The Influence of Soil Moisture on the Planetary Boundary Layer and On

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The Inﬂuence of Soil Moisture on the Planetary Boundary Layer and on Cumulus Convection over an Isolated Mountain. Part I: Observations

XIN ZHOU AND BART GEERTS University of Wyoming, Laramie, Wyoming

(Manuscript received 18 May 2012, in ﬁnal form 29 August 2012)

ABSTRACT

Data collected around the Santa Catalina Mountains in Arizona as part of the Cumulus Photogrammetric, In Situ and Doppler Observations (CuPIDO) experiment during the 2006 summer monsoon season are used to investigate the effect of soil moisture on the surface energy balance, boundary layer (BL) characteristics, thermally forced orographic circulations, and orographic cumulus convection. An unusual wet spell allows separation of the two-month campaign in a wet and a dry soil period. Days in the wet soil period tend to have a higher surface latent heat flux, lower soil and air temperatures, a more stable and shallower BL, and weaker solenoidal forcing resulting in weaker anabatic flow, in comparison with days in the dry soil period. The wet soil period is also characterized by higher humidity and moist static energy in the BL, implying a lower cumulus cloud base and higher convective available potential energy. Therefore, this period witnesses rather early growth of orographic cumulus convection, growing rapidly to the cumulonimbus stage, often before noon, and producing precipitation rather efficiently, with relatively little lightning. Data alone do not allow discrimination between soil moisture and advected airmass characteristics in explaining these differences. Hence, the need for a numerical sensitivity experiment, in Part II of this study.

1. Introduction (e.g., Sellers et al. 1992). Thus, soil moisture is a critical parameter determining CBL characteristics (e.g., Zhang Warm-season deep convection in the western United and Anthes 1982). However, the impact of soil moisture States is strongly diurnally modulated, with cumulus on mountain-scale circulations and on the growth of convection initiating close to solar noon and a maximum orographic convection is less obvious. That is the topic in lightning and precipitation in the midafternoon of the present study. This paper (Part I) explores ob- (Watson et al. 1994b; Nesbitt and Zipser 2003; Carbone servations. A companion paper (Part II) uses numerical et al. 2002; Demko et al. 2009). Satellite imagery shows simulations. how this convection almost invariably initiates over This study should be framed in the broader question mountain ranges and not the surrounding lower terrain about the relationship between soil moisture and warm- (Banta and Schaaf 1987). Clearly the convection and season precipitation. Many numerical experiments have mountain-scale circulations that control this spatial been designed to address this question (e.g., Sun and distribution are tightly coupled to the land surface, es- Ogura 1979; Carlson et al. 1983; Rowntree and Bolton pecially the surface sensible heat flux that controls the 1983; Atlas et al. 1993; Koster et al. 2004). Observational daily evolution of the convective boundary layer (CBL). studies on this topic are relatively scarce, especially in The sensible heat flux mainly depends on the magnitude complex terrain. The simulated soil moisture feedback of the surface net radiation, and on soil moisture, which on precipitation over flat terrain can be positive or is the primary control in the partitioning of the avail- negative, mainly depending on scale, but also depending able energy between sensible and latent heat fluxes on numerical factors such as the surface and boundary layer schemes and the cumulus parameterization method (Pan et al. 1996; Gallus and Segal 2000; Hohenegger et al. Corresponding author address: Bart Geerts, Department of At- mospheric Science, University of Wyoming, 6034 Engineering 2009). Building, Laramie, WY 82071. The main positive feedback mechanism is that moist E-mail: [email protected] soil enriches BL air with moist static energy, which

DOI: 10.1175/MWR-D-12-00150.1

Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/01/21 05:36 AM UTC 1062 MONTHLY WEATHER REVIEW VOLUME 141 reduces convective inhibition (CIN) and increases convective available potential energy (CAPE; Betts et al. 1996; Eltahir 1998; Scha¨r et al. 1999; Pal and Eltahir 2001; Findell and Eltahir 2003a,b; Wood 1997; Hohenegger et al. 2009). Another positive feedback is that increased soil moisture decreases surface albedo (Idso et al. 1975) and thus increases net radiation [and boundary layer (BL) moist static energy] under clear skies (Eltahir 1998). Boundary layer clouds may complicate this response: the addition of water vapor into the CBL may increase low-level cloudiness and stability (Hohenegger et al. 2009), but the resulting surface net shortwave radiation reduction may be compensated by a reduced net longwave radiation loss, unless the clouds are con- fined to the daytime (Scha¨r et al. 1999; Pal and Eltahir 2001). The main negative feedback mechanism is that moist soil reduces the sensible heat flux, and thus the daytime maximum temperature, the CBL depth, and the ability of air parcels to overcome CIN (Findell and Eltahir FIG. 1. Topography of the Santa Catalina Mountains, showing 2003a,b; Ek and Holtslag 2004; Hohenegger et al. 2009). the location of 10 ISFF stations (triangles are regular stations, di- amonds are stations with flux and soil measurements), the highest Another negative feedback relates to spatial heteroge- peak Mt. Lemmon (2791 m MSL), the flux tower at Mt. Bigelow neity: in a flat landscape with mesoscale soil moisture (2583 m MSL), the two stereo-cameras mounted on a rooftop at variations, the wetter soil regions will be relatively cool, the University of Arizona, and the National Weather Service resulting in mesoscale subsidence as BL air diverges KTUS radiosonde launch site in 2006. toward the drier, warmer patches, where the CBL becomes deeper and deep convection may erupt (Walker and Rowntree 1977; Sun and Ogura 1979; Carlson et al. and orographic precipitation is complicated also be- 1983; Taylor et al. 2007; Nair et al. 2011). The flow to- cause deep convection produces outflow boundaries that ward warmer patches is density driven (solenoidal). The can trigger secondary convection (Demko and Geerts minimum scale supporting significant solenoidal BL 2010b), and because the complex terrain may aid or stifle circulations is at least an order of magnitude larger than mesoscale convective organization (Nesbitt et al. 2008; the CBL eddies [i.e., at least O(10 km)], depending Hauck et al. 2011). mainly on surface contrast, ambient wind, and BL depth To make progress on the topic of soil moisture impact (Chen and Avissar 1994). on warm-season precipitation in complex terrain, this These and other factors need to be considered in study focuses on the rather simpler component questions complex terrain. Elevated surface heating leads to sig- of how soil moisture affects CBL growth, mountain-scale nificant pressure perturbations (Geerts et al. 2008) re- convergence, and initial cumulus growth. Part I of this sulting in mountain-scale solenoidal BL circulations study is entirely observational. Its primary data source is (e.g., Banta 1986) especially when soils are dry (Ookouchi a network of surface weather and flux stations deployed et al. 1984). The initiation of cumulus convection over or for two months (July–August 2006) around a rather iso- near the high terrain is due to a combination of terrain- lated mountain in southern Arizona. induced convergent flow (Banta 1984; Kottmeier et al. A 5-day period of heavy rainfall occurred across 2008; Barthlott et al. 2011) and elevated heating and southeast Arizona in late July 2006, yielding widespread early elimination of CIN (Demko et al. 2009; Demko near-saturated soil conditions. About 118 mm of rain and Geerts 2010a,b). Clearly orographic convection fell between 27–31 July around the Santa Catalina would be most effective if the surrounding valley soil is Range (SCR; Fig. 1), mostly at night, causing some rare moist (with lush vegetation) and the mountain soil dry, local flooding (Damiani et al. 2008). This event was since the solenoidal circulations due to the terrain and driven by a synoptic-scale moisture surge, which is not due to soil moisture would have the same sign, but in uncommon in Arizona in summer (e.g., Adams and reality a history of precipitation mainly over mountains Comrie 1997). This spell was unusual because of a typically leads to an opposite soil moisture (and vege- series of nocturnal organized convective systems, aided tation) distribution. The relation between soil moisture by high surface humidity and a weak but persistent

