure 4(b) shows the locations of the wide intense ture content to build up extreme buoyancy via sensible convective echoes, which consist of groups of intense heat fluxes as the air moves over the flat desert region of cells comprising portions of mesoscale convective sys- the northwestern subcontinent. The lid effectively delays tems (MCSs; Houze, 2004). Similar to deep intense con- the release of the deep convective instability. Sawyer and vective echoes, the wide ones concentrate in the north- Carlson et al. pointed out that intense convection may western indentation (cf. Figures 4(a) and (b)). However, occasionally break through the lid or break out along the the wide intense convective echoes tend also to occur in the central subregion, in front of the central, convex, lid edge (Figure 5(c)). steep, wall-like Himalayan barrier. In the case of severe convection over the US High Plains, as analyzed by Carlson et al. (1983), deep con- 3.3. Occurrence of broad stratiform events relative to vective updraughts are frequently triggered near a ‘dry- terrain line’, where the top of the moist layer intersects the sloping terrain (Bluestein 1993, pp. 282–290). By anal- Figure 4(c) shows that the broad stratiform precipitation ogy, we might expect intense deep convection to break events occur primarily in the eastern and central subre- out just upstream of or over the lower portion of the gions, roughly at the terminus of the axis of mean south- mountain barrier, as seems to be the case in Figures 4(a) westerly low-level flow, in the zones of rapidly rising and (b), and as will be shown statistically in Section 7. terrain immediately northeast and northwest of the Bay of Dryline-type convection, however, breaks out only when Bengal (Figure 1(a)). They are clustered upstream of the the stable lid is subjected to organized lifting (e.g. the mountain barrier and are grouped within a concave inden- upward motion ahead of a short-wave trough). By anal- tation in the mountain barrier. Compared to the western ogy, in the Himalayan region deep convection might thus subregion, where low-level flow from the Arabian Sea be expected to erupt where the potentially unstable col- experiences a long passage over the desert, the mountain umn defined by the previously capped moist monsoonal barrier and the sharp indentation of lower terrain extend- layer underrunning the warm dry layer is subjected to oro- ing northeastwards from the Bay of Bengal between the graphic lifting. Such lifting would be expected directly Himalayan Barrier and the Arakan Mountains of Myan- over the foothills (in connection with upslope flow) or mar, are much closer to where the monsoon flow makes immediately upwind of the mountain barrier towards landfall. The preferential occurrence of well-defined strat- which the moist layer is flowing, by diurnal processes iform regions in the northeastern subcontinent suggests

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1395

Figure 5. Sawyer’s (1947) conceptual model of the monsoon environment over the northwestern subcontinent, as observed during 23 August–1 September 1945. (a) Synoptic and orographic context. The wide arrow indicates the general direction of the low-level monsoon ﬂow. The cross-sections in (b), along AB, and in (c), along CD, are traced from the original paper.

flashes/km-2/day-1 that convective cloud systems upstream of the mountains 0.3 in this region retain a maritime character. Schumacher 35°N and Houze (2003a) showed that the stratiform compo- 0.25 30 nent of tropical rain tends to be substantially greater over 0.2 oceans than over land. 25 In addition, the convection in the eastern region occurs 0.15 20 primarily in conjunction with synoptic-scale Bay of Ben- 0.1 gal monsoon depressions (Ramage, 1971; Rao, 1976; 15 Godbole, 1977; Sikka, 1977; Shukla, 1978; Sanders, 0.05 1984; Johnson and Houze, 1987; Douglas, 1992a,b). 10 0 These depressions occur in conjunction with the advance 60°E 70 80 90 100 of the active monsoon into the northernmost Bay of Ben- gal (Lawrence and Webster, 2002; Wang et al., 2005). Figure 6. TRMM Lightning Imaging Sensor (LIS) data (ﬂashes per The highly stratiform character of the clouds and precip- square kilometre per day) averaged for June–September 1998–2004. itation in one such depression was documented by aircraft

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1396 R. A. HOUZE JR, ET AL. radar and microphysical measurements in the Summer Monsoon Experiment of 1979 (Houze and Churchill, 1987).

4. Vertical dimensions of intense convective echoes Altogether, 117 deep intense convective echoes (as deﬁned in Section 2.3.2) were observed in our TRMM PR dataset (see Wilton, 2005, Appendix B, Table B.1, for details on the times and locations).

