Extreme enhancements 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 (H₂O) in the North American (NA) summer lowermost stratosphere (LMS) results in significant outliers in the 100-hPa H₂O measurements from the Aura Microwave Limb Sounder (MLS). Statistics from 15 years of MLS data confirm that the NA region contains over 23% of the global 100-hPa H₂O observations >8 ppmv, despite having only 1.8% of total observations. A profile sampled in August 2019 stands out, with 26.3 ppmv of H₂O, far exceeding the prior record NA enhancement and the 6.8 ppmv enhancements typically associated with overshooting convection in NA. This particular humidity outlier in the LMS is associated with a large overshooting convective event (OCE) that spans multiple US states and lasted for several hours prior to the MLS overpass. Colocation of the full MLS data record over NA with cloud observations from Aqua's Moderate Resolution Imaging Spectroradiometer 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 an MLS profile. Analysis confirms that close proximity to exceptionally large OCEs is associated with 100-hPa H₂O >10 ppmv.