Unauthenticated | Downloaded 10/01/21 05:36 AM UTC MARCH 2013 Z H O U A N D G E E R T S 1063 upper-tropospheric cyclone. During this spell the air and humidity sensors and 3D sonic anemometers at a mass was as moist tropical as commonly observed far- height of 10 m above ground level (AGL). The latent ther south along the Sierra Madre Occidental in summer and sensible heat (LH and SH) fluxes are estimated (e.g., Higgins et al. 2006). The National Weather Service using the eddy correlation technique at 5-min intervals (NWS) forecast office at Tucson, Arizona (KTUS), re- based on 30-min running averages. The SH flux is pro- corded a total of 214 mm of rain in July and August portional to the buoyancy flux; that is, it includes the 0 0 2012, which is 193% of normal. While this unusual wet virtual temperature correction (w uy), where w is verti- spell makes it difficult to understand the more typical cal velocity and uy virtual potential temperature. The nature of land–atmosphere coupling in this region soil heat flux is computed using the gradient method. We (Small 2001), it does provide a strong regional-scale also use corresponding data at 10 m AGL from a flux signal to examine soil moisture impact. Clearly this study tower located on Mt. Bigelow, near the top of the SCR. does not examine the role of soil moisture in this flooding This station is at an elevation 1432 m higher than that of event—that event was largely driven by large-scale mois- the surrounding 10 ISFF stations. ture advection. Instead, it examines how the changes in soil The evolution of orographic cumuli was captured by moisture resulting from this event affect diurnal changes at two pairs of cameras, one located to the northwest, the the surface and in the boundary layer, and the resulting other to the southwest of the mountain (Damiani et al. thermally driven orographic circulations and orographic 2008). The pair located 30 km southwest of Mt. Lemmon convection. (Fig. 1) is used herein, because of the prevailing southerly Observations alone do not allow discrimination be- flow during CuPIDO. Each camera pair was optimally tween soil moisture impact and changes due to large- spaced for stereo-photogrammetry over the SCR, and scale advection. A model sensitivity study, in which soil thus cloud edges can be geolocated in three dimensions moisture is controlled, does allow a quantification of the at high temporal resolution (Zehnder et al. 2007). This soil moisture impact, but only if the model is realistic. method is used to obtain the height of the highest cu- This can be checked by validating that the regional mulus cloud top over the SCR. model, driven by observed initial and boundary condi- The National Lightning Detection Network (NLDN) tions, accurately simulates atmospheric and land surface data are used to examine lightning patterns over the conditions over the two-month period. Part II of this SCR. Upper-air data come from the KTUS radiosondes study does exactly that. at 0000 UTC [4.5 h after local solar noon (LSN)] and Section 2 describes the data sources. These data are 1200 UTC (shortly before sunrise). These radiosondes analyzed in section 3. The influence of soil moisture were released from the valley floor ;40 km south- is summarized in section 4. Conclusions are given in southwest of the highest point in the SCR (Fig. 1). On section 5. 16 days, Mobile GPS Advanced Upper -Air Sound- ing System (MGAUS) soundings were launched at 45– 2. Data sources 90-min intervals from a location just upstream of the mountain top [see Table 1 in Damiani et al. (2008)]. We The Cumulus Photogrammetric, In Situ, and Doppler also use the KTUS level-III radar data (http://www. Observations (CuPIDO; http://www.eol.ucar.edu/projects/ ncdc.noaa.gov/nexradinv/) for the detection of precipi- cupido/) campaign was conducted around the SCR near tation echoes (radar fine lines marking outflow bound- Tucson in southeast Arizona between 1 July and 29 aries were rarely seen), the Climate Prediction Center’s August 2006 (Damiani et al. 2008). This range is rela- (CPC) station-based 0.25830.258 gridded daily preci- tively isolated with a horizontal scale of ;30 km and pitation data (http://www.esrl.noaa.gov/psd/) and the a maximum height of ;2000 m above the surrounding 25 km 3 25 km gridded Advanced Microwave Scan- plains. These plains are part of the Sonoran Desert, with ning Radiometer for Earth Observing System (AMSR-E)/ sparse vegetation. Ten Integrated Surface Flux Facility Aqua daily L3 soil moisture product (http://nsidc.org/). (ISFF) stations were positioned around the SCR (Fig. 1). All stations collected key meteorological data such as 10-m wind and 2-m temperature and humidity. These 3. Results data were averaged to a 5-min time resolution. Four a. Bifurcation based on soil moisture ISFF stations measured soil moisture and temperature at 5 cm below the surface, and all surface energy balance Data from the 10 ISFF stations within 2 h of LSN terms, including the surface heat fluxes. High-rate tem- (1930 UTC 6 2 h) are examined for July–August 2006 perature, humidity, and 3D wind data were measured at (Fig. 2). A 4-h period around noon is chosen because the these stations by means of fast-response temperature net radiation and surface heat fluxes peak in that period.

