Extreme outliers in lower stratospheric water vapor over North America observed by MLS: Relation to overshooting convection diagnosed from colocated Aqua-MODIS data

Convectively injected water vapor (H2O) in the North American (NA) summer lower17most stratosphere results in significant outliers in the 100-hPa H2O measurements from18 the Aura Microwave Limb Sounder (MLS). MLS statistics from 15 years confirm that19 the NA region contains over 60% of global 100-hPa H2O> 12 ppmv, despite having only20 ∼ 1.8% of all MLS observations. A profile sampled in August 2019 stands out, with H2O = 26.3 ppmv,21 far exceeding the prior record and the median ∼ 4.5 ppmv abundance in NA. This par22ticular outlier is associated with a large overshooting convective event (OCE) that spanned23 multiple US states and persisted for several hours. Colocation of the MLS data over NA24 with cloud observations from Aqua’s Moderate Resolution Imaging Spectroradiometer25 reveals the unique character of this case, as only 2.3% of MLS profiles are as close to an26 OCE and only 0.024% of OCEs cover as large an area within a 500 km perimeter of a27 profile.