DECEMBER 2015 V I R T S A N D HOUZE 4739
KATRINA S. VIRTS AND ROBERT A. HOUZE JR. Department of Atmospheric Sciences, University of Washington, Seattle, Washington
(Manuscript received 1 May 2015, in ﬁnal form 5 August 2015)
Observations from A-Train satellites and other datasets show that mesoscale convective systems (MCSs) affect the water vapor and ice content of the tropical tropopause transition layer (TTL). The largest MCSs with radar reﬂectivity characteristics consistent with the presence of large stratiform and anvil regions have the greatest impact. Most MCSs are associated with clouds in the TTL. Composites in MCS-relative co- ordinates indicate enhanced cloudiness and ice water content (IWC) extending toward the cold-point tro- popause (CPT), particularly in large and connected MCSs. Widespread anvils in the lower TTL are evident in the peak cloudiness diverging outward at those levels. Upper-tropospheric water vapor concentrations are enhanced near MCSs. Close to the centers of MCSs, water vapor is suppressed at TTL base, likely because of the combined effects of reduced moistening or dehydration at the higher TTL relative humidities and sub- sidence above cloud top. Weak moistening is observed near the CPT, consistent with sublimation of ice crystals at the tops of the deepest MCSs. In the outﬂow region, moistening is observed in the lower TTL near the largest MCSs. Enhanced water vapor in the upper troposphere and lower TTL extends beyond the area of substantially enhanced cloudiness and IWC, in agreement with the observed radial outﬂow, indicating that MCSs are injecting water vapor into the environment and consistent with the possibility that MCS devel- opment may be favored by a premoistened environment.
1. Introduction is a positive maximum in the upper troposphere, with the strongest heating near anvil cloud top (Houze 1982). Deep convective clouds in the tropics sometimes ag- Observations from the CloudSat satellite, launched in gregate and develop extensive regions of stratiform pre- 2006, have permitted the analysis of the vertical structure cipitation and nonprecipitating anvil. Large, organized of clouds around the world. Cetrone and Houze (2009) cloud features of this type are called mesoscale convective and Yuan et al. (2011, hereafter YHH11) have used systems (MCSs; Houze 2014, chapter 9). Over half of CloudSat to examine characteristics of tropical MCS an- tropical precipitation and latent heating is produced by vils. The distribution of CloudSat reﬂectivities varies with MCSs (Yuan and Houze 2010, hereafter YH10). The la- anvil thickness and distance from the MCS center and is tent heating in MCSs is top heavy as a result of their sig- consistent with the detrainment of ice particles from the niﬁcant stratiform precipitation areas (Houze 1982, 1989), MCS updrafts into the anvils and the sedimentation, or and this top-heavy heating is important in forcing the gravitational settling and eventual removal, of the larger large-scale tropical circulation (Hartmann et al. 1984; particles as the anvil age increases. Both studies noted Schumacher et al. 2004). The net radiative heating in MCSs contrasts between the anvil structures of land and oceanic MCSs. Land MCSs exhibit broader and higher reﬂectivity distributions consistent with more intense convection and less extensive stratiform rain areas, while oceanic MCSs Denotes Open Access content. exhibit narrower distributions with modal intensity strongly decreasing with height, consistent with the de- velopment of broad stratiform areas. Corresponding author address: Katrina Virts, Department of Atmospheric Sciences, University of Washington, 408 ATG Bldg., MCS vertical structure in the troposphere has impli- Box 351640, Seattle, WA 98195-1640. cations for the systems’ impacts on the overlying tropical E-mail: [email protected] tropopause transition layer (TTL). The TTL extends
Ó 2015 American Meteorological Society 4740 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 from around 14 km, just above the level of main convective 2006). These observations suggest that tropical convec- outﬂow, to the highly stable stratosphere above 18 km tion affects the moisture budget of the TTL. (Fueglistaler et al. 2009; Randel and Jensen 2013). Air be- Analysis of the impact of deep convection on water low the TTL undergoes net radiative cooling as it subsides vapor proﬁles in the TTL has addressed the fundamental toward the ground, while air in the TTL tends to ascend question of whether the net contribution is to moisten or slowly while undergoing net radiative heating. The TTL dehydrate the environmental air. Relative humidity with links the rising branch of the tropospheric Hadley circula- respect to ice increases with height in the upper tropo- tion, whose ascent is concentrated in areas of tropical moist sphere and TTL because of the decrease in saturation convection, and the broader rising branch of the strato- mixing ratio in the ascending cold air, and supersatura- spheric Brewer–Dobson Circulation. The TTL is charac- tion frequently occurs in the TTL (Luo et al. 2007; terized by extremely low temperatures (Anthes et al. 2008; Fueglistaler et al. 2009). Previous studies have empha- Kim and Son 2012), and air ascending through the TTL sized the importance of the ambient relative humidity undergoes freeze drying, producing the low stratospheric in determining the impact of convection: convection in water vapor concentrations noted by Brewer (1949). subsaturated regions moistens the environment, while in The mean level of neutral buoyancy (LNB) in con- supersaturated regions net dehydration is observed, be- vective regions of the tropics lies between 12 and 14 km, cause excess water vapor is deposited onto the ice parti- just below TTL base (Takahashi and Luo 