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FIG. 2. Evolution of key atmospheric variables throughout the two-month period. Most values are averaged 62 h around local solar noon (1930 UTC). The precipitation is the 24-h total (0000–0000 UTC). All ISFF data are 10 station averages, except soil moisture, SH and LH fluxes, which are obtained from the four stations with flux measurements (see Fig. 1). The red/blue banner on top in this and other figures marks the dry and wet soil periods.

The very wet period of 27–31 July is evident in Fig. 2c, (Figs. 3b,c) and with the AMSR-E/Aqua soil moisture but there are three wet days in early July, and several product (Figs. 3f,g). That product is based on the space- days with more than 5 mm of rain in August. The time borne measurement of upwelling microwave radiation at series of precipitation at the ISFF stations corresponds several frequencies (Njoku 2004). It indicates a strong well with the fully independent CPC U.S. daily gridded west–east soil moisture gradient across southern Arizona, dataset (Fig. 3a). The ;25-km CPC data suggest that the with an ;3% higher volumetric soil moisture to the east. SCR is the wettest in the region (Figs. 3b,c), with a peak The east–west gradient is broadly consistent with CPC value located just southeast of Mt. Lemmon. precipitation data, although the AMSR-E soil moisture Soil moisture (Fig. 2c) jumps up in each wet spell, and data fail to capture the widespread anomalously high soil then tapers off gradually over several days in the fol- moisture from late July into early August 2006 across the lowing dry spell, mainly due to evapotranspiration domain. (Grayson et al. 1997). Assuming that this decay is ex- To examine the impact of soil moisture in a statisti- ponential, the data suggest a soil moisture ‘‘half life’’ of cally signiﬁcant way, we divide the two-month period 1.7 days, at a depth of 5 cm, for the four-station average in two parts: the wet soil period (WSP; 38 days) and dry around the SCR. The moistest soils are found on the soil period (DSP; 22 days) (Fig. 2c). The second period southeast side of the SCR (station south-southeast in (27 July–29 August) clearly is wetter than the ﬁrst pe- Fig. 1), and the driest soils on the other side (station riod, but the brief wet spell in early July moistened the north-northwest), consistent with the CPC precipitation soils enough to compel us to include the period 5–8 July

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FIG. 3. (a) Time series of daily (1200–1200 UTC) precipitation from CPC [(b),(c) spatially averaged over the domain] and from the 10 ISFF stations. In the six color panels below (a), the DSP is shown on the left, the WSP on the right. These panels are geolocated by latitude and longitude, and terrain is contoured in blue at 0.5-km intervals. (b),(c) CPC, ISFF, and Bigelow mean daily precipitation in a small domain [(d),(e) shown as a box]. (d),(e) CPC mean daily precipitation in a larger domain. (f),(g) AMSR-E/Aqua volumetric soil moisture in the larger domain.

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TABLE 1. Comparison of the dry and wet soil periods. All pa- little, thus the LH flux is largely controlled by surface rameters are averages for all ISFF stations, unless stated otherwise. and near-surface water (Kurc and Small 2004). Seasonal Values are averaged for a 4-h period centered on local solar noon greening does occur in response to significant rainfall (1930 UTC 6 2 h), except daily precipitation, which is a 24-h total. Some parameters involve other data sources, as listed [Bigelow (Meńdez-Barroso et al. 2009), such as the late July 2006 (BGL)]. wet spell. This vegetation response may serve to extend the memory of precipitation pulses (Watts et al. 2007). Difference Property DSP WSP (WSP 2 DSP) b. Soil moisture, surface energy balance, and airmass 5-cm soil moisture (vol %) 6.6 15.6 9.0 characteristics Air temperature (8C) 33.7 27.4 26.3 5-cm soil temperature (8C) 40.7 31.8 28.9 The surface and lower atmosphere differ substantially ue (K) 346.5 351.1 4.6 between the two periods, especially around noon (Fig. 2 u (K) 317.7 311.2 26.5 and Table 1). The higher soil moisture (Fig. 2c) implies Du (K) (BGL 2 ISFF) 0.9 2.2 1.3 a higher LH flux (Fig. 2f)1 and a lower land surface Mountain-scale convergence 1.1 0.6 20.5 2 2 temperature (Fig. 2a) during the WSP. The 6.3-K lower (10 4 s 1) 2 Vapor mixing ratio (g kg 1) 9.2 13.3 4.1 air temperature during the WSP (Table 1) is at least RH (%) 26 53 27 partly due to the reduced SH, especially in late July and LCL (m MSL) 4045 2568 21498 early August as heavy precipitation was recorded across Daily precipitation (mm) 0.9 5.3 4.4 south and central Arizona. The Bowen ratio (SH:LH CPC daily precipitation (mm) 1.1 5.5 4.4 flux) averages just 0.7 during the WSP, compared to 6.2 Surface albedo (%) 20 17 23 2 Net SW radiation (W m 2) 694 568 2126 during the DSP. 2 Net LW radiation (W m 2) 2201 2100 101 The SH and LH fluxes plus the soil heat flux may be 2 2 Net radiation (W m 2) 481 453 228 underestimated: their sum is about 140 W m 2 below 2 Soil heat flux (W m 2) (GH) 275 247 28 the measured net radiation (Table 1). Several other (,0 / into soil) 2 studies have documented a significant imbalance be- SH flux (W m 2) 205 101 2104 2 LH flux (W m 2) 58 168 110 tween net radiation and the three flux terms, most likely 2 SH 1 LH 2 GH (W m 2) 338 316 222 due to eddy correlation technique uncertainties (e.g., Panin et al. 1998; Twine et al. 2000; LeMone et al. 2002; Finnigan et al. 2003; Oncley et al. 2007). The lack of closure of the surface energy balance is more likely over in the WSP. That leaves a relatively short DSP, from 1 to the complex terrain where the flow is neither stationary 4 July and from 9 to 26 July. We emphasize that this nor horizontally homogenous (Panin et al. 1998). The study uses soil moisture to distinguish the wet period, flux deficit is about the same in the DSP and the WSP, instead of atmospheric parameters such as precipitation thus the relative magnitude of the heat fluxes probably is or dewpoint. Thus, our wet period is distinct from the about right. Arizona ‘‘monsoon period,’’ which the National Weather The higher water vapor mixing ratio r during the WSP Service bases on the dewpoint data. In 2006 the WSP (Table 1) is in part due to higher regional-scale evapo- started the day after significant rains, and included sev- transpiration from the surface (water vapor recycling), eral days without rainfall. although synoptic-scale airmass advection likely ex- The contrast between the dry and wet periods is ob- plains a dominant part of the difference between the two vious also in a regional context (Figs. 3d,e): the daily periods, as it does in typical Arizona ‘‘monsoon’’ periods precipitation across southern Arizona during the DSP is (e.g., Adams and Comrie 1997). That is because clima- only 38% of that during the WSP, which experiences tologically, in summer, southern Arizona is close prox- heavy rain mainly in southeast Arizona, yielding a imity to a rather deep high-moisture air mass to the strong west–east gradient with the SCR at the western south. edge of the wet region. Around the SCR the daily preci- As a result of the lower temperature and higher pitation is about fivefold higher during the WSP (Table 1). mixing ratio, the relative humidity in the WSP is about In the soils around the four ISFF stations, a maximum soil moisture content of about 22.6% is recorded at a depth of 5 cm, after the heavy rains of late July. During 1 the WSP (DSP) the soil moisture averages 69% (29%) Note that the LH and SH traces in Fig. 2f are discontinuous because the average is shown only if all four stations report. The of this peak value. Many Sonoran desert plants depend all-station requirement is not imposed on averages based on all 10 on moisture deep below the surface (e.g., Shreve and ISFF stations in Fig. 2, since 9 reporting stations (and on 1 day just Wiggins 1964), but the vegetation is sparse and transpires 8) still give a robust average.