4.1. Example of a deep intense convective element over lower terrain An example of a deep intense convective echo forming at the boundary of the low-level moist flow from the Ara- bian Sea and large-scale leeside downslope dry flow from the Afghan Plateau (Section 3.1) occurred in northwestern India at ∼0900 UTC 14 June 2002. The flow resembled the climatological patterns in Figure 1 (Figures 7(a) and (b)). The low-level (10 m) winds show southwesterly flow entering northwestern India/Pakistan from the Ara- bian Sea, coming ashore and crossing the inland desert in a strong pulse. The deep intense convective echo occurred near the terminus of this moist low-level southwesterly jet (star in Figure 7), and at the southern edge of the midlatitude westerly jet (Figure 7(b)). The 10 m winds indicate that northwesterly flow (on a dry downslope trajectory from the Afghan Plateau) met the southwesterly moist monsoon flow near the deep intense convec- Figure 7. NCMRWF reanalysis of (a) 10 m winds and (b) 200 mb tive echo. Since these maps are for the afternoon (1200 winds for 1200 UTC 14 June 2002. The star shows the location of = deep intense convective echo. The locations of sounding stations are UTC 1730 local standard time, LST), it is unlikely that indicated by P (Patiala), D (Delhi), and J (Jodhpur). the downslope flow was due to the diurnal heating cycle over the plateau (Murakami, 1987). intense convective cells to be a ubiquitous aspect of Soundings at 0000 and 1200 UTC (9 h prior to and TRMM PR data in northern India, consistent with the 3 h after the TRMM overpass in which the deep intense fact that the convection over and just upstream of convective echo was observed) were consistent with the Himalayan barrier occurs generally in a region of the observed convective development (Figure 8). The weak shear, south of the upper-level westerly jet and soundings in Figure 8 display the two-layered structure north of the upper-level easterly jet (Figure 1(b)). The described in Section 3.1, as a low-level moist layer topography of the subtending terrain, shown in red at originating over the Arabian Sea underran dry warm air the bottom of Figure 9(b), further confirms that this advected downstream from the Plateau, in the manner convective event was clearly not triggered directly over described by Sawyer (1947) and Carlson et al. (1983). the mountains. The sounding at Patiala at 0000 UTC (Figure 8(a)) showed relatively large convective available potential −1 4.2. A deep intense convective element over high energy (CAPE; Houze 1993, chapter 8) of 2700 J kg . terrain Figure 9 shows the TRMM PR reflectivity associated with the deep intense convective cell at 0900 UTC (1430 While most monsoon convection occurs just upstream of LST) 14 June 2002. The vertical section shows that or over the lower slopes of the Himalayas, deep intense the cell was over 18 km in height, and that the 40 convective echoes do occur occasionally over the high dBZ echo reached 16 km. Also notable is the vertical terrain of the Tibetan Plateau (Figure 4(a)). Wide intense alignment (erect structure) of the deep intense convective convective echoes were generally not observed over the echo, as opposed to the more tilted structure often Plateau (Figure 4(b)), indicating that the convection over associated with robust thunderstorms and squall lines the Plateau tends to be unable to develop mesoscale (Houze, 1993, chapters 8 and 9). The erect cell structure organization, possibly because of insufficient moisture in in Figure 9(b) is consistent with the wind data showing the environment (generally <5gkg−1 of water vapour; little shear at the convective location well south of Wang and Gaffen, 2001). We suggest that the semi- the core of the midlatitude westerly jet (Figure 7(b)). arid conditions over the southern portion of the Plateau We have found this vertically upright structure of deep (the part included in our study) are adequate to support

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1397

(a)Patiala 00 UTC (b) Patiala 12 UTC 500

600

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(c)Delhi 00 UTC (d) Delhi 12 UTC 500

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Pressure (mb) 900

(e)Jodhpur 00 UTC (f) Jodhpur 12 UTC 500

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10 20 30 40 10 20 30 40 Temperature (°C)

Figure 8. Rawinsonde data in skew T –log p format for Patiala, India on 14 June 2002 at (a) 0000 UTC and (b) 1200 UTC. (c)–(d) and (e)–(f) are as (a)–(b), but for Delhi and Jodhpur, respectively. Adapted from University of Wyoming Department of Atmospheric Science Upper Air Data website (http://weather.uwyo.edu/upperair/).

(a) (b) 20 32°N dBZ 70 16 62 31 54 12 46 38 30 8 30 Height (km) 22 4 29 14 0 6 0 25 50 75 100 28 Distance (km) 74°E 75 76 77

Figure 9. TRMM PR reflectivity for 0900 UTC 14 June 2002. (a) Horizontal pattern at 4 km. (b) Vertical cross-section running left to right along the red line in (a). The red line in the cross-section plot indicates the height of the subtending terrain. The colour scale indicates reflectivity in dBZ. Note that dBZ values <17 are the result of interpolation of the gridded data. The lowest measured value is ∼17 dBZ. the occurrence of occasional single deep intense isolated that their lower portions are truncated (Figures 10(b) and convective cells but inadequate to support a cloud system (c)). of greater breadth. Convection over the Plateau tends to be scattered cells, mostly isolated but sometimes 4.3. Statistics of heights of intense convective echoes aligned. An example of convective echoes over the Plateau (Figure 10(a)) shows that intense convective Figure 11(a) shows the cumulative frequency distribution echoes (including deep ones, with 40 dBZ extending of the heights of all the identified intense convective above 10 km) occur over the Plateau and look similar echoes for the 2002 and 2003 monsoon seasons. The to the echoes over the lowlands and foothills except bulk of the intense convective echoes in all subregions

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1398 R. A. HOUZE JR, ET AL.

(a) (b) 16 33°N 12 dBZ 8 67 31 4 59 51 0 29 0 200 400 600 800 43 (c) 35

Height (km) 16 27 27 12 19 8 11 25 4 3

° 0 79 E 81 83 85 87 89 050100 150 Distance (km)

Figure 10. TRMM PR reflectivity for 1227 UTC 14 July 2002. (a) Horizontal pattern at 6.5 km. (b) and (c) show vertical cross-sections running left to right along the red lines in (a). Other details are as in Figure 9. are below the 0 °C level, which is generally located between 5.5 and 6.0 km over the study region. Since 40 dBZ is not the highest reflectivity found in the intense convective echoes, this result is consistent with the findings of Maheshwari and Mathur (1968) who found that the maximum reflectivity in Indian monsoon convective storms was generally at or below the 0 °C level. This figure also demonstrates that the eastern subregion has more frequent occurrence of 40 dBZ echo top below the 0 °C level. The occurrence of the echo maximum at low levels is a typical characteristic of convective precipitation in airmasses of maritime-tropical origin (Caracena et al., 1979; Szoke and Zipser, 1986; Szoke et al., 1986). The eastern subregion, being closer to the sea, evidently had convection resembling that over tropical oceans. Although the intense convective echoes most frequently have tops below 5 or 6 km, a surprisingly large number of the 40 dBZ echo tops over northern India can reach extremely high levels. Seshadri (1963) and Ghosh (1967) studied echoes observed by ground-based radar at Delhi and found echoes occasionally reaching 17 km. To focus on the deeper echoes seen by the TRMM PR, Figure 11(b) shows only those 40 dBZ intense convective echoes with tops at or above 10 km. The occurrence of these high reflectivities in the upper-level ice region are likely explained by the presence of larger ice particles, which may be too large to be in the Rayleigh scattering regime for the 2 cm wavelength of the TRMM PR. Grau- pel particles of the order of 1 cm in diameter are evidently Figure 11. (a) Normalized frequency distribution of 40+ dBZ intense lofted to high altitudes by extremely strong updraughts convective echo. (b) Inset showing normalized frequency distribution + in these cases. Rayleigh scattering requires the particle of 40 dBZ intense convective echo tops only for intense convective echoes extending above 10 km amsl. size to be much less than the radar wavelength. It is further evident from Figure 11(b) that the highest frequency of very deep 40 dBZ intense convective echoes occurs in the western subregion, with some intense convective far from the ocean, and the convection is of a more con- echo-top heights extending to over 16 km! This region is tinental nature, with stronger updraughts. The convection

