Extreme Outliers in Lower Stratospheric Water Vapor Over North America Observed by MLS: Relation to Overshooting Convection Diagnosed From Colocated Aqua‐MODIS Data

Abstract

Convectively injected water vapor (H 2 O) in the North American (NA) summer lowermost stratosphere results in significant outliers in the 100-hPa H 2 O measurements from the Aura Microwave Limb Sounder (MLS). MLS statistics from 15 years confirm that the NA region contains over 60% of global 100-hPa H 2 O > 12 ppmv, despite having only ∼1.8% of all MLS observations. A profile sampled in August 2019 stands out, with H 2 O = 26.3 ppmv, far exceeding the prior record and the median ∼4.5-ppmv abundance in NA. This particular outlier is associated with a large overshooting convective event (OCE) that spanned multiple U.S. states and persisted for several hours. Colocation of the MLS data over NA with cloud observations from Aqua's Moderate Resolution Imaging Spectroradiometer (MODIS) reveals the unique character of this case, as only 2.3% of MLS profiles are as close to an OCE and only 0.024% of OCEs cover as large an area within a 500-km perimeter of a profile. Plain Language Summary Large-scale convection over the North American continent during the summer can significantly moisten the lowermost stratosphere. This study provides statistical analysis of 15 years of humidity outliers observed from space, including a recent event that yielded an outlier that far exceeded typically observed background values and summer enhancements. It is shown that this sampled air mass was in close proximity to an exceptionally large and long-lived convective event. Further analysis reveals that these two factors help explain the largest humidity outliers. Using global Version 3 observations from the Aura Microwave Limb Sounder (MLS) over the years 2005-2012, Schwartz et al. (2013) ("S2013" hereinafter) revealed that OCEs over the North American monsoon area ("NA") yield elevated H 2 O in the LMS in up to 1% of profiles. While numerous studies discuss RESEARCH LETTER

Persistence of moist plumes from overshooting convection in the Asian monsoon anticyclone

“…Their convective overshooting origin is unambiguously supported by both the enhanced isotopic ratios in the moist plumes and by their traceability to convective events. Notably, the occurrence of lower stratospheric moist plumes above the monsoon regions is also supported by satellite observations (Fu et al, 2006;Schwartz et al, 2013;Werner et al, 2020), whereas the enhanced water isotopic ratios observed over these regions (Hanisco et al, 2007;Randel et al, 2012) support the role of overshooting convection in maintaining the water vapour maximum in the summer monsoon anticyclones. This process adds to 455 the radiatively-driven slow ascent of wet air through the warm tropopause in the northern part of AMA.…”

Hunga Tonga-Hunga Ha'apai Hydration of the Stratosphere

“…Three natural pathways for direct injection of H 2 O into the stratosphere exist: overshooting convection, pyrocumulonimbus (pyroCb) storms, and volcanic eruptions. The previous stratospheric H 2 O record measured by MLS was 26.3 ppmv at 100 hPa associated with an overshooting convective event in August 2019 that spanned thousands of square kilometers and persisted for several hours (Werner et al, 2020). Two pyroCbs stand out in the MLS H 2 O record: the 2017 Pacific Northwest (Pumphrey et al, 2021) and the 2019/2020 Australian New Year's (Kablick et al, 2020;Khaykin et al, 2020;Schwartz et al, 2020) events.…”

The Hunga Tonga‐Hunga Ha'apai Hydration of the Stratosphere

“…Three natural pathways for direct injection of H 2 O into the stratosphere exist: overshooting convection, pyrocumulonimbus (pyroCb) storms, and volcanic eruptions. The previous stratospheric H 2 O record measured by MLS was 26.3 ppmv at 100 hPa associated with an overshooting convective event in August 2019 that spanned thousands of square kilometers and persisted for several hours (Werner et al., 2020 ). Two pyroCbs stand out in the MLS H 2 O record: the 2017 Pacific Northwest (Pumphrey et al., 2021 ) and the 2019/2020 Australian New Year's (Kablick et al., 2020 ; Khaykin et al., 2020 ; Schwartz et al., 2020 ) events.…”