2014). Di- cles, which may be removed via sedimentation (Jensen vergent outﬂow from regions of tropical deep convection et al. 2007; Wright et al. 2009; Hassim and Lane 2010). occurs in the upper troposphere and at TTL base Chae et al. (2011), analyzing water vapor anomalies from (Highwood and Hoskins 1998; Folkins and Martin 2005). the Microwave Limb Sounder (MLS) below the cloud- Based on CloudSat observations of anvil characteristics, top heights identiﬁed in data from the Cloud–Aerosol Takahashi and Luo (2012) concluded that maximum Lidar and Infrared Pathﬁnder Satellite Observations mass detrainment from convection is around 11 km, just (CALIPSO) satellite, observed strong moistening in the below the LNB determined from atmospheric soundings. upper troposphere and decreased moistening above The geographic distribution of cirrus in the upper tro- TTL base, shifting to dehydration above 16 km if the posphere strongly resembles the distribution of tropical tops of clouds were above this level, consistent with less precipitation, consistent with cirrus forming in and effective moistening with increasing background hu- spreading out from the upper portions of deep convec- midity. They furthermore emphasized that the mecha- tion (Virts et al. 2010). Convectively generated cirrus nism they posited is not restricted to convective towers also occur in the TTL (Massie et al. 2002; Luo and and cirrus outﬂow but could apply to any cirriform Rossow 2004), although in situ formation in ascending cloud; the mechanism could occur in situ or in the upper layers becomes increasingly important at higher alti- reaches of stratiform or anvil tops of MCSs. tudes (Riihimaki and McFarlane 2010; Virts et al. 2010). As mentioned above, a fraction of deep convective Trajectory analysis suggests that cirrus can persist up to clouds penetrate the CPT. The penetrating towers con- 2 days after convection weakens and be advected up to tain extremely cold, dry air and have been hypothesized 1000 km from the region of the parent convection (Luo as a mechanism for lower-stratospheric dehydration and Rossow 2004). As this advection occurs, sublimation (Danielsen 1993; Sherwood and Dessler 2000). How- of the smaller ice crystals adds vapor to the environment ever, modeling studies of such clouds indicate net hy- (Soden 2004). dration because they leave behind small ice particles Observations across the tropics indicate that some that undergo sublimation in the relatively warmer deep convection overshoots its LNB and penetrates into stratosphere (Chaboureau et al. 2007; Jensen et al. the TTL. Overshooting occurs over both ocean and land, 2007). These modeling results have been conﬁrmed by although the most vigorous convection is concentrated observational evidence (e.g., Corti et al. 2008; Khaykin over land (Alcala and Dessler 2002; Liu and Zipser 2005; et al. 2009). However, the horizontal scale of the pene- Zipser et al. 2006). Alcala and Dessler (2002) found that trating cores of active convection is very small, and it approximately 5% of convective elements observed by seems unlikely that they alone can account for the water the Tropical Rainfall Measuring Mission (TRMM) ra- vapor properties of the TTL. dar penetrated into the TTL. Takahashi and Luo (2012) Rossow and Pearl (2007) concluded that most con- reported that mean cloud-top height of the deepest vection penetrating near the CPT is associated with or- convective cores observed by CloudSat is 15 km, within ganized (i.e., large) MCSs; however, to date little the TTL. However, less than 1% of deep convective analysis has been attempted of MCS characteristics in clouds reach the cold-point tropopause (CPT; Alcala the TTL or how MCSs might affect the moisture content and Dessler 2002; Gettelman et al. 2002; Dessler et al. of the TTL. Yet MCSs dominate much of the upper-level DECEMBER 2015 V I R T S A N D HOUZE 4741
TABLE 1. Methodology used by YH10 to identify cloud and precipitation features and MCSs, based on MODIS Tb11 and AMSR-E AE_ Rain data. The cloud-top minimum temperature Tb11RC1min is deﬁned as the mean Tb11 of the coldest decile of the largest raining core (RC1). Adapted from YHH11.
Abbreviation Name Deﬁnition
HCC High cloud complex Region of MODIS Tb11 contained within a single 260-K isotherm HCS High cloud system Portion of HCC associated with a particular minimum value of Tb11 2 PF Precipitation feature Region of AMSR-E AE_Rain parameter surrounded by 1 mm h 1 contour RC Raining core Portion of any PF overlapping and/or located within an HCS 2 HRA Heavy rain area Portion of PF with rain rate greater than 6 mm h 1 MCS Mesoscale convective Any HCS whose largest RC satisﬁes the following criteria: system 1) Exceeds 2000 km2 in total area 2 2) Accounts for more than 70% of the total area with rain rate greater than 1 mm h 1 inside the HCS
3) Minimum cloud-top temperature above the RC (indicated by Tb11RC1min) is less than 220 K 4) More than 10% of RC is occupied by HRAs SMCS Separated MCS The largest RC of the MCS is part of a PF that contains less than three dominant RCs of any MCS CMCS Connected MCS The largest RC of the MCS is part of a PF that contains dominant RCs of at least three MCSs cloud coverage of the tropics. The stratiform portions of distribution of clouds, ice, and water vapor in MCSs in MCSs are orders of magnitude larger in scale than the the TTL is presented in section 5. Conclusions are in convective towers embedded in the MCSs, and the strat- section 6. iform regions are buoyant and have very cold tops rising and diverging over wide areas, suggesting that they might have a substantial effect on the moisture of the TTL. In 2. Data this paper, we investigate the water vapor and ice content a. Observations from A-Train satellites of MCSs in the upper troposphere and TTL to determine