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

Combined airborne, shipboard, and satellite measurements provide the first observational 25 assessment of all major terms of the vertically integrated water vapor (IWV) budget for a 26 150×160 km region within the core of a strong atmospheric river over the northeast Pacific 27 centered on 1930 UTC on 5 February 2015. Column integrated moisture flux convergence is 28 estimated from eight dropsonde profiles, and surface rain rate is estimated from tail doppler radar 29 reflectivity measurements. Dynamical convergence of water vapor (2.20±0.12 mm-hr−1) nearly 30 balances estimated precipitation (2.47±0.41 mm-hr−1), but surface evaporation (0.0±0.05 mm-31 hr−1) is negligible. Advection of drier air into the budget region (−1.50±0.21 mm-hr−1) causes 32 IWV tendency from the sum of all terms to be negative (−1.66±0.45 mm-hr−1). An independent 33 estimate of IWV tendency obtained from the difference between IWV measured by dropsonde 34 and retrieved by satellite three hours earlier is less negative (−0.52±0.24 mm-hr−1), suggesting 35 the presence of substantial temporal variability that is smoothed out when averaging over several 36 hours. The calculation of budget terms for various combinations of dropsonde subsets indicates 37 the presence of substantial spatial variability at ~50 km scales for precipitation (0.5 to 9.8 mm-38 hr−1), moisture flux convergence (−7.4 to 10.0 mm-hr−1), and IWV tendency (−8.3 to 3.7 mm-39 hr−1) that is smoothed out when averaging over the full budget region. Across subregions, surface 40 rain rate is linearly proportional to dynamical convergence of water vapor. These observational 41 results improve our understanding of the thermodynamic and kinematic processes that control 42 IWV in atmospheric rivers and the scales at which they occur.