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1399 in this region is known for its intensity and high frequency of lightning, as noted by Barros et al. (2004) (Figure 6).

5. Horizontal dimensions of intense convective echoes Altogether 121 of the intense convective echoes included in this study qualiﬁed as wide intense convective echoes (as deﬁned in Section 2.3.2). Generally these echoes are complexes of interconnected echoes, which can be regarded as MCSs.

5.1. Example of a wide but amorphous intense convective element

A wide intense convective echo occurred in the concave indentation of the northwestern end of the Himalayas at ∼1300 UTC 22 July 2002. Figure 12 shows the synoptic conditions on that day. Similar to the climatological patterns in Figure 1, a strong low-level southwesterly flow of moist air from the Arabian Sea was crossing the northwest coast of India and extended all the way to the foothills of the Himalayas (Figure 12(a)). The wide intense convective echo (star) occurred in the convergent region north of the terminus of the 10 m southwesterly flow, between the southwesterly flow and dry downslope flow from the north. Similar to the case described in Section 4(a), this convection occurred in the Figure 12. NCMRWF reanalysis of (a) 10 m winds and (b) 200 mb local afternoon, and dryline-type behaviour is suggested winds for 1200 UTC 22 July 2002. The star shows the location of by the 10 m flow and its relation to the adjacent steeply wide intense convective echo. The location of the sounding station is indicated by J (Jodhpur). sloped terrain. From the 200 mb winds (Figure 12(b)), it appears that the flow was again south of an upper-level part of an organized line of similar echo structures. In westerly jet, and that the shear was not especially strong. plan view, it was amorphous. Soundings prior to the wide intense convective echo The vertical cross-section in Figure 14(b) shows about at Jodphur resembled those of a classic severe storm six vertically oriented cells (best identified by their situation over the Great Plains of the USA (Fawbush protruding tops in the 30–35 dBZ contour range). These and Miller, 1954; Newton and Fankhauser, 1964; Palmen´ cells were vertically erect and packed close together and Newton, 1969; Bluestein, 1993, pp. 448–451). These to form a contiguous 40+ dBZ echo over a horizontal soundings indicated a strong capping inversion atop the distance of ∼90 km. The vertically erect structure of the moist boundary layer (Figure 13). The strength of the cells seen in this case was typical of all the embedded capping inversion lessened from 0000 to 1200 UTC cells of all the wide convective echoes that we have and thus evolved towards a sounding where vigorous convection was more likely to break out. These soundings examined, consistent with the generally weak shear in are consistent with upper-level warm dry flow from the this region during the monsoon. The cells could have Afghan Plateau overriding the low-level moist monsoonal been still growing or beginning to collapse. In either case flow and providing a cap that allows the low-level they were likely headed toward formation of a stratiform warm moist flow from the Arabian Sea to build up region later in the lifetime of the complex of cells, in instability as it moves farther inland until being released the manner normally expected as cells weaken in a MCS by convergence directly over or just upstream of the (Gupta et al., 1955; Houze, 1993, 1997, 2004). Himalayas. Of the 121 wide intense convective echoes observed At 1300 UTC (1830 LST), the TRMM PR detected a in this study, 20 (including the example above) were wide intense convective echo over the plains of the north- also classified as deep convective echoes. Fourteen of western subcontinent, within the concave indentation of those doubly intense cases (i.e. both deep and wide) were the Himalayan barrier (Figure 14(a)). The 40 dBZ echo located in the western subregion; only six were located covered 3475 km2 at an altitude of 3.5 km. It contained in the central subregion, while no echoes in the eastern multiple more intense cells in its interior. However, it subregion were simultaneously classified as both deep showed no elongation to form an echo band, nor was it and wide.

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Soundings for 22 July 2002

(a)Jodhpur 00 UTC (b) Jodhpur 12 UTC 500

600

700 800

Pressure (mb) 900

10 2030 40 1020 30 40 Temperature (°C)

Figure 13. Rawinsonde data in skew T –log p format for Jodhpur, India, on 22 July 2002 at (a) 0000 UTC and (b) 1200 UTC, from the same source as Figure 8.

(a) 36°N 35 dBZ 34 (b) 16 70 33 62 12 54 32 46 31 8 38 30

30 Height (km) 4 22 29 14 0 6 28 0 50 100 150 200 27 Distance (km)

71°E 72 74 77

Figure 14. As Figure 9, but for 1309 UTC 14 June 2002.