the impact of these larger convective entities on the water NASA’s A-Train constellation operates in a sun- content at upper levels. synchronous orbit, crossing the equator around 1:30 To identify MCSs, we use the dataset of YH10, who and 13:30 LT. The A-Train instruments used in this introduced a methodology for identifying MCSs in ob- study, including the YH10 MCS database derived from servations from NASA’s A-Train satellites (L’Ecuyer observations from two of those instruments, are de- and Jiang 2010): high, cold cloud tops observed by the scribed in this section. Moderate Resolution Imaging Spectroradiometer 1) MCS DATABASE (MODIS) and precipitating areas observed by the Ad- vanced Microwave Scanning Radiometer for Earth Clouds and precipitating areas are two prominent Observing System (AMSR-E; see section 2 for details). MCS components readily observed by satellites. The Other instruments aboard A-Train satellites observe YH10 methodology begins with the identiﬁcation of cloud vertical structure (CloudSat), including that of these components in data from two instruments on the optically thin cirrus layers (CALIPSO), and measure Aqua satellite: MODIS 10.8-mm brightness tempera- proﬁles of ice water content (IWC) and water vapor tures (TB11) and the AMSR-E AE-Rain product concentration in the TTL (MLS). Collectively, these (Kummerow et al. 2001; Wilheit et al. 2003; Kummerow observations offer a unique opportunity to analyze the and Ferraro 2006). Their methodology is summarized in vertical structure of MCSs and their contribution to Table 1. clouds and moisture in the TTL. We analyze these ob- YH10 then divide MCSs into two categories. Sepa- servations of water vapor and ice in the vicinity of each rated MCSs (SMCSs) are those in which the pre- MCS in the YH10 database. cipitation feature (PF) containing the largest raining The remainder of the paper is organized as follows: the core (RC) contains at most one other dominant RC of YH10 MCS identiﬁcation scheme is described in section any MCS. Long-lived MCSs in close vicinity sometimes 2, along with the other satellite datasets used in this study. merge to form larger precipitating systems (Williams The climatological distributions of MCSs and of clouds and Houze 1987; Mapes and Houze 1993). This type of and moisture in the TTL are shown in section 3.MCS system is designated by YH10 as a connected MCS vertical structure and microphysical characteristics in (CMCS), in which a rain area contains the largest RC of the troposphere are discussed in section 4, while the at least three MCSs. SMCSs occur more frequently and 4742 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 exhibit a greater range of sizes than CMCSs, so they are 4) MLS ICE WATER CONTENT AND WATER VAPOR further subdivided into small SMCSs (the smallest 25% The MLS aboard the Aura satellite observes thermal by area, with HCSs less than approximately 11 000 km2) microwave emissions in ﬁve spectral regions. We ana- and large SMCSs (the largest 25%, with HCSs greater lyze MLS version 3 vertical proﬁles of IWC and water than approximately 41 000 km2). In this study, we ana- vapor mixing ratio, whose validations are described by lyze all MCSs identiﬁed in the tropics (308N–308S) Read et al. (2007) and Wu et al. (2008), respectively. during 2007–10. Proﬁles are provided at 1.58 intervals along track and 2) CLOUDSAT REFLECTIVITY have vertical resolutions of 4 and 3 km, respectively, and horizontal resolutions of approximately 7 km CloudSat carries a 94-GHz cloud proﬁling radar across track and approximately 300 km along track. (CPR) that measures backscatter from clouds (Stephens The useful altitude ranges are from 215 to 83hPa for et al. 2002; Marchand et al. 2008). The backscatter IWC and above 316 hPa for water vapor. The un- proﬁles are available every 2.5 km along track and certainties for water vapor are 20% below 100 hPa and have a vertical resolution of 240 m at nadir. A cloud 10% at 100 hPa and above, while the uncertainty for mask is provided in the CloudSat geometric proﬁle IWC is a factor of 2, largely owing to the uncertainty product (2B-GEOPROF). In this study, observations from particle size assumption (Jiang et al. 2012). The within clouds are identiﬁed by screening for cloud mask data are screened following the recommendations in values greater than or equal to 20. Livesey et al. (2011). The CloudSat proﬁles analyzed in this study are those MLS also observes atmospheric temperature, with identiﬁed by YHH11 as sampling some portion of an recorded biases of about 1.5 K in the TTL (Schwartz MCS. In that study and in YH10, it was noted that et al. 2008). For this paper, we apply an offset at each clouds with tops above 10 km identiﬁed by CloudSat pressure level based on comparison with global posi- fall into two categories: precipitating clouds with bases tioning system (GPS) temperature observations, as near the surface and elevated anvils with bases pri- given by Chae et al. (2011). The water vapor and cor- marily above the freezing level. Accordingly, they de- rected temperature observations are then used to cal- ﬁned anvil clouds as having base above 3 km and top culate relative humidity with respect to ice (referred to above 10 km. The same deﬁnition is used in this study. herein as relative humidity). Because it is dependent We will also present statistics on CloudSat proﬁles upon the resolution of the temperature observations, the sampling nonanvil regions of the MCS. This category is vertical resolution of the relative humidity is coarser dominated by proﬁles sampling the deep convective or than that of water vapor: approximately 5 km in the TTL stratiform precipitating areas of the system and is re- (Schwartz et al. 2008; Livesey et al. 2011). ferred to herein as the precipitating category.
