The Observed Water Vapor Budget in an Atmospheric River over the Northeast Pacific

Abstract

Combined airborne, shipboard, and satellite measurements provide the first observational assessment of all major terms of the vertically integrated water vapor (IWV) budget for a 150×160 km region within the core of a strong atmospheric river over the northeast Pacific centered on 1930 UTC on 5 February 2015. Column integrated moisture flux convergence is estimated from eight dropsonde profiles, and surface rain rate is estimated from tail doppler radar reflectivity measurements. Dynamical convergence of water vapor (2.20±0.12 mm-hr−1) nearly balances estimated precipitation (2.47±0.41 mm-hr−1), but surface evaporation (0.0±0.05 mm-hr−1) is negligible. Advection of drier air into the budget region (−1.50±0.21 mm-hr−1) causes IWV tendency from the sum of all terms to be negative (−1.66±0.45 mm-hr−1). An independent estimate of IWV tendency obtained from the difference between IWV measured by dropsonde and retrieved by satellite three hours earlier is less negative (−0.52±0.24 mm-hr−1), suggesting the presence of substantial temporal variability that is smoothed out when averaging over several hours. The calculation of budget terms for various combinations of dropsonde subsets indicates the presence of substantial spatial variability at ~50 km scales for precipitation, moisture flux convergence, and IWV tendency that is smoothed out when averaging over the full budget region. Across subregions, surface rain rate is linearly proportional to dynamical convergence of water vapor. These observational results improve our understanding of the thermodynamic and kinematic processes that control IWV in atmospheric rivers and the scales at which they occur.