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TABLE 2. Average stability parameters for all 59 individual WSP (moisture term: 19.9 K); the lower WSP potential 0000 UTC KTUS soundings. The KTUS ground level is at 751 m MSL. temperature (26.5 K) is insufﬁcient to offset the hu- u u Parameters (No. DSP WSP Difference midity effect on e. The WSP e excess is consistent with of soundings) (21) (38) (WSP 2 DSP) Watson et al. (1994a), who found that the 850-mb ue is CBL height (m MSL) 4431 2818 21613 8 K higher during monsoon ‘‘bursts’’ than during mon- LCL height (m MSL) 4241 2872 21369 soon ‘‘breaks’’ over Tucson. LFC height (m MSL) 4904 3587 21317 The net surface radiation [the sum of net surface LNB height (m MSL) 10 988 12 689 1701 2 shortwave (SW) radiation and net surface long-wave CAPE (J kg 1) 420 707 287 2 (LW) radiation] is lower during the WSP than the DSP, CIN (J kg 1)69734 Precipitable water (mm) 29.0 34.5 5.5 although the difference is relatively small (Table 1). ue @ 900 mb (K) 344.4 347.6 3.2 These ﬁndings disagree with the theory of Eltahir (1998) and Pal and Eltahir (2001), which expects not only a

higher ue, but also a higher net radiation and when the soil is moister, irrespective of cloudiness. This suggests twice that in the DSP, and the lifting condensation level that the WSP ue excess primarily is due to advection, and (LCL), measured AGL, is about half as high in the WSP. not primarily driven by the surface. High ue moisture The LCL (Fig. 2d) is calculated from surface tempera- surges from the south are the primary drivers of North ture and dewpoint, assuming well-mixed conditions to American monsoon variability (Higgins and Shi 2000). cloud base. This is a fair assumption as cumuli are present The surface albedo is slightly lower during the WSP almost every day around LSN, even during the DSP, (Table 1), as moist soils tend to look darker (Idso et al. and as the MGAUS soundings reveal a deep CBL 1975). But the net SW radiation is lower during the around noon. WSP, and so is the net LW radiation loss. This suggests On many days in the WSP, cumulus clouds obscure higher low-level cloud coverage around LSN during the the top of the SCR. On these days Mt. Bigelow experi- WSP. This is qualitatively confirmed by webcam data ences near-100% relative humidity, and the LCL esti- and visible satellite imagery. mated from the ISFF stations is substantially below that from the Bigelow temperature and dewpoint values c. Soil moisture and thunderstorm potential (Fig. 2d). The potential temperature excess at Bigelow We now explore the relation between soil moisture over the ISFF stations (Du) is also much higher on these and deep-convective parameters. We use the 0000 UTC days (Fig. 2e and Table 1), either because the CBL top KTUS soundings2 since typically the CBL is still well over the ISFF stations is below Mt. Bigelow, or else developed at that time, although it may modified by because u is increased by condensation of water vapor orographic deep convection. The composite soundings (i.e., the equivalent potential temperature u is more e for the WSP and the DSP in Fig. 4 are computed by closely conserved than u in a cloud-topped CBL). Only interpolating data from individual soundings to fixed on days with an LCL and a CBL top well above the levels and then averaging. All available soundings (59 in elevation of Mt. Bigelow, is Du a good measure of so- total) are used in Fig. 4 and Table 2. Significant light- lenoidal forcing strength. ning activity and radar-detected near-surface precip- The near-surface air has a higher u during the WSP e itation was present over the SCR between LSN and (Table 1). As will be shown later, the u excess of 4.6 K e 0000 UTC on 16 of these 59 days, most of them during during the WSP is not driven by diurnal surface heating, the WSP (see below). Afternoon thunderstorms over as it is persistent throughout the night. Thus, it is an air- the SCR may have spread cold pools across the Tuc- mass characteristic. A u excess (;3 K) during the WSP is e son valley, increasing CIN and decreasing CAPE. None found also in the 0000 (Table 2) and 1200 UTC KTUS of the 0000 UTC soundings reveal a shallow cold pool, soundings in the boundary layer. One can partition Du e and KTUS radar data do not reveal any outflow into temperature and moisture terms by linearizing the boundaries propagating over the sounding site between differential expression for u : e 2200–0000 UTC. The exclusion of these 16 soundings decreases the average CAPE a little in both periods, on Du ’Du 1 L D e r, (1) Cp where L is the latent heat of condensation and Cp is the 2 0000 UTC is 4.5 h after LSN. World Meteorological Organi- u specific heat under constant pressure. The e excess re- zation (WMO) radiosondes are normally released ;50 min before sults from a persistently higher BL humidity during the the nominal time (i.e., closer to LSN).