3) CALIPSO CLOUD OCCURRENCE b. ERA-Interim y The Cloud–Aerosol Lidar with Orthogonal Polar- The horizontal winds (u, ) and vertical velocities in v ization (CALIOP) is a two-wavelength polarization pressure coordinates ( ) in this study are from the lidar carried aboard CALIPSO. Cloud layer base and European Centre for Medium-Range Weather Fore- top heights are identiﬁed from the lidar backscatter casts (ECMWF) interim reanalysis (ERA-Interim; and are provided at a resolution of 60 m in the vertical Dee et al. 2011). ERA-Interim ﬁelds are available four 8 in the TTL and 5 km along track. Following Fu et al. times daily at 1.5 resolution for ﬁve pressure levels in (2007), opaque features (those that completely at- the upper troposphere and TTL (200, 175, 150, 125, and y tenuate the lidar signal) are assumed to be deep 100 hPa). The u and ﬁelds are used to calculate the convective clouds and assigned a cloud base at radial wind—that is, the component of the horizontal Earth’s surface. While CloudSat’s estimated opera- wind directed toward or away from the center of tional sensitivity (approximately 232 to 230 dBZ)is each MCS. insufﬁcient to see clouds with small IWC, CALIPSO c. Compositing technique can detect cloud layers with optical depths as low as 0.01 or less (Winker et al. 2007). In this study, we For the results presented in section 5, each MCS in the take advantage of the complementary capabilities of YH10 database is assigned a separate coordinate sys- these instruments, using CloudSat to examine the tem, where the origin corresponds to the center of the vertical structure of the deep convective clouds and largest raining core. Observations from CALIPSO, CALIPSO to analyze the extent of cirrus shields in MLS, and ERA-Interim are extracted and stored in the TTL. system-relative coordinates—that is, as a function of DECEMBER 2015 V I R T S A N D HOUZE 4743
FIG. 1. Density of (top) small SMCSs, (middle) large SMCSs, and (bottom) CMCSs for the years 2007–10, expressed as the number of systems observed in some portion of a 18318 grid box per year (note that the color scales differ). vertical height or pressure level and distance from the convection over the tropical continents and the warm MCS. For MLS and ERA-Interim, the dataset pressure pool. Clouds and IWC are more strongly focused in levels and horizontal sampling frequency (every 1.58) areas with high annual-mean precipitation and frequent are retained. As described earlier, CALIPSO observa- MCS occurrence (Fig. 1). Water vapor and relative hu- tions are available at higher spatial resolution; however, midity exhibit more diffuse distributions, indicating the in order to reduce noise, we calculate cloud occurrence advection of water vapor beyond the areas of frequent for 0.25 km vertical and 18 horizontal bins. No further deep convection as well as the effect of the large-scale spatial ﬁltering is applied to the data. When indicated, temperature ﬁeld. anomalies are calculated by subtracting the climatolog- The climatological, zonal-mean vertical structure of ical monthly mean at each location prior to compositing. tropical clouds and moisture is shown in Fig. 3. In the upper troposphere, clouds and ice are most frequent around 78N—the latitude of the rising branch of the 3. Climatology of MCSs and the TTL annual-mean Hadley circulation. A secondary maxi- The annual distribution in Fig. 1 indicates that all mum in IWC is observed south of the equator, associated three types of MCSs are observed throughout the deep with convection in the austral summer Hadley cell. IWC convective regions of the tropics—the tropical conti- concentrations decrease with height above ;170 hPa, nents, the intertropical convergence zones (ITCZs), and while cloud occurrence broadens and becomes more ubiquitously over the western Paciﬁc warm pool. How- equatorially symmetric in the TTL, where cirrus can ever, as shown by YH10, both large and small SMCSs form either through the shearing off of convective anvils are more frequently observed over land or near-coastal or in situ in rising, adiabatically cooling layers. regions, while the merging of systems to produce Water vapor concentrations decrease by about three CMCSs occurs mostly over the oceans, especially the orders of magnitude from the upper troposphere to the warm pool. lower stratosphere (Fig. 3c). This strong decrease with Annual-mean distributions of clouds and moisture at height reﬂects the temperature dependence of the 147 hPa, near TTL base, are shown in Fig. 2. Maxima in saturation mixing ratio, particularly at the low tem- each ﬁeld are located over the regions of intense peratures observed in the TTL. The decrease in the 4744 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72
23 FIG. 2. Annual-mean (a) CALIPSO cloud fraction, (b) MLS IWC (mg m ), (c) MLS water vapor mixing ratio (ppmv), and (d) MLS-relative humidity with respect to ice (%) at 147 hPa. Relative humidity values have been corrected for MLS temperature bias [see section 2a(4)]. infusion of water vapor into the environment by con- and Houze 1995; Masunaga et al. 2008). Cloud micro- vection also contributes to the water vapor distribution at physical processes can then be inferred from the re- higher altitudes. Despite the low water vapor concen- ﬂectivity distributions. While YHH11 analyzed CFADs trations in the TTL, relative humidities are high (Fig. 3d), of anvil characteristics as functions of geographic re- with zonal-mean values in excess of 80% because of the gion, anvil thickness, and distance from the MCS center, extremely low temperatures. The high relative humidities we examine reﬂectivity characteristics for both the in the TTL promote cirrus occurrence. precipitating portions and the nonprecipitating anvils of Before considering the contribution of MCSs to each category of MCS (Fig. 4). The primary features of clouds and water vapor in the TTL, it is useful to analyze the CFADs are shared by each type of MCS: in the their characteristics in the troposphere, speciﬁcally their precipitating areas, peak reﬂectivities of 10–15 dBZ are vertical structure and microphysical attributes. In the observed in the melting layer (around 4–5 km). Re- following section, these characteristics are investigated ﬂectivities decrease below the melting layer as a result using CloudSat observations. of signal attenuation by the precipitation. High re- ﬂectivities dominate the distribution through most of the troposphere, indicating the presence of large drop- 4. Vertical structure of MCSs in the troposphere lets in the convective updraft below the melting layer The distribution of CloudSat reﬂectivities in MCSs and the formation of graupel and large ice particles as a function of height can be represented using con- above. In the upper troposphere, above 9–10 km, re- toured frequency by altitude diagrams (CFADs; Yuter ﬂectivities generally decrease with height because of DECEMBER 2015 V I R T S A N D HOUZE 4745
anvils are lower, generally below 10 dBZ (Fig. 4). Peak reﬂectivities are observed in the lower portions of MCS anvils, around 6–10 km, and the median reﬂectivity de- creases with height to approximately 220 dBZ near 13 km. Vertical velocities in anvils are generally weak, such that ice particles tend to drift downward (Houze 2014, chapter 6). The presence of larger particles near anvil cloud base reﬂects this sedimentation, as well as ice particle growth by aggregation (McFarquhar and Heymsﬁeld 1996). YHH11 further noted weaker re- ﬂectivities and a narrowing of the reﬂectivity distribu- tion in anvils as distance from the MCS core increased, consistent with aging. While the CFADs for the three MCS categories share these basic similarities, key differences also exist. In the raining cores of small SMCSs, the reﬂectivity distribu- tion exhibits a more vertical orientation, with median reﬂectivities remaining high (greater than 0 dBZ)upto 13 km. In contrast, in large and connected MCSs, re- ﬂectivities decrease more rapidly with height. To more clearly contrast the MCS types, difference plots are shown in Fig. 5. These plots were constructed by sub- tracting the CFADs for small SMCSs from those for large SMCSs, such that blue shading indicates the re- ﬂectivities proportionately more likely to be observed in small systems. In Fig. 5, above the melting layer, high reﬂectivities are more commonly observed in small SMCSs, indicative of strong convection and less de- veloped stratiform regions. The reverse is observed above the melting layer in large SMCSs, where the narrower reﬂectivity distribution is consistent with the presence of stratiform rain areas. A narrower distribu- tion is also observed in the anvils of large SMCSs com- pared to those in small SMCSs, as shown by the pattern of blue shading in Fig. 5b. The greatest contrast is ob- served in the occurrence of high reﬂectivities above 9 km, which are much more common in the small SMCSs, as the larger particles formed in the intense convective updrafts ﬁnd their way into the anvils of the small MCSs.
