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Clouds and Water Vapor in the Tropical Tropopause Transition Layer Over Mesoscale Convective Systems

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Clouds and Water Vapor in the Tropical Tropopause Transition Layer Over Mesoscale Convective Systems DECEMBER 2015 V I R T S A N D HOUZE 4739 Clouds and Water Vapor in the Tropical Tropopause Transition Layer over Mesoscale Convective Systems 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) ABSTRACT 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 DOI: 10.1175/JAS-D-15-0122.1 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.
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