5.2. Examples of wide intense convective echoes with MCS structure. (Squall lines may be more common in the line organization pre-monsoon season, when the westerly jet is positioned farther south, and the shear is greater; however, these On some occasions, the wide intense convective echoes early-season storms would be excluded from our study showed line organization. One recurring type is illustrated since we are considering only the monsoon months.) by Figure 15. A contiguous line of high-reﬂectivity echo These squall lines are not as well developed as those over roughly paralleled the steep Himalayan barrier. A cross- the central USA (Houze et al., 1989, 1990) or west Africa section along the convective line showed vertically erect (Hamilton and Archbold, 1945; Fortune, 1980; Houze cells (consistent with the weak shear) interconnected by and Betts, 1981; Hodges and Thorncroft, 1997; Schu- strong (>40 dBZ) echo (Figure 15(c)). Stratiform echo is macher and Houze, 2006), where stronger shear prevails. located along the northern red line in Figure 15(a). The An example in northern India is shown in Figure 16. It vertical cross-section in Figure 15(b) shows bright-band exhibits an arc-shaped line roughly perpendicular to the echo surrounding a convective cell (at 85 km on the hor- steep wall of the main Himalayan barrier. The bow of the izontal scale). The stratiform echo was likely formed as line in Figure 16(a) was pointing generally to the south- cells similar to the one at 85 km collapsed. (The history east. Time-lapse looping of Meteosat-5 infrared satellite of the echoes cannot actually be determined because the imagery shows that this system was propagating south- TRMM PR provides snapshot coverage only 2–3 times eastwards, in the direction of the convex bow of the line. per day over the region of study.) The stratiform precipi- The PR data show that a stratiform echo was trailing tation likely formed as the vertically erect cells collapsed. behind, giving the echo a classic squall-line structure Collapse of convective echo cores is a well-known pro- (Figure 16(b)). This day was unusual in that a strong cess of generating areas of stratiform precipitation in midlevel jet lay over and parallel to the Himalayan bar- MCSs in low-shear environments (Houze, 1993, 1997) rier (Figure 17(b)). This jet lent the convection a sheared and had been noted in India by Gupta et al. (1955). environment and apparently played a role in organizing The central subregion is the only zone of our South a rear-inﬂow jet (Smull and Houze, 1987), in the way Asian study area in which we have noted propagat- that the midlevel African Easterly Jet helps to organize ing arc-shaped leading-line/trailing-stratiform squall-line squall lines in West Africa. The midlevel jet seen in

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(b) 20 16 (a) dBZ 12 70 35°N 8 62 4 54 34 0 46 0 50 100 150 38 (c) 33 20 30 Height (km) 16 22 32 14 12 6 8 31 71°E 72 73 74 75 76 4 0 050100 150 200 Distance (km)

Figure 15. TRMM PR reﬂectivity for 2208 UTC 3 September 2003. (a) Horizontal pattern at 4 km. (b) and (c) contain vertical cross-sections running left to right along northern and southern red lines in (a), respectively. Other details are as in Figure 9.

(a)

29°N (b) dBZ 16 67 59 28 12 51 43 27 8 35 27 Height (km) 19 26 4 11 0 3 25 0 50 100 150 85°E 86 87 88 89 Distance (km)

Figure 16. As Figure 9, but for 2017 UTC 5 June 2003.

Figure 17(b) persisted for three days, and squall lines heights are below the melting level and consistent with occurred in the pre-Himalayan channel on each day. In the low-level echo centroid heights found by Szoke et al. our data sample, we found one additional instance of a (1986) and Szoke and Zipser (1986) in tropical maritime similar squall line occurring along the Himalayas in an convection and with the results of the analysis of ground- analogous synoptic regime, with a midlevel jet overlying based radar in India by Maheshwari and Mathur (1968). the Himalayan escarpment. Apparently the convection The wide areas of convection were all classiﬁed as favoured in the region just upstream of and over the convective precipitation by the 2A23 algorithm, thus foothills of the Himalayas can become organized into eliminating the possibility that the horizontal scale of a propagating squall line in conjunction with these rela- the echoes could have been determined by radar bright- tively rare synoptic events exhibiting strong shear. band echoes. The portion of Figure 18(a) including echoes >1000 km2 is replotted in Figure 18(b), which 5.3. Statistics of the horizontal dimensions of intense shows that widest areas of convection exhibited a higher convective echoes frequency of occurrence in the western subregion, with a tendency to decrease eastwards. This same regional Figure 18(a) shows the cumulative distribution function behaviour was noted for intense convective echo height + of the horizontal area of 40 dBZ intense convective (cf. Figure 11(b)). echoes. This sample was extracted from the set of all intense convective echoes observed by the TRMM PR over the western, central, and eastern subregions 6. Characteristics of stratiform echo regions in the during the 2002 and 2003 summer monsoon seasons. monsoon The average height at which the areal maximum was located was approximately 2.5 km. The highest altitude We have intensively investigated the 10 largest broad at which the areal maximum occurred was 4.5 km. These stratiform regions (as deﬁned in Section 2.3.3) in our

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1402 R. A. HOUZE JR, ET AL.

(a) 1.0 .95 .90 .85 .80 .75 .70

.65 Western sub-region .60 Central sub-region .55 Eastern sub-region .50 0 500 1000 1500 2000 2500 (b)

