Atmospheric Water Vapor Transport and the Water Balance of North America

The atmospheric water vapor flux divergence and certain aspects of the water balance of North America are investigated, using data from the period May 1, 1958–Apr. 30, 1963. The mean vertical distribution of flux divergence is computed for the United States for the months of January (1962, 1963) and July (1961, 1962). Strong flux convergence in the lowest kilometer and divergence in the remainder of the troposphere were found in July. Flux convergence was found throughout the troposphere over the eastern half of the area in January, with a maximum between 900 and 950 mb.; while in the west, convergence (with no particularly pronounced maximum) was found above 800 mb., with weak divergence below. Corresponding features of the profiles were found at higher elevations over the west, where the flux divergence above 500 mb. is quite significant. Particular emphasis is placed on computation of the vertically integrated flux divergence, and its use in estimating P-E, the mean difference between precipