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

The strength of a landfalling AR depends in part on the rate of column moisture gain versus column moisture loss. Here, airborne, shipboard, and satellite measurements are combined to provide the first observational assessment of all major terms of the vertically integrated water vapor (IWV) budget for a 150 km × 160 km region within the core of a strong atmospheric river over the northeast Pacific. Column integrated moisture flux convergence is obtained from eight dropsonde profiles, precipitation rate is estimated from tail doppler radar reflectivity measurements, surface evaporation rate is obtained from shipboard flux measurements, and IWV time tendency is obtained from sum of terms and by the time difference between dropsonde measurements and retrieved values from a satellite pass three hours earlier. For this case, it is found that the estimated precipitation rate exceeds the column integrated moisture convergence, with negligible contribution from surface evaporation. Moisture flux convergence is substantially lower than precipitation rate due to advection of drier air into the budget region. Setting aside advection and considering only the change in IWV due to dynamical convergence yields a value that is nearly the same as the precipitation rate. Calculation of water vapor budget terms for subregions within the larger region indicates the presence of substantial spatial heterogeneity in precipitation, moisture flux convergence, and instantaneous IWV tendency that is smoothed out when averaging over the larger region. The IWV tendencies calculated from the difference between dropsonde and satellite data exhibit much less variability across subregions, indicating that the large magnitudes of the instantaneous tendencies are not sustained over several hours. Precipitation rate within subregions appears to increase linearly with the change in IWV due to dynamical convergence but is unrelated to the change in IWV due to advection. If broadly representative, these results suggest that IWV in an atmospheric river generally decreases over time as precipitation removes the increase in IWV produced by dynamical convergence in the lower troposphere but evaporation is too small to offset the decrease in IWV produced by mass divergence in the lower troposphere.