Frequency 1.00 .999 .998 .997 .996 .995 .994

.993 Western sub-region .992 Central sub-region .991 Eastern sub-region .990 1000 2000 3000 4000 5000 Area of 40 dBZ core (km2) Figure 17. NCMRWF reanalysis of (a) 10 m winds and (b) 500 mb winds for 1200 UTC 5 June 2003. Figure 18. Cumulative distribution of 40 dBZ convective areal maxima (a) between 25 and 2500 km2 and (b) between 1000 and 5000 km2. dataset. Of these, six were associated with well-defined monsoon depressions. The remaining four were associ- through the troposphere. This sounding typifies the upper- ated with what we would call quasi-depressions, which air conditions in the depressions we have examined. had all the characteristics of a Bay of Bengal depres- Figure 22(a) shows the radar echo sampled during a sion except that the vortex in the low-level wind was not TRMM PR overpass of the Bay of Bengal depression. completely closed. The PR echo (250 km ×∼500 km) represents only a small portion of the overall cloudy region associated with 6.1. Example of a broad stratiform region in a Bay of the depression. The areas of enhanced upper-level cloud Bengal depression and their associated radar echoes in the depressions never Figure 19 is a composite showing how the Bay of Bengal take on the synoptic-scale dimension of the depression. depression is connected with the monsoon intraseasonal They appear to be MCSs embedded within and proba- oscillation (Webster, 2006). The four panels show the bly abetted by the parent Bay of Bengal depression. The progression over a 13-day period. Day 0 is defined as widespread upward motion of the synoptic system evi- the occurrence of 10 mm day−1 of rain at 90 °E. The dently destabilizes the air and provides a moist environ- cyclonic gyre over the head of the Bay of Bengal on day ment favourable for MCSs. The distribution of MCS-like 8 (Figure 19(d)) has propagated northeastwards into the structures within the depression was reminiscent of the Bay of Bengal as part of an equatorial Rossby wave. The highly stratiform MCSs analyzed by Houze and Churchill arrival of the gyre in the northernmost portion of the Bay (1987). The flow over the terrain likely enhances these marks the onset of an active monsoon period. mesoscale precipitation areas. Figure 22(b) exemplifies On 11 August 2002, the PR detected a broad stratiform how the mesoscale precipitation features on the north region in connection with such a Bay of Bengal depres- side of the depression often tend to nestle into the eastern sion. The large-scale flow (Figure 20) broadly resembles concave indentation of the Himalayan range, where large the climatological patterns in Figure 1. In the low-level stratiform echoes occur most frequently (Figure 4(c)). winds (Figure 20(a)), the Bay of Bengal depression was The stratiform precipitation echo appears to have been centred at the head of the Bay (about 22 °N, 90 °E). The composed of weakened convective cells (Figure 22(c)). sounding for Tengchong (Figure 21, station location in This structure was typical of the mesoscale precipita- Figure 20(a)) shows saturated moist adiabatic conditions tion areas in all the depressions we have examined in

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1403

Composite 925 mb wind vectors and precipitation (>8mm day–1) (a) (b) 30°N

30°S

30°S 30°E 60 90 12030°E 60 90 120 5 m s-1 8 1012141618 mm day-1

Figure 19. Evolution of the rainfall and 925 mb wind ﬁeld of the monsoon intraseasonal oscillation. Rainfall is based on Microwave Sounding Unit data. Winds are based on NCEP reanalysis. Composite of 39 events from 1985 to 1995. (a)–(d) show composite distributions relative to day 0, deﬁned as the occurrence of >8mmday−1 rainfall at 80–90 °E/5 °N–5°S persisting for three days. The numbers in the upper left of each panel indicate the number of days before (negative) and after day 0. The annual cycle has been removed. Adapted from Webster and Tomas (1997). the PR data, whether the precipitation features were over A large number of the mesoscale patches of enhanced the sea, the lowlands, or the foothills of the Himalayas. high cloudiness in Figure 22(a) are south of the depres- Occasionally they contained active, deep convective echo sion centre within the low-level southwesterly regime cells, which apparently later collapsed to form the broader upstream of the Myanmar coast. The strong upper-level background of stratiform precipitation (Houze, 1997), and easterlies prevailing at this comparatively low latitude are active deep cells only occupy a small fraction of the rain producing streamers of westward anvil blow-off from the areas at any given time. (The existence of ‘generating precipitating cloud lines off Myanmar. A PR observation cells’ associated with a layer of convective overturning of some of these precipitation echoes over the northern aloft, as is frequently seen in midlatitude cyclones, cannot Bay of Bengal is shown in Figures 23(a) and (b). A ver- be ruled out as an alternative explanation for the observed tical cross-section shows convective echo cells of rather echo structure. In the tropical oceanic environment, where moderate depth, some of which have collapsed into strat- the convection in the depression takes the form of dis- iform echo (Figure 23(c)). Zuidema (2003) also found crete MCSs, it seems more likely that the echo pattern is the clouds upstream of the Myanmar coast to be of small a manifestation of collapsed deep penetrative convective to moderate depth and width in comparison to the over- cells.) all population of clouds seen in infrared imagery of the Figure 19 shows how the cyclonic gyre within the region. overall low-level wind pattern of the Rossby wave structure that propagates into the Bay of Bengal typically 6.2. Statistics of stratiform area sizes induces strong southwesterly winds across the Myan- mar coast. Zuidema (2003) showed that clouds tend to Figure 4(c) showed that broad stratiform regions, i.e. concentrate upstream of the coast, as they do upstream the largest stratiform regions, preferentially occur in of the western Ghats and other mountain ranges of the the eastern subregion. The cumulative distribution in region (Grossman and Durran, 1984; Xie et al., 2006). Figure 24 further shows that stratiform areas of all sizes

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non-interpolated TRMM PR 2A25 radar reflectivity values for the entirety of the 2002 and 2003 monsoon seasons and inclusive of all three subregions (west, central, and east). The contours represent the frequency of occurrence of a given reflectivity at a given height, f (dBZ,z). The entire distribution was normalized by dividing by the maximum absolute frequency of the samples within the region of analysis, fmax(dBZ,z). Thus, the contours are labelled in values of f/fmax. This method permits the comparison of CFADs between regions despite the different absolute frequencies. The contours are truncated at lower reflectivity values as a result of the reduced sensitivity to low rain rates of the TRMM PR. Figure 25 is characterized by three statistically distinct regimes, viz. those located above, at, and below the climatological 0 °Clevel(∼5.5–6). Below the 0°C level, the echoes are produced by raindrops and have a median reflectivity of ∼25 dBZ (corresponding to a rain rate of about 1–2 mm h−1). The wide spread of the distribution below the 0 °C level is consistent with rain falling from convection in various stages of development. Above the 0 °C level, in the ice region, the median reflectivity value is ∼20 dBZ. At (or actually just below) the 0 °C level, the tendency for a melting-layer bright band to occur in the echoes is evident in the statistics in the form of a distinct rightward shift in the CFAD contours toward higher dBZ values at ∼5 km. Subsequent CFADs add detail to the regional trends evident in Figure 4 by breaking down the Figure 20. NCMRWF reanalysis of (a) 10 m winds and (b) 200 mb winds for 11 August 2002. The location of the sounding station is CFAD statistics by subregion, rainfall type, and terrain indicated by T (Tengchong, China). height. Figures 26(a)–(c) are generally consistent with more 200 intense convection in the west. They further show that the western subregion had higher reflectivity at upper levels, indicating a tendency for the deepest convection 300 to occur in the western subregion and for the maximum

