Enhanced stratospheric water vapor over the summertime continental United States and the role of overshooting convection

Abstract

Abstract. The NASA ER-2 aircraft sampled the lower stratosphere over North America during the field mission for the NASA Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC 4 RS). This study reports observations of convectively influenced air parcels with enhanced water vapor in the overworld stratosphere over the summertime continental United States and investigates three case studies in detail. Water vapor mixing ratios greater than 10 ppmv, which is much higher than the background 4 to 6 ppmv of the overworld stratosphere, were measured by the JPL Laser Hygrometer (JLH Mark2) at altitudes between 16.0 and 17.5 km (potential temperatures of approximately 380 to 410 K). Overshooting cloud tops (OTs) are identified from a SEAC 4 RS OT detection product based on satellite infrared window channel brightness temperature gradients. Through trajectory analysis, we make the connection between these in situ water measurements and OT. Back trajectory analysis ties enhanced water to OT 1 to 7 days prior to the intercept by the aircraft. The trajectory paths are dominated by the North American monsoon (NAM) anticyclonic circulation. This connection suggests that ice is convectively transported to the overworld stratosphere in OT events and subsequently sublimated; such events may irreversibly enhance stratospheric water vapor in the summer over Mexico and the United States. A regional context is provided by water observations from the Aura Microwave Limb Sounder (MLS).

Convective Hydration of the Upper Troposphere and Lower Stratosphere

“…High-altitude aircraft has observed localized convective hydration (Anderson et al, 2012;Corti et al, 2008;Danielsen, 1993;Hanisco et al, 2007;Herman et al, 2017;Khaykin et al, 2009;Kley et al, 1982;Smith et al, 2017); however, our calculations show that these events contribute <1% to the global stratospheric water vapor budget. Even if we narrow the range of impact to the tropical lower stratosphere (less than 30°latitude) the impact is 1-2%, much smaller than the 5-10% tropical increase computed by S14 using MERRA reanalysis.…”

“…Both convection and diabatic upwelling supply water vapor to the upper tropical troposphere and play a role in determining the altitude of the tropopause (Randel & Wu, ; Son & Lee, ; Thuburn & Craig, ; Zhou & Holton, ). Midlatitude aircraft measurements suggest that overshooting convection can also move water directly into the stratosphere bypassing dehydration by the cold trap (Anderson et al, ; Danielsen, ; Hanisco et al, ; Herman et al, ; Smith et al, ). Recent tropical aircraft measurements show tropical convective overshoots reaching into the lower stratosphere (Corti et al, ; Khaykin et al, ).…”

“…water directly into the stratosphere bypassing dehydration by the cold trap (Anderson et al, 2012;Danielsen, 1993;Hanisco et al, 2007;Herman et al, 2017;Smith et al, 2017). Recent tropical aircraft measurements show tropical convective overshoots reaching into the lower stratosphere (Corti et al, 2008;Khaykin et al, 2009).…”

Effects of convective ice evaporation on interannual variability of tropical tropopause layer water vapor

“…Previous work has shown that TTL water vapor variability is mainly controlled by TTL temperature variability (Mote et al, 1996;Holton and Gettelman, 2001;Fueglistaler et al, 2009). In particular, variations in the BDC and QBO play key roles (Fueglistaler and Haynes, 2005;Geller et al, 2002;Liang et al, 2011;Liess and Geller, 2012;Calvo et al, 2010;Randel et al, 2006;Dessler et al, 2013Dessler et al, , 2014Tao et al, 2015).…”

“…Air enters the stratosphere from the tropical troposphere mainly through the tropical tropopause layer (TTL, ∼ 15-18 km) (Sherwood and Dessler, 2000;Fueglistaler et al, 2009), which serves as a transition region between the troposphere and stratosphere. It is generally recognized that the coldest temperatures in the TTL act like a "cold trap" that provides primary control on the amount of water vapor entering the lower stratosphere (Mote et al, 1996;Holton and Gettelman, 2001). Large interannual variations of TTL water vapor have been observed and attributed to a set of physical processes that affect water vapor by varying TTL temperatures, such as the quasi-biennial oscillation (QBO) (Geller et al, 2002;Fueglistaler and Haynes, 2005;Liang et al, 2011;Liess and Geller, 2012;Randel and Jensen, 2013;Wang et al, 2015;Tao et al, 2015) and the Brewer-Dobson circulation (BDC) (Randel et al, 2006;Calvo et al, 2010;Dessler et al, 2013Dessler et al, , 2014Fueglistaler et al, 2014;Gilford et al, 2016).…”