A case-study analysis of convectively sourced water vapor plumes in the overworld stratosphere over the continental U.S. observed in situ during the SEAC4RS mission

Abstract:

Water vapor in the upper troposphere and lower stratosphere (UTLS) is important both radiatively and chemically. However, the processes that control the distribution and phase of water in this region of the atmosphere are not well understood. This is especially true at mid-latitudes where several different dynamical mechanisms are capable of influencing UTLS water vapor concentrations. Identifying and quantifying these mechanisms and understanding how these processes will change in response to anthropogenic climate forcing is a critical challenge facing the atmospheric chemistry and climate community.

The Harvard Water Vapor (HWV) instrument has repeatedly observed moist layers in the summertime extra-tropical lower stratosphere, with some present well into the stratospheric overworld (potential temperature >380 K) over the continental U.S. Water vapor mixing ratios in these layers are elevated by several ppmv above the nominal background, with the magnitude of the enhancement diminishing with altitude. The present analysis utilizes high resolution in situ data acquired during the SEAC4RS mission in August of 2013, to describe encounters with elevated concentrations of water vapor in the overworld stratosphere over North America that show evidence of deep convection as their source. The plume observed over the Great Lakes on August 27, 2013, represents the largest enhancement (~10 ppmv) observed in situ between 400 K and 420 K. Trajectory calculations link these plumes to specific convective storms identified in NEXRAD radar data and in GOES satellite infrared imagery. It is theorized that ice lofted in summertime deep convective storm systems rapidly evaporates in the under-saturated stratosphere and provides a means of significantly elevating water vapor, with a minimal perturbation to other trace species.

The in situ and remote data sets, in combination with trajectory analyses, are then used to describe in greater detail the spatial extent, water vapor content and lifetime of these events. Through this strategic combination of data products, we address questions regarding the frequency and distribution of these convective penetrations, and discuss the potential for deep convective storms to contribute to the water vapor budget, and more generally impact the chemical composition of this region.