400 echo heights to be less in the eastern subregion. The eastern subregion CFAD exhibited a more pronounced 500 rightward shift in reﬂectivity in the melting layer than

Pressure (mb) 600 in the central and western subregions, consistent with a 700 greater tendency for stratiform precipitation in the eastern 800 900 subregion. When the CFADs are further subdivided according -40 -30 -20 -10 0 10 20 30 40 to precipitation type (Figure 27), the regional differ- Temperature (°C) ences become even more evident. The convective echoes (Figure 27(a) and (b)) showed a tendency to have both Figure 21. Rawinsonde data in skew T –log p format for Tengchong, China, on 11 August 2002 at 0000 UTC, from the same source as higher overall reflectivity values and higher reflectivities Figure 8. at upper levels in the western region. The CFAD contours indicate the occurrence of up to 40 dBZ echo at tend to be more frequent in the eastern subregion, 12 km in the west, while 40 dBZ echoes are statistically where synoptic forcing by Bay of Bengal depressions evident only to about 9 km in the eastern subregion. The and proximity to the ocean should both favour the stratiform partitions (Figures 27(c) and (d)) show a more development of MCSs with larger stratiform precipitation pronounced bright-band signature in the more maritime areas. regime of the eastern subregion. When the CFADs are stratified according to subtending terrain, they show that convection over the lowlands 7. Probability distributions of Himalayan region tended to be slightly deeper than that over the foothills radar echoes in all three subregions (Figure 28(a)–(f)). Evidently, the Figure 25 is a Contoured Frequency by Altitude Dia- net effect of the Himalayan range in triggering convection gram (CFAD; Yuter and Houze, 1995) containing the was most pronounced over the lowlands just upstream of

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1405

(a) (b) ir 350 30°N 30°N 340 330 320 310 25 300 290 280 270 20 260 250 25 240 15 230 220 210 200 10 190 80°E 85 90 95 100 105 90°E 95 dBZ (c) 16 67 59 12 51 43 8 35 27 Height (km) 4 19 11 0 3 0100 200 300 400 500 Distance (km)

Figure 22. TRMM PR reflectivity for 0252 UTC 11 August 2002. (a) Horizontal swath of PR reflectivity (green and yellow shades) at 4 km is superimposed on the Meteosat-5 infrared temperature field. Grey shades show the infrared temperature in K, with red and yellow shades highlighting the coldest cloud tops. (b) Zoomed-in view of the PR reflectivity in (a). (c) Vertical cross-section running left to right along the black line in (a) and (b). Meteosat-5 data were obtained from http://www.eumetsat.int. Other details are as in Figure 9.

(a) (b) 350 40°N 340 330 20°N 35 320 310 300 30 290 280 25 270 260 20 250 240 15 230 15 220 10 210 200 190 70°E9075 80 85 90 95100 105 °E 95 dBZ (c) 16 67 59 12 51 43 8 35 27 Height (km) 4 19 17 0 3 075 150 225 300 Distance (km)

Figure 23. TRMM PR reflectivity for 0455 UTC 11 August 2002. (a) Horizontal swath of PR reflectivity (green and yellow shades) at 2 km is superimposed on Meteosat-5 infrared temperature field. Other details are as in Figure 22.

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(a) Western sub-region 14 12 10 8 6 4 2 1.0 0 0.9

(b) Central sub-region 0.8

14 0.7 12 Figure 24. Cumulative distribution of area of stratiform precipitation. 0.6 10 CFAD for all regions 8 0.5 1.0 14 6 0.9 Altitude (km) 0.4 12 0.8 4 2 10 0.7 0.3 0.6 0 8 0.5 0.2 (c) Eastern sub-region 6 0.4

Altitude (km) 0.3 4 14 0.1 0.2 2 0.1 12 0.0 0 0.0 10 15 20 25 3035 40 45 Reflectivity (dBZ) 8 6 Figure 25. Contoured frequency by altitude diagram (CFAD) of TRMM 4 PR reflectivity data for June–September 2002–2003 for western, central, and eastern subregions combined. Contours represent the 2 frequency of occurrence relative to the maximum absolute frequency. 0 Altitudes are geopotential height (km amsl) relative to the surface of 15 20 25 30 35 40 45 50 the earth ellipsoid. The ordinate of the CFAD is altitude (in 250 m Reflectivity (dBZ) increments, or bins) and the abscissa is reflectivity in dBZ (in 1 dBZ bins). Figure 26. Contoured frequency by altitude diagram (CFAD) of TRMM PR reflectivity data for June–September 2002–2003 for (a) western, the mountains. This result suggests that the formation of (b) central, and (c) eastern subregions separately. Contours and other the most intense convection was focused at lower ele- details are as in Figure 25. vations immediately upstream of the mountains, i.e. near the leading edge of the warm, dry air advected downslope in remote regions of complex terrain has given us fresh from the Tibetan Plateau, in a manner possibly similar insight into two longstanding questions: to dryline convective triggering observed in the central (1) the behaviour of highly convective clouds in a moist USA (Section 3.1). Convection located directly over the flow impinging on a major mountain barrier, and foothills, though still strong, already has (on average) lost (2) the particular structure and organization of summer some buoyancy – conceivably because the near-surface monsoon convection over the subcontinent of South moisture has been removed from the airstream flowing Asia. up the terrain. The availability of low-level moisture and ability to form deep convection is apparently nearly com- The first question has general applicability to under- pletely absent over the higher terrain. The echoes seen in standing the role of orography in affecting precipitation, the mountain CFADs (Figure 28(g)–(i)) were composed while the second is vital to understanding and predicting predominantly of stratiform echo, probably advected monsoon clouds and precipitation. The schematic draw- from the tops of the lowland and foothill convection. ing in Figure 29 suggests some of the conclusions we have drawn from an extensive examination of the vertical structure of Himalayan region convection as seen in the 8. Conclusions three-dimensional TRMM PR data. The schematic does The ability of the TRMM PR to provide vivid and not depict every observed scenario but rather illustrates detailed images of the vertical structure of convection scenes typical of each of the subregions.