FIG. 3. Annual-mean, zonal-mean (a) CALIPSO cloud fraction, 23 (b) MLS ice water content (mg m ), (c) MLS water vapor mixing 5. MCS impacts on the TTL ratio (ppmv; note the log scale), and (d) MLS-relative humidity with respect to ice (%). Relative humidity values have been cor- Having examined MCS vertical structure in the tro- rected for MLS temperature bias [see section 2a(4)]. For (a), the posphere, we now investigate the impact of MCSs on the vertical coordinate is given in both pressure (hPa) and altitude (km, rounded to the nearest 0.5 km). overlying TTL. In this section, we present analyses of clouds, wind, IWC, and water vapor in the vicinity of the MCSs, with the variables composited in system-relative lower concentrations of particles of generally smaller coordinates as described in section 2c. dimension. a. Clouds and wind As air from the MCS raining cores is incorporated into the anvils, microphysical processes modify the Cross sections of composited cloud fraction in the characteristics of the ice particles. Reﬂectivities in MCS upper troposphere and TTL near MCSs are shown in 4746 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72
FIG. 4. CFADs of CloudSat reﬂectivities in (a),(d) small SMCSs, (b),(e) large SMCSs, and (c),(f) CMCSs, using proﬁles sampling (top) the precipitating portions of the MCSs and (bottom) the anvil portions. Each CFAD is normalized such that its maximum value is 1.
the left column of Fig. 6. The basic cloud distribution is SMCS compared to a small SMCS. These differences similar for all MCS types: while the largest cloud frac- lessen with distance, and at 1500 km from the MCS, tions are observed lower in the troposphere, cloud oc- composite cloud fractions are ;0.2 in each case. At such currence is frequent in the upper troposphere near the large distances, CALIPSO is likely observing neigh- MCS center. About 74% of CALIPSO proﬁles within boring clouds. To reduce the impact of clouds that may 100 km of the center of an MCS observed a cloud top not be part of the MCSs, we subtract the climatological above 150 hPa, at TTL base, and about half of such monthly mean cloud fraction at each grid point prior to layers were opaque to the lidar (not shown). A cloud compositing. Anomalous cloud fractions for each MCS top above 120 hPa was observed in about 47% of the type are shown in the right column of Fig. 6. Here, the proﬁles. Above that level, cloud occurrence decreases contrast between small SMCSs and large and connected more rapidly with height. Beginning within about MCSs is even more striking. Enhanced cloudiness near 200 km of the MCS center and extending farther away, small SMCSs is observed through the depth of the TTL an outward bulge in cloud fraction indicates the but is conﬁned to within about 200 km of the MCS spreading of cirrus anvils. This bulge is centered center, and the magnitude of the anomalies is small: less ;170 hPa near the MCS core and shifts upward to than 0.1. In large and connected MCSs, however, ;150 hPa at greater distances. cloudiness anomalies are larger and extend higher in the Large and connected MCSs are associated with sub- TTL. This result is in agreement with Rossow and Pearl stantially greater cloud fractions in the upper tropo- (2007), who reported an increasing likelihood of con- sphere and TTL than small SMCSs, particularly in the vective intrusions of the TTL with increasing cloud area within a few hundred kilometers of the MCS center, system size and that such intrusion was observed in al- where the probability of CALIPSO observing a cloud in most all systems with radii . 500 km. We also observe the upper troposphere is almost double near a large enhanced cloud occurrence to distances of 600 km and DECEMBER 2015 V I R T S A N D HOUZE 4747
MCSs are associated with both stronger and broader ascent than small SMCSs, and their anomalous di- vergence extends into the cloud-free environment. b. Ice Ice associated with MCSs can be derived from MLS observations, and IWC anomalies calculated as de- scribed in section 2c are shown in Fig. 7. IWC anomalies for the subset of levels above TTL base are shown in the right column. The largest ice concentrations and anomalies in these cross sections are in the upper tro- posphere near the MCS convective cores and extend upward into the TTL. Large and connected MCSs inject larger quantities of ice into the upper troposphere and
FIG. 5. Difference between CFADs for large and small SMCSs, TTL than small SMCSs. IWC decreases with distance using proﬁles sampling (a) the precipitating portions of the MCSs from the MCS core, indicating the removal of larger ice and (b) the anvil portions. Blue shading indicates where reﬂectivity particles via sedimentation and/or sublimation in the values are proportionately more likely to be observed in divergent airﬂow associated with the stratiform and a small SMCS. anvil regions. While peak enhanced cloudiness in the MCS outﬂow lies at or above TTL base, as illustrated by the overlaid cloud fraction anomalies, the largest IWC beyond in large and connected MCSs. The outward values are at the bottom of the cross section. This is bulge in the anomaly patterns is at a slightly higher al- partly a result of the relatively coarse vertical resolution titude than that in the composites in Fig. 6a, indicating of the IWC data. In addition, CALIPSO’s sensitivity that the MCS anvil cirrus and outﬂow are slightly higher enables it to detect cirrus layers with IWC too small to than what is typically observed in the tropics. be detected by MLS. As discussed in section 1, various studies have ana- lyzed how frequently deep convection reaches into the c. Water vapor tropical stratosphere. Our analysis indicates that 11% of CALIPSO proﬁles within 100 km of the center of an As shown in Fig. 2, water vapor near TTL base is more MCS indicate a cloud top above 95 hPa, near the CPT, broadly distributed than ice. The strong decrease of and approximately 1% indicate a cloud top above water vapor