Adjustments for Wind Sampling Errors in an Estimate of the Atmospheric Water Budget of the Mississippi River Basin

Abstract

The spurious mass divergence found in observed and analyzed atmospheric wind fields causes errors in estimated energy flux divergence. An objective method for correcting these errors in radiosonde-based atmospheric water and energy budgets is developed. The proposed method is compared with other correction methods for estimating the moisture budget over the Mississippi River basin. The proposed method adjusts the spatial weights applied at each radiosonde station in the calculation of divergence. The adjustment is done in a least squares sense to minimize the modification to the weights under the constraint of mass conservation (accounting for surface pressure tendency) by use of Lagrange multipliers. The weight adjustment requires fewer and less arbitrary assumptions than existing methods. The adjustment scheme is particularly useful for diagnosing different components of energy transport (e.g., eddy, mean advection, mean divergence) because the adjustment is applied equally to all terms, unlike other correction methods. For the Mississippi River basin, the magnitudes of the correction range from 20% for the annual mean to as much as 100% for the individual months. With or without wind adjustments, however, estimates of long-term mean water vapor flux convergence from twice-daily soundings differ substantially from streamflow estimates. Given the large diurnal range of water vapor flux convergence in the summer, it is not surprising that the surface and atmospheric moisture budgets do not match. Examination of the diurnal range of each component of transport indicates that the diurnal variability of the moisture flux divergence is almost entirely due to the diurnal cycle of the mean wind divergence profile. It is speculated that auxiliary information on this cycle could be used to remove the bias in budget estimates made from twice-daily soundings.