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(a) Western sub-region (convective)(b) Eastern sub-region (convective)

16 1.0 14 12 0.9 10 8 0.8 6 0.7 4 2 0.6 0 (c) Western sub-region (stratiform)(d) Eastern sub-region (stratiform) 0.5

Altitude (km) 16 0.4 14 0.3 12 10 0.2 8 6 0.1 4 2 0.0 0 15 20 25 30 35 40 4550 55 15 20 25 30 35 40 4550 55 Reflectivity (dBZ)

Figure 27. Contoured frequency by altitude diagram (CFAD) of TRMM PR reﬂectivity data for June–September 2002–2003 for (a) western subregion convective echoes, (b) eastern subregion convective echoes, (c) western subregion stratiform echoes, and (d) eastern subregion stratiform echoes. Contours and other details are as in Figure 25.

(a)Western lowlands (b) Central lowlands (c) Eastern lowlands 16 14 12 10 8 1.0 6 4 0.9 2 0 0.8 (d)Western foothills (e) Central foothills (f) Eastern foothills 0.7 16 14 0.6 12 10 0.5 8 6 0.4 Altitude (km) 4 2 0.3 0 (g) (h) (i) Western mountains Central mountains Eastern mountains 0.2 16 14 0.1 12 10 0.0 8 6 4 2 0 15 20 25 30 35 40 45 50 15 20 25 30 35 40 45 50 15 20 25 30 35 40 45 50 Reflectivity (dBZ)

Figure 28. Contoured frequency by altitude diagram (CFAD) of TRMM PR reﬂectivity data for June–September 2002–2003 for categories of subregion and height of subtending terrain. Contours and other details are as in Figure 25. Terrain height categories are deﬁned in Figure 4.

Our analysis of PR echoes identiﬁed by the TRMM Himalayan region most commonly have tops below 2A23 algorithm as being convective in nature shows the 0 °C level, consistent with early studies of radar that convective echoes >40 dBZ in intensity in the reﬂectivity structure in the region (Maheshwari and

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1408 R. A. HOUZE JR, ET AL.

Mathur, 1968). Also consistent with early investigations The TRMM PR data further show the spatial distribu- (Seshadri, 1963; Ghosh, 1967), the TRMM PR data tion of extreme convective events. Deep intense convec- occasionally show much more extreme echo heights, and tive echoes (40 dBZ echo tops >10 km) occur especially we term echoes of 40 dBZ or more extending above frequently in the western subregion, primarily within the 10 km ‘deep intense convective echoes’. The Himalayan concave indentation in the western end of the Himalayan region of this study lies between the upper-level westerly range. The frequent occurrence of deep intense convective echoes lofting graupel to high altitude in the con- jet to the north and the easterly jet to the south. In cave indentation of the Himalayan barrier in the western this setting of weak winds aloft and weak shear, the subregion is consistent with the frequent occurrence of deep intense convective echoes usually exhibit no tilt. lightning in this region (Barros et al., 2004). + These vertically erect 40 dBZ intense convective echoes The deepest intense convective echoes tend to be sometimes extend above 17 km. Such high reflectivity located just upwind of the Himalayas or over the values extending to such extreme altitudes suggests foothills (Figures 4(a), 28). These extremely deep con- graupel particles of substantial size being lofted to great vective events appear to occur where the advance edge heights by intense convective updraughts. of the low-level moist southwesterly monsoon flow from the Arabian Sea meets dry air descending from the adja- (a) cent Afghan or Tibetan Plateau (Figure 29(a)). The loca- 20 tion of the extremely deep convective outbreaks suggests a possible similarity to dryline convection. The moist 15 southwesterly low-level flow coming from the Indian Ocean is capped by an associated inversion as it under- 10 runs dry air advected from the Afghan or Tibetan Plateau. Sawyer (1947) and Carlson et al. (1983) suggested that 5 intense convection may occasionally penetrate this ‘capping inversion’ or (more frequently) break out near the leading edge of the warm, dry air advected in from the elevated plateau, where the lower moist layer is not capped. Our results are consistent with these ear- (b) lier studies. This scenario is particularly applicable over 20 the northwestern subcontinent, where the moist low-level flow from the Arabian Sea must traverse the hot arid 15 plain of the northwestern subcontinent before reaching the vicinity of the Himalayan barrier. The similarity to some aspects of severe convective formation over the US 10 Great Plains in the region east of the Rocky Mountains suggests a behaviour that is general and repeatable from