concentrations with altitude in the TTL 80 hPa, near the top of the TTL. Of the cloud layers dominates cross-sectional composites of water vapor with tops above 95 hPa within 100 km of the center of relative to MCS location (not shown). It is more in- an MCS, 65% have a sufﬁciently low optical depth that formative to present the composites as anomalies from they are not opaque to the lidar. These nonopaque the climatological monthly mean (Fig. 8). Each MCS layers have a median thickness of 1.8 km (not shown). type is associated with enhanced water vapor in the Thus, while some portion of convection in MCSs ex- upper troposphere. The anomalies decrease with dis- tends into the lower stratosphere, the majority of the tance from the MCS but extend radially outward beyond extremely high-topped clouds observed by CALIPSO the area of substantially enhanced cloudiness or IWC near MCSs are thin anvil clouds or cirrus layers over- (cf. Fig. 7). As noted above, the anomalous upper- lying the deep convective clouds, as discussed in tropospheric divergence also extends beyond the Garrett et al. (2004). cloudy region, particularly in large and connected The overlaid vectors in Fig. 6 show the radial winds MCSs. The true strength and extent of the MCS outﬂow and vertical velocities from ERA-Interim. Strong ascent can only be estimated from the reanalysis winds. Be- is observed in the convective core of the MCS, and its cause A-Train orbits repeat only every 16 days, we also magnitude is underestimated as a result of the relatively cannot determine whether premoistening of the upper coarse resolution of the reanalysis data. The vertical troposphere occurred prior to MCS formation. How- velocities weaken rapidly with height in the stably ever, the results in Fig. 8 indicate that water vapor stratiﬁed TTL. Upper-level divergence and outﬂow spreads laterally from the MCSs and into the sur- from the MCS are reﬂected in the outward direction of rounding cloud-free region prior to MCS dissipation. the composite radial winds. A similar pattern is ob- Previous studies have shown that diverging cirrus can served in the anomaly wind ﬁelds. Large and connected persist for up to 2 days after convective dissipation and 4748 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72
FIG. 6. (left) Composite CALIPSO cloud fraction [colors and contours; contour interval (CI) 5 0.1] and ERA- Interim vertical velocity and horizontal radial wind (vectors) in the upper troposphere and TTL as a function of distance from MCS center, for (a),(b) small SMCSs, (c),(d) large SMCSs, and (e),(f) CMCSs. (right) Anomalies of cloud fraction (CI 5 0.05) and wind relative to the MCS center. be advected up to 1000 km and that elevated upper- occurs in response to strong divergence in the cloud tropospheric water vapor concentrations persist longer outﬂow. They point out that these water vapor patterns than the cirrus (Luo and Rossow 2004). are observed with any type of cloud in the TTL: deep Water vapor values in the TTL are too small to be seen convective, convectively generated cirriform, or in situ in the left column of Fig. 8, which is dominated by the cirriform. It is therefore reasonable that the processes moister upper troposphere. Anomalies for the subset of they describe should be associated with the MCSs dis- altitudes in the TTL (above ;150 hPa) are shown in the cussed herein, thus producing the negative water vapor right column of Fig. 8. Contrasting characteristics are anomalies in the lower TTL in Fig. 8. The cirriform upper- observed near the MCS centers (within a radius of about level clouds of MCSs and their anvils are produced by the 200 km) and at distances greater than about 400 km. Near combination of convective and mesoscale dynamics that the convective core, water vapor concentrations are be- characterize MCSs (Houze 2004, 2014, chapter 9). We low the climatological mean in the lowermost TTL, par- further observe weak moistening in the upper TTL above ticularly in large and connected MCSs. This altitude can the MCS centers, consistent with the observation of Corti be above, at, or below the cloud top, as indicated by the et al. (2008) that the deep clouds that extend upward overlaid cloud fraction anomalies and the discussion in beyond the CPT leave behind ice particles which sub- section 5a. Chae et al. (2011) noted rapidly decreasing limate and moisten the environment. moistening within clouds above about 14 km and weak Outside the convective core, at distances of about dehydration just below cloud-top level when the cloud 400 km from the MCS center and extending outward, extended above 16 km, as the environmental relative enhanced water vapor concentrations are observed in humidity became supersaturated. They also noted strong the lower TTL as well as the upper troposphere (Fig. 8), dry anomalies in the lower TTL when those altitudes lay indicating that outﬂow from the MCSs and evaporative just above the convective cloud tops, where subsidence processes are moistening the environment. The water DECEMBER 2015 V I R T S A N D HOUZE 4749
23 FIG. 7. (left) Composite MLS ice water content anomalies (colored shading; mg m ) as a function of distance from MCS center, for (a),(b) small SMCSs, (c),(d) large SMCSs, and (e),(f) CMCSs, overlaid with cloud fraction (contours; CI 5 0.05) and circulation anomalies (vectors) from Fig. 6. (right) As in (left), but for subset of altitudes in the TTL. vapor anomalies in this region decrease more rapidly moistening of a broad region by the smallest MCSs. with height than the environmental water vapor (cf. with Rather, it appears that small SMCSs tend to form in Fig. 3), consistent with decreased convective inﬂuence large-scale environments with a slightly moistened TTL. and less effective moistening in the TTL. The moisten- ing appears to extend upward to ;120 hPa; however, 6. Conclusions observations with ﬁner vertical resolution would be needed to examine the precise vertical extent of the In this study, the