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FIG. 4. (a) Composite 0000 UTC KTUS soundings in the WSP and DSP. (b) Potential temperature and (c) mixing ratio gradient of this composite sounding in the lower troposphere.

Unauthenticated | Downloaded 10/01/21 05:36 AM UTC MARCH 2013 Z H O U A N D G E E R T S 1069 account of the higher preexisting CAPE on days with underestimated due to a rather shallow stable layer afternoon thunderstorms. within a possibly much deeper CBL (e.g., on 15 July). In The WSP profile is more similar to a typical monsoon such cases we ascertained that this stable layer is not the sounding in the Sierra Madre Occidental farther south in top of a convective cold pool. Mexico (Higgins and Gochis 2007). It suggests a history The precipitable water is higher and the LCL lower of modification by deep convection. Compared to the during the WSP (Table 2), consistent with surface ob- DSP profile, it has a slightly higher tropopause, a mid- servations (Table 1). On many WSP days, the SCR top is tropospheric temperature lapse rate close to the moist obscured by cumulus clouds. The CBL water vapor gen- adiabatic rate, and higher mixing ratio in the mid- and erally suffices for some CAPE to be present throughout upper troposphere. But the largest differences between the two-month period: surface-based CAPE is present on the two periods are found near the surface (Fig. 4). all but 3 days in DSP and all but 2 days in the WSP (Fig.

Both the uy and r profiles indicate that the CBL is 5b). The CAPE and near-surface (900 mb) ue are higher generally deeper in DSP, consistent with the higher SH during the WSP at 0000 UTC, suggesting that more fuel flux in that period. The smaller slope of the uy profile remained for evening convection at that time. Thus, the during the DSP (Fig. 4b) indicates a weaker static sta- WSP has a higher potential of deep convection and heavy bility. The lapse rate is superadiabatic [(›uy/›z) , 0] precipitation. during this period, not just near the ground (which can The KTUS soundings in both soil periods suggest that be an instrument bias, related to the lack of ventilation the initiation of deep convection over the valley must be 2 just before the sonde is released), but also well above the rare. The CIN is rather substantial (70 J kg 1) over surface, up to 1.0 km AGL. This and the more uniform r KTUS at 0000 UTC, about thrice the average CIN in the (near-zero slope) suggest more vigorous mixing through MGAUS sounding released closer to the mountain the CBL during the DSP, consistent with a higher sur- closer to local noon. This is unlikely because of earlier face SH flux (Table 1). The high r lapse rate during convection (as cold pools were rarely observed over the WSP suggests a high LH flux (Fig. 4c); this lapse rate KTUS at 0000 UTC), but rather because of a tendency is almost as large near the surface as that in the en- for subsidence to occur over the Tucson valley. This also trainment zone near the CBL top. The CBL top in the explains the better definition of the CBL top at KTUS. composite sounding (Fig. 4) is not sharply defined, as Also, the distance from the LCL to the LFC is quite a peak in the u rise rate and/or the r lapse rate. It is high, about 700 m on average, at KTUS in both periods defined practically as the first level with a significant (Table 2). This makes it unlikely for a cumulus cloud to 2 increase of u [0.1 K (100 m) 1] and a significant de- reach the LFC, lacking buoyancy and suffering from 2 2 crease of r [0.2 g kg 1 (100 m) 1]. The CBL top is gen- entrainment. In fact the average CBL top remains well erally much more obvious in individual KTUS soundings. below the LFC. The CBL top also is generally better defined at KTUS d. Soil moisture and diurnal evolution of the at 0000 UTC, than over the SCR closer to local noon. boundary layer This may be because of subsidence over the Tucson valley, and ascent over the SCR. The soundings over We now examine the difference between the DSP and the SCR are MGAUS units, released from various points WSP diurnal cycles. The surface energy parameters vary around the mountain between 1500–2100 UTC, on just as expected, with a peak around LSN and very small 16 days in the two-month period [i.e., the CuPIDO in- values at night (Figs. 6e–j). The net shortwave radiation tensive operations periods (IOPs) (more on this below)]. (and also the net radiation) peak slightly before LSN, Six different balloon launch sites were used, all within because cloudiness rapidly increases over the SCR 10 km from the mountain top (Mt. Lemmon; Fig. 1 in during the 4 h centered at LSN, according to visible Damiani et al. 2008). They were selected in order to satellite imagery. The diurnal temperature range (Fig. capture the upwind side of the mountain. KTUS is lo- 6a) is larger during the DSP, on account of the large cated farther from the SCR (;40 km) in a broad valley daytime SH flux and the lower PW and cloudiness at (Fig. 1). night, leading to a higher longwave radiation loss. The The CBL top and other sounding parameters in Table same applies to the soil temperature range (Fig. 6d), 2 are computed from the individual KTUS soundings which is driven by a larger soil heat flux (Fig. 6h). The and then averaged for the DSP and the WSP. The CBL diurnal water vapor range is larger as well during the is a remarkable 1.6 km deeper during the DSP, and DSP (Fig. 6b), because the daytime CBL grows deeper, extends well above the SCR top. The day-to-day vari- and thus more dry free-tropospheric air is entrained ability of the CBL depth during the DSP is consi- into the BL. However, the diurnal range of ue is slightly derable (Fig. 5c). In some cases the CBL top may be smaller during the DSP (Fig. 6c), as the temperature and