Height (km) 5 one region to another. The prevalent occurrence of the deepest and widest intense convective echoes just upstream of and over the lower elevations of the Himalayas – not over the higher terrain – has been seen in other precipitation regimes, (c) e.g. the western Ghats (Grossman and Durran, 1984) 20 and the European Alps (Houze et al., 2001). However, in the Himalayan region considered here, we also ﬁnd 15 that the most intense of the convection occurs upstream of and not over the higher terrain. The processes leading 10 to this behaviour in the Himalayan region are likely at least partially different from the processes governing 5 the precipitation in other regions. Volumetric statistical summaries (CFADs) compiled from all the TRMM PR overpasses of the study area show that the convective echoes tend to be slightly deeper and more intense over the lowlands than over the foothills of the Himalayas, Figure 29. Schematic illustrations of typical extreme precipitation indicating that the low-level conditions favouring deep events in each subregion of the study: (a) A deep intense convective convection are maximized just upstream (i.e. south) of echo in the western subregion, (b) a wide intense convective echo the mountains. Apparently, the full complement of low- (mesoscale convective system) over the lower terrain of the central subregion and deep intense convective echoes over the plateau, and level moisture and heat content present over the plains is (c) a broad stratiform precipitation area over the lower terrain of the not present over the foothills and higher terrain, so that eastern subregion and a deep intense convective echo over the plateau. the greatest orographic effect on convective precipitation

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj MONSOON CONVECTION IN THE HIMALAYAN REGION 1409 is felt just upstream of the mountain barrier. The dry (3) Arc-shaped lines quasi-perpendicular to the Him- air off the Afghan or Tibetan Plateau overriding the alayan barrier and propagating towards the southeast leading portion of the low-level monsoon flow caps the with a trailing region of older stratiform echo. These convection in the region possessing this stratification, occur only rarely such as when anomalously strong and a triggering mechanism is favoured immediately midlevel flow and associated shear becomes aligned upstream of or over the lower foothills of the mountain with the Himalayan escarpment. range. The triggering might involve upstream lifting (Grossman and Durran, 1984). However, diurnal heating Broad stratiform echoes occur primarily in the east- and cooling over the mountain barrier likely play a role ern and central subregions (Figure 29(c)). They occur in the triggering near the barrier. The exact triggering primarily upstream of and over the lower mountainous mechanism is a question to be resolved in future studies. terrain in association with Bay of Bengal depressions. The intense convective echoes sometimes organize Their centroids are most highly clustered within the con- upscale to form wide convective elements in which cave indentation at the eastern end of the Himalayan several cells with vertically erect intense convective barrier. This suggests that the tendency of the depression echoes become interconnected by weaker echo and cloud to promote convection is enhanced by moist flow over (Figure 29(b)). In extreme cases the interconnecting echo the topography of the Himalayas. Vertical cross-sections is of high intensity (>40 dBZ) and the embedded cells in MountainZebra show that the broad stratiform echoes even more intense and very deep. We have called these in the depression have widespread well-defined bright extreme cases ‘intense convective echoes’, defined as bands. The appearance of embedded cellular echoes in areas of echo exceeding 40 dBZ over a horizontal area the detailed cross-sections further suggests that broad >1000 km2. Such wide intense echoes are elements of stratiform echoes are, at least in part, the product of mesoscale convective systems (MCSs; Houze, 2004). In the weakening of earlier convective cells, in the manner several respects, the geographical pattern of occurrence described by Houze (1997). This latter supposition is dif- of the wide intense convective echoes detected by the ficult to verify in this study since the TRMM PR provides PR resembles that of the deep intense convective echoes: only snapshots. As pointed out by Houze (1993), strati- viz. the preferred location was in the western subregion, form precipitation may form in response to convection, and their occurrences were most frequent in the concave if either the convection is tilted by shear or if convective indentation of the Himalayan barrier in that region. cells repeatedly die out and new cells form in the same However, the pattern of occurrence of wide intense mesoscale vicinity. In the latter case, the older, weak- convective echoes differs in two respects from that of the ened cells coalesce to form a stratiform region in the deep intense convective echoes as defined in this study: vicinity of newer, active cells. Since the monsoon convection appears to have been predominantly non-tilted and occurring in a generally weakly sheared environ- (1) The wide intense convective echoes rarely, if ever, ment, it is likely that the stratiform regions were primarily occur over the Tibetan Plateau, whereas isolated deep conglomerations of older, weakened cells. Cross-sections intense convective echoes do occasionally appear such as Figures 22(c) and 23(c) are typical and qualita- over the Plateau (Figures 29(b) and (c)). Wide MCSs tively suggest that the bright band is the net result of contain large stratiform regions, which are favoured dying cells. in more oceanic conditions (Schumacher and Houze, CFADs show the bright-band signature to be much 2003a). We suggest that the arid conditions over the sharper and better defined in the eastern subregion than in Plateau (Wang and Gaffen, 2001) make it difficult for the western subregion. This observation, together with the convection to grow easily upscale to form MCSs. more frequent occurrence of large stratiform regions in (2) The wide intense convective echoes have a tendency the eastern subregion, suggests that the deep convection to appear in the central subregion, along the convex in the eastern subregion was of a more maritime char- mid-portion of the steep Himalayan barrier. They acter. By contrast, convection in the western subregion most frequently occur along the foothills and just was of a distinctly continental nature. The large strati- upstream of the steep terrain (Figure 29(b)). form regions associated with Bay of Bengal depressions observed over land by the TRMM PR appear to be simi- The wide intense convective echoes in our dataset lar to those previously observed over the Bay (Houze and exhibit three repeatable types of structure: Churchill, 1987). Apparently these depressions continue to provide a moist maritime monsoonal environment sup- (1) Multiple intense, vertically erect cells amassed within porting the convection as their parent circulations move a horizontally amorphous mesoscale echo; over land. In the eastern subregion, the monsoonal flow (2) Several intense, vertically erect cells grouped in a associated with depressions traverses only rather moist line parallel to the Himalayan barrier, either just land surfaces before encountering high terrain, in contrast upstream of the barrier or over the foothills. As the to the low-level southwesterly flow entering northwestern cells along these barrier-parallel lines weaken, they India from the Arabian Sea which must first traverse the form stratiform regions within the wide contiguous extremely hot and extensive desert region of the north- precipitation feature; and western subcontinent.

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 1389–1411 (2007) DOI: 10.1002/qj 1410 R. A. HOUZE JR, ET AL.

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