vertical structure and microphysical moistening. As in the upper troposphere, enhanced composition of MCSs, as well as their role in detraining water vapor in the TTL extends laterally into cloud-free ice particles and water vapor in the TTL, have been regions. Weak dry anomalies are observed near the investigated using a suite of satellite and reanalysis ob- CPT, near the tops of the highest clouds (cf. with Fig. 6). servations. We have examined three types of MCSs, as The water vapor anomaly pattern near small SMCSs identiﬁed in observations from MODIS and AMSR-E exhibits different characteristics than that for large and (YH10): small and large separated MCSs and connected connected MCSs. Primarily, it is less robust, indicating systems mostly likely formed via the merging of multiple that it is the largest MCSs that are important to the MCSs. MCSs are ubiquitous in the convective regions of moistening of these upper levels. Positive anomalies the tropics (Fig. 1), including East Asia, the western associated with the small SMCSs are observed through Paciﬁc warm pool, and Central and South America, most of the TTL, with peak values at 121 hPa. Laterally, which have been identiﬁed as major source regions the anomalies extend beyond the narrow core of en- for air parcels eventually entering the stratosphere hanced cloudiness and several hundred kilometers into (Schoeberl et al. 2013). SMCSs occur more frequently the region of suppressed cloudiness (i.e., beyond the red over the tropical landmasses, while the merging of line in Fig. 8b). As previously noted, the radial outﬂow longer-lived systems to form CMCSs is more common in associated with small SMCSs is weak; hence, it is un- the oceanic ITCZs and over the western Paciﬁc warm realistic to interpret the anomaly pattern in Fig. 8b as pool (Fig. 1). 4750 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72
FIG.8.AsinFig. 7, but colored shading is MLS water vapor mixing ratio anomalies (ppmv).
CloudSat reﬂectivity distributions in the precipitating but also a prominent outward bulge in cloud occur- regions of active MCSs indicate high reﬂectivities in the rence near TTL base associated with anvil cirrus. Most midtroposphere, indicative of large ice and graupel MCSs are associated with clouds in the TTL: 74% of particles forming in the convective updrafts and evolv- CALIPSO proﬁles within 100 km of the center of an ing into stratiform precipitation as the updrafts weaken MCS exhibited a cloud top above TTL base (about or being detrained into neighboring stratiform regions 150 hPa; Fig. 6). Large and connected MCSs are asso- of the MCSs (Fig. 4). Reﬂectivities in the MCS anvils are ciated with more frequent cloud occurrence in the TTL generally lower and decrease rapidly with altitude in the than small SMCSs, in agreement with the inference of upper troposphere, consistent with sedimentation and Rossow and Pearl (2007) that the likelihood of cloud aggregation within the anvils and the removal of larger penetrating upward into high levels increases with graupel prior to air entering the anvil, as previously system size. Rossow and Pearl (2007) and Takahashi shown by Cetrone and Houze (2009) and YHH11. Small and Luo (2014) both report that penetration of the TTL SMCSs exhibit characteristics of intense, young con- tends to occur in the growing stage of the cloud system, vection and are consequently more likely than large and although the latter noted signiﬁcant penetration during connected MCSs to contain large ice and graupel par- the mature stage. Our results associate the most ex- ticles in the upper troposphere (Fig. 5). Larger MCSs tensive cloudiness in the TTL speciﬁcally with large exhibit narrower, more strongly tilted reﬂectivity dis- MCSs, in which anvils and overlying cirrus occur in tributions in the upper troposphere, consistent with the greater quantities. ‘‘Overshooting’’ convective ele- ‘‘size sorting’’ processes characteristic of mature anvils ments are actually less likely in these more widespread (McFarquhar and Heymsﬁeld 1996). MCSs; for example, Virts and Houze (2015) showed Composites of cloud occurrence observed by that the larger MCSs have less vigorous convective CALIPSO near MCSs (Fig. 6) reveal that cloudiness elements, as indicated by lightning occurrence. It is the anomalies in small SMCSs are primarily vertically ori- combination of convective and stratiform dynamics of ented and conﬁned near the MCS cores, while large and MCSs that make them robust in affecting cloudiness in connected MCSs exhibit not only broader core regions the TTL. DECEMBER 2015 V I R T S A N D HOUZE 4751
MCSs inject ice into the upper troposphere and TTL, greater importance than isolated overshooting cells be- as shown by composites of anomalous MLS IWC with cause they are large enough in horizontal scale to be respect to MCS location (Fig. 7). The largest ice quan- highly impactful on the water vapor ﬁeld. Quantiﬁcation tities are near the centers of the MCSs, but elevated ice of the comparative contributions of convection of vari- concentrations also extend laterally into the stratiform ous sizes and organization to the total water vapor and anvil regions. Large and connected MCSs contrib- content of the TTL and lower stratosphere is needed but ute the greatest quantities of ice to the TTL. These is not straightforward given the water vapor data cur- systems contain predominately smaller ice crystals and rently available. less graupel than the small SMCSs (Fig. 5). The less MCS evolution cannot be examined using A-Train rapid sedimentation rates of the smaller particles allow satellite observations, which provide only snapshots in them to remain aloft in the stratiform and anvil regions. time. However, the results presented in this paper sug- We note that the IWC anomalies decrease nearly gest that small SMCSs are primarily young, developing monotonically with height in the upper troposphere and systems, while large and connected MCSs represent the TTL, while the