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FIG. 5. Time series of 0000 UTC KTUS sounding parameters. The dashed horizontal lines indicate the mean values in the respective periods (DSP and WSP). The 22 Jul sounding is missing. humidity vary in opposition, and thus two relatively during the WSP, and the difference is only meaningful large terms in Eq. (1) oppose each other during the DSP. during the brief period when the CBL is well developed The heat flux terms at Mt. Bigelow are shown also in (as evident from Fig. 6a), the diurnal cycle of SH flux Fig. 6. The main reason is to demonstrate the impor- at Mt. Bigelow (Fig. 6j) is a more useful measure, as tance of heating over elevated terrain to drive oro- it drives elevated heating that drives this solenoidal graphic cumulus growth. To quantify the evolution of forcing. the solenoidal forcing, we could examine the potential The Mt. Bigelow heat flux terms appear to be quite temperature excess at Mt. Bigelow over the foothill noisy, and are available over only 2/3 of the two-month stations (Du, Table 1). But this is a suitable measure of period. The large perturbations in SH and LH fluxes solenoidal forcing only when the CBL is well mixed and are due to rather low-frequency terrain-driven co- deeper than Mt. Bigelow, and when the cloud base is herent eddies, according to the vertical velocity power above Mt. Bigelow. This is the case on all days in the spectra (not shown): the Bigelow flux tower is located DSP (Figs. 5a,c), at least during part of the day, but not on a narrow, steep ridge, which explains the small di- on many WSP days. On these days the CBL top does not urnal temperature range (Fig. 6a). These eddies are exceed the elevation of Mt. Bigelow and/or the cloud more intense during the day as thermals grow in size base is below Mt. Bigelow. Thus, we can estimate the toward the CBL top, but they may occur at night as peak solenoidal forcing (i.e., the isobaric temperature well, in stratified flow impinging on the mountain. So difference at Mt. Bigelow pressure level, between the Bigelow flux data carry considerable uncertainty, mountain and adjacent valley) to be about 1.0 K during but basic patterns are consistent with the ISFF record the DSP (Table 1). Since this estimate is not available (Figs. 6i,j).

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FIG. 6. (a)–(j) Diurnal evolution of composite atmospheric and soil parameters in the two periods at the ISFF stations and Mt. Bigelow (BGL). In (a)–(d) the diurnal range is listed in the top-right corner, in red(DSP)andinblue(WSP).Allparametersarebased on 60 days, except the Bigelow LH and SH ﬂuxes (40 days).

OneremarkablefeatureisthepeakingoftheSH simulations indicate significant doming of the CBL flux well before solar noon at Mt. Bigelow in both top over the SCR, albeit less than the terrain doming periods (Fig. 6j), even earlier than the SH flux peak (De Wekker 2008; Demko and Geerts 2010a). Thus, at the ISFF stations. The Bigelow ridge is east–west terrain height matters in examining CBL height (AGL). (or east-southeast–west-northwest) oriented (Fig. 1), The higher launch points yield a lower CBL height and the prevailing low-level flow was from the south- (Fig. 7), but the launch sites did not change during southeast, so the asymmetry of SH around solar noon is IOPs. unlikely to be due to terrain facing the morning sun. In short, there is a clear CBL deepening trend at all Instead, it is due to a drop in SW radiation and thus net sites during the CuPIDO IOPs. This trend continued to radiation around noon, associated with orographic Cu 0000 UTC, especially during the DSP, given the large development (Figs. 6e,g). The near-surface SH flux in increase in CBL depth from the MGAUS soundings the morning hours over the mountain drives a mountain- (Figs. 7a,b) to the KTUS soundings (Table 2), which scale solenoidal circulation (Demko and Geerts 2010a) were released ;5 h later in the afternoon, on average. and destabilizes the atmosphere, leading to orographic Cu. This deepening averages 2.0 km on the 4 IOP days in the The surface SH flux drives the development of the DSP, and 1.2 km on the 10 WSP IOP days. Both the CBL. The CBL clearly deepens around solar noon CBL depth and the cloud base (LCL) are much greater (Figs. 7a,b). In Fig. 7 the CBL top is expressed AGL, in the DSP than in the WSP, but in both periods the air because MGAUS launch sites vary in elevation. CBL tends to reach saturation at or very near the CBL top, depth is affected not just by time of day, but also by not only according to KTUS (Table 2) soundings, but local terrain and prevailing wind. Unfortunately no also in the MGAUS soundings. Camera animations and time-matched mountain–valley soundings are avail- visible satellite imagery reveal that on most DSP days able to examine the CBL ‘‘topography,’’ but numerical towering Cu pop up over the high terrain, while on many

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FIG. 7. (a),(b) MGAUS sounding CBL height and (c),(d) CAPE evolution based on 14 CuPIDO IOPs. Only those IOPs with three or more MGAUS soundings are used. Six different launch sites were used, all in the Santa Catalina Mountains, as shown in Fig. 1 in Damiani et al. (2008).

WSP days more widespread shallow Cu clouds develop, converging over the mountain, and upper-level di- even over the low terrain. vergence near the CBL top (e.g., Fig. 1 of Geerts et al. The larger diurnal range of BL depth, u (Fig. 6) and 2008). One reason for the location of the ISFF stations in CBL depth during the DSP may imply that the diurnal the foothills surrounding the SCR during CuPIDO was variation of CAPE is larger as well, and, in turn, that to estimate the presence and strength of this thermally convective precipitation is more strongly diurnally direct circulation, or at least its surface component. Not modulated during the DSP. Indeed, there is a clear trend surprisingly, the kinetic energy of this circulation is in CAPE around noon during the DSP, and no such overwhelmed by turbulent kinetic energy in the CBL. trend in the WSP (Figs. 7c,d). We believe this difference So, following Geerts et al. (2008) and Demko et al. is signiﬁcant (although the MGAUS-measured change (2009), we examine the integrated convergence along in CAPE is limited to the 14 CuPIDO IOPs and a nar- the polygon deﬁned by the 10 ISFF stations around 2 row part of the diurnal window) as it is consistent with the mountain. The ‘‘mountain scale’’ convergence (s 1) the difference in the timing of convective precipitation, is computed as as discussed below. þ 2$ v 5 1 v ’ 1 åv d e. Diurnal variation of thermally forced mountain n ds n s. (2) circulation A A

Elevated surface heating leads to a thermally forced Here v is the horizontal wind vector; vn(vn) is the ve- orographic circulation with low-level anabatic wind locity normal to the closed loop (polygon), whose