cloud occurrence exhibits an outward mature stage of the MCS life cycle (Houze 2004, 2014, bulge associated with the spreading anvils. This differ- chapter 9). Small SMCSs contain vigorous convection ence is largely due to the coarser vertical resolution of that produces large ice particles, especially graupel. the IWC data (approximately 4 km in the TTL; Wu et al. These systems have limited stratiform and anvil regions, 2008) but is also consistent with lower ice concentrations but the anvils that do exist contain large particles de- in the higher cirrus layers observed by CALIPSO. trained from the convective core. Water vapor also de- Composites based on MLS water vapor mixing ratios trains from the convective region, moistening the upper (Fig. 8) demonstrate that MCSs moisten the upper tro- troposphere, while reduced moistening in convection posphere. Moreover, stronger moistening is associated penetrating the TTL combined with subsidence above with the large and connected MCSs. In the TTL, a cloud top produces negative water vapor anomalies near complex structure is observed that is consistent with TTL base. As MCSs mature (and in some cases merge), previous studies of dehydration and hydration at upper their convection becomes less vigorous, resulting in re- levels. Near the MCS cores, water vapor concentrations duced ice and graupel size. This ice is incorporated into are suppressed near TTL base. Higher relative humidity the extensive stratiform and anvil regions (either by in the TTL (Fig. 3) reduces the possible amount of detrainment or by cells weakening and becoming strat- moistening, and subsidence above cloud top has been iform), where it undergoes sedimentation and aggrega- observed to produce strong drying in the lower TTL, as tion, such that smaller ice particles dominate near cloud shown by Chae et al. (2011). Weak moistening is ob- top. At this stage, MCSs are characterized by extensive served in the upper TTL, near the CPT, as the deepest upper-level divergence. Anvils extend radially outward penetrating convection leaves behind small ice crystals via this divergence, with peak cloud occurrence near that sublimate and moisten the environment. A reversed TTL base shifting to slightly higher altitudes with dis- pattern is observed from approximately 400 km out- tance from the MCS core. The MCSs thus moisten the ward, where the spreading anvils of large and connected upper troposphere via sublimation, and water vapor is MCSs moisten the lower TTL, and weak drying is ob- detrained laterally into the cloud-free air. At larger served at the CPT. The water vapor and radial wind distances from the convective core, where the above- anomalies in the upper troposphere and lower TTL ex- cloud subsidence is weaker, this moistening extends into tend into the cloud-free areas, indicating that the MCS the TTL. outﬂow is moistening the environment. We also em- Our discussion has focused on contrasts between phasize the possibility that a premoistened environment small SMCSs and large and connected MCSs. More favors the development of mature MCS stratiform and subtle differences exist between large SMCSs and anvil regions, but this cannot be investigated here owing CMCSs. As shown in Fig. 1, large SMCSs occur more to the limitations of satellite data. often over or near land, while most CMCSs are observed Our conclusion that large and connected MCSs more over the oceans. Thus, in a broad sense, differences strongly moisten the upper troposphere and lower TTL between these two MCS categories represent the dif- than small SMCSs contrasts with the notion that ferences between the largest MCSs over land and moistening of the TTL is primarily caused by isolated oceans. Examining Figs. 4 and 6–8, it can be seen that overshooting convective cells. The area covered by compared to large SMCSs, CMCSs are slightly larger overshooting convective cells is many orders of magni- (Yuan and Houze 2013), exhibit somewhat stronger tude smaller than the area occupied by MCSs. Our re- outﬂow in the upper troposphere, and are associated sults indicate that the largest, mature MCSs are of with larger ice and water vapor anomalies. However, 4752 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 these differences are slight, and our results indicate that Dessler, A. E., S. P. Palm, and J. D. Spinhirne, 2006: Tropical mature MCSs, whether they form over land or ocean, cloud-top height distributions revealed by the Ice, Cloud, and are major contributors to water vapor in the upper tro- Land Elevation Satellite (ICESat)/Geoscience Laser Altime- ter System (GLAS). J. Geophys. Res., 111, D12215, posphere and TTL. doi:10.1029/2005JD006705. Folkins, I., and R. V. Martin, 2005: The vertical structure of tropical Acknowledgments. The authors thank three reviewers convection and its impact on the budgets of water vapor and for their helpful suggestions and Beth Tully for processing ozone. J. Atmos. Sci., 62, 1560–1573, doi:10.1175/JAS3407.1. the graphics. This work was supported by the National Fu, Q., Y. Hu, and Q. Yang, 2007: Identifying the top of the tropical tropopause layer from vertical mass ﬂux analysis and CALIPSO Aeronautics and Space Administration (Grants lidar cloud observations. Geophys.Res.Lett., 34, L14813, NNX10AM28G and NNX13AQ37G) and the U.S. De- doi:10.1029/2007GL030099. partment of Energy (Grant DE-SC008452). CALIPSO Fueglistaler, S., A. E. Dessler, T. J. Dunkerton, I. Folkins, Q. Fu, data are available from the NASA Langley Research and P. W. Mote, 2009: Tropical tropopause layer. Rev. Geo- Center Atmospheric Science Data Center (http://www- phys., 47, RG1004, doi:10.1029/2008RG000267. Garrett, T. J., A. J. Heymsﬁeld, M. J. McGill, B. A. Ridley, D. G. CloudSat calipso.larc.nasa.gov/), data are available from Baumgardner, T. P. Bui, and C. R. Webster, 2004: Convective the CloudSat Data Processing Center (http://www. generation of cirrus near the tropopause. J. Geophys. 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