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FIG. 8. Diurnal cycle of composite mountain-scale convergence, based on 10 ISFF stations. Positive value indicates convergence and negative value indicates divergence. perimeter is defined by ds (ds); and A is the polygon’s The flow even becomes slightly divergent in the early area. Station data such as the wind speed normal to a afternoon (Fig. 8). This reflects a tendency of orographic polygon side are most representative of a polygon side thunderstorms to be maturing and producing divergent if that polygon connects the midpoints of 10 pairs of cold pools in the afternoon. Indeed, some relation be- adjacent stations, rather than the stations themselves tween the afternoon divergence spell (Fig. 9b) and [see Fig. 3 in Demko et al. (2009)]. Station temperature precipitation (Fig. 9c) is evident in both periods: the and humidity data can be used in equations similar to brief recovery of convergent flow in the late afternoon Eq. (2) to estimate the heat and moisture convergences coincides with a lull in precipitation and lightning fre- (see Demko et al. 2009). These convergences are gen- quency (Fig. 9b). This recovery of both convergence and erally proportional to (air mass) convergence [Eq. (2)], temperature becomes more evident in a composite of as they are controlled by the sign and strength of the thunderstorms where time is expressed relative to the normal wind. Focusing on three case studies, Demko storm’s first lightning strike over the SCR (Fig. 10). et al. (2009) show that mountain-scale convergence de- Nocturnal divergent flow around the mountain es- velops during the daytime until orographic deep con- tablishes before sunset. Significant precipitation occurs vection matures. Cooler storm outflows may lead to in the evening in both periods, and throughout the night temporary divergent flow around the mountain. They during the WSP, on account of a few long-lived meso- also show that periods of strong mountain-scale con- scale convective systems (MCSs) in late July. In general vergence are unlikely to trigger or intensify orographic the early afternoon thunderstorms tend to be small, convection, which may be counterintuitive. Thus, they while nighttime systems tend to be larger and less con- hypothesize that convective initiation tends to result fined to the mountain footprint, so the impact of thun- from surface heating over the mountain during lulls in derstorms on mountain-scale convergence is evident mountain-scale convergence. only during the afternoon. The two-month record shows that nocturnal and f. Diurnal variation of orographic Cu growth, early-morning drainage flow (which implies mountain- lightning, and precipitation scale divergence) tends to be stronger during the DSP (Fig. 8), consistent with the slightly higher longwave A stereo-pair of cameras was operational from roof- radiation loss and cooling rate (Fig. 6). Yet the daytime tops on the southwest side of the SCR during CuPIDO convergent flow tends to be stronger during the DSP, (Fig. 1), in order to geolocate orographic Cu clouds peaking ;1 h before LSN. In general, the weaker (Quicktime animations can be found at the CuPIDO mountain-scale convergence during the WSP is consis- archive at http://www.eol.ucar.edu/projects/cupido/). On tent with the numerical study of Ookouchi et al. (1984), some days low-level or widespread cloudiness prevented which documents weaker elevated heating, weaker so- the identification of orographic clouds. The daily evo- lenoidal forcing, and a weaker orographic circulation lution of cumulus cloud growth could be tracked on most under higher initial soil moisture. CuPIDO days. For those days, the height of the tallest During the WSP, mountain-scale convergence signif- Cu tower over the SCR was estimated manually, at icantly leads the ISFF SH flux, which peaks around LSN 10-min intervals. As soon as a spreading cumulonimbus (Fig. 6j), and its trend is rather asymmetric, with a rapid anvil forms, the cloud-top height recording is termi- rise until 2–3 h before LSN, and a gradual decline nated, but may be resumed if the anvil vanishes and new thereafter. This matches the asymmetry in the Bigelow towers grow. The cloud-top determination is discon- SH flux (Fig. 6j), confirming that mountain-scale continued also when orographic convection becomes too vergence is driven by surface heating over the mountain. complex or becomes obscured by interspersed clouds.

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FIG. 9. (a) Stereo-grammetrically estimated cumulus cloud top over the Santa Catalina Mountains, as seen from the southwest side (Fig. 1) on those days on which the view was not obscured by low-level clouds, plus LCL-estimated cloud base and NLDN lightning strike frequency on matching days, for the WSP and the DSP. (b) Diurnal cycle of NLDN lightning strikes on all days in a box conﬁned by stations north, east-northeast, west, and south (i.e., ;20 km of Mt. Lemmon). (c) Rain rate at the ISFF sites. (d) Day-to-day diurnal cycle of the lightning strikes.

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An afternoon summer precipitation maximum in the southern Arizona is well established (e.g., Wallace 1975; Watson et al. 1994b; Carbone et al. 2002; an animation of the diurnal variation of radar-based precipitation in summer can be viewed at http://locust.mmm.ucar.edu/ episodes/Hovmoller). Precipitation is diurnally more evenly distributed during the WSP. The nocturnal precipitation maximum during the WSP is surprising. This maximum derives almost entirely from a series of nocturnal MCSs between 27 and 31 July 2006. Such episodes are unusual but not unprecedented (e.g., McCollum et al. 1995). This nocturnal precipitation maximum is not mirrored by an equally pronounced peak in lightning frequency FIG. 10. Composite evolution of (a) average temperature at the 10 ISFF stations and at Mt. Bigelow (expressed as a departure from during WSP nights (Fig. 9). The storms on 27–31 July the 4-h mean) and (b) mountain-scale convergence, for select 2006, unlike storms on other days in the 60-day period, thunderstorm events over the SCR. Time is relative to the first light- were almost all nocturnal (Fig. 9c). They had a large, ning strike. Storms are selected to occur between 1800–0000 UTC, continuous anvil on satellite infrared imagery, with and are relatively isolated (no lightning ,1 h before the first light- widespread precipitation according to the KTUS radar. ning and ,1 h after its end). The relatively lack of lightning strikes (Fig. 9b) under copious rainfall (Fig. 9c) suggests that these noc- We also track occurrences of lightning strikes over the turnal MCSs were relatively devoid of lighting, possibly SCR, using the NLDN database. because of a lower aerosol concentration in the BL Cumulus cloud tops generally grow rapidly before (Williams et al. 2002), or because of weaker updrafts LSN in both periods, but the cumulus deepening process and/or shallower cloud tops (Nesbitt et al. 2008; Rowe and growth in lightning frequency tend to occur 1–3 h et al. 2012). The overall number of daily lightning strikes earlier during the WSP than during the DSP (Fig. 9). over the SCR is about the same during both periods This is consistent with the higher CAPE (Table 2) and (12% difference), yet the daily precipitation rate is the higher humidity above the cloud base (Fig. 4) during 5 times higher during the WSP than the DSP. We do not the WSP. The latter implies lower cumulus erosion attempt to estimate precipitation efficiency, but the by entrainment of dry air in the midtroposphere (e.g., composite soundings suggest that the DSP convection is Wang et al. 2009). The cloud depth growth is offset less efficient, given the deep, dry BL (Fig. 4). Convection somewhat by the increase in cloud-base height during shortly after sunset (;0300–0600 UTC) generates co- the morning hours, on account of surface heating (Fig. pious lightning during the DSP (on 26 July, Fig. 9d), but 9a). Note that the cloud-top height is an average only, appears to be inefficient in terms of precipitation. The during the prelightning growth phase. During the DSP, WSP also shows a precipitation peak with much lightning convective cloud tops grow, at first with almost no around this time: this is due to an event on 9 August, lightning, until ;2.5 h after LSN, at which time lightning whose sounding is similarly dry at low levels as the cor- is remarkably frequent, not just for the 44 days for which responding one in the DSP (i.e., 26 July). both lightning and cloud-top data are available (Fig. 9a), but also in the 60-day record (Fig. 9b). Around this time 4. Discussion (2100–2200 UTC), the mountain-scale convergence and the Bigelow SH flux become suppressed, not just in the Some of the above results suggest a negative relation DSP, but also in the WSP. The late-afternoon lull in between soil moisture and the parameters that affect precipitation (around 0000 UTC) is consistent with a convective initiation: the WSP corresponds with lower reduced lightning frequency, a lower cloud-top height, net radiation, weaker sensible heat flux, lower CBL and a recovery in mountain-scale convergence. temperature, more stable profile within the lowest Precipitation correlates rather well with the lightning 1000 m AGL, and shallower CBL. Yet in effect, deep strikes in the afternoon and early evening. It is strongly convection tends to grow earlier in the day over the diurnally modulated during the DSP (Fig. 9c), with vir- mountain and produce more surface precipitation during tually no precipitation between midnight and LSN, the WSP, on account of a lower cloud base, and higher a clear afternoon maximum (consisting of two separate precipitable water, equivalent potential temperature, peaks), and a weak maximum a few hours after sunset. and CAPE.

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The relationship between soil moisture and convec- 5. Conclusions tion over complex terrain clearly is not straightforward. A two-month series of surface station and sounding Airmass characteristics such as precipitable water, u , e data, collected around the Santa Catalina Mountains in and cloud-active aerosol concentration are not primarily Arizona during the summer monsoon, was used to study driven by soil moisture, but rather by advection, espe- the effect of soil moisture on boundary layer evolution, cially in view of the more tropical airmass in close thermally driven circulations, and cumulus development proximity to the south. But the variation of other char- over complex terrain. The 60-day period was divided in acteristics, such as SH flux, diurnal temperature range, two periods, depending on local soil moisture: the DSP solenoidal forcing, and mountain-scale convergence are and the WSP. The main findings are as follows: strongly impacted by soil moisture. Variations in cumulus development and orographic precipitation are affected d Wet soil conditions correspond with a dominance of by both large-scale advection and regional soil moisture latent over sensible heat flux at the surface and a variations. slightly lower net radiation during the daytime be- Orographic thunderstorms are common during the cause of the larger cloud amount. This yields a smaller Arizona monsoon during both dry and wet soil periods, diurnal temperature range and a lower daytime sur- but the nature of the convection is different. This dif- face temperature. Also, compared to the DSP, the ference is at least partly due to soil moisture, as re- CBL is more stable and shallower around noon, and vealed in this study. Orographic deep convection can the solenoidal forcing weaker, resulting in weaker be triggered fairly easily in dry soil conditions, on anabatic flow. account of a higher surface SH flux, more elevated d The WSP air mass has more water vapor, and thus heating, and stronger solenoidally forced circulation. a higher equivalent potential temperature, a lower It occurs mostly in the afternoon and early evening, cumulus cloud base, and more CAPE. Orographic Cu and produces much lightning, but little precipitation. towers appears rather early during the day, and in Under moist soils orographic deep convection tends to a high-humidity environment they grow rapidly to the initiate rather early in the day, not because convergent, cumulonimbus stage with divergent cold pools, on anabatic flow develops earlier, but rather because average 2–3 h earlier than during the DSP. The higher CAPE is larger, the low- to midtroposphere more humid humidity during the WSP makes nocturnal organized (reducing cumulus erosion by dry-air entrainment), and convection more likely, when a suitable tropospheric thus growth from shallow to deep orographic convection flow pattern arises. more rapid. d Orographic precipitation occurs in the afternoon and Several nights with organized convection occurred early evening during the DSP, from thunderstorms during the WSP in 2006. Clearly the moist soils were not that tend to be prolific lightning producers. Yet, with the cause of the organization or longevity of the con- a high cloud base, the storms’ precipitation efficiency vection, rather, the MCSs caused moist soils. But in is relatively low. turn, the moist soils alter the surface energy balance d Observations alone do not allow discrimination be- and daytime orographic cumulus growth, which af- tween soil moisture and advected airmass character- fects the lightning frequency and precipitation effi- istics in explaining these differences; hence, the need ciency of thunderstorms. for a numerical sensitivity experiment. We repeat that the observed differences between the In Part II of this study, high-resolution numerical WSP and the DSP cannot exclusively be attributed to simulations will be validated in terms of observed pre- soil moisture. In Part II of this study, the two-month cipitation, soil moisture, surface, and upper-air charac- period will be numerically simulated and, following teristics, and then analyzed further to explore the effect validation against the observations presented in Part I, of soil moisture on BL development, thermally forced a sensitivity experiment will be discussed in which the circulations, and orographic cumulus convection. soil moisture level is controlled, in order to isolate the impact of soil moisture. Finally, we need to emphasize that the contrast be- Acknowledgments. This work was funded by National tween the wet and dry soil periods within the 2006 Science Foundation (NSF) Grants ATM-0444254 and Arizona monsoon was unusually large, and thus this ATM-0849225. The authors thank the National Center for study does not represent typical conditions. A multiyear Atmospheric Research Earth Observing Lab for collect- field campaign would be needed to reveal climatologi- ing the MGAUS and ISFF data, and Joseph Zehnder for cally representative differences, especially in terms of collecting and analyzing the stereo-grammetric cloud data the diurnal cycle. for the CuPIDO-06 campaign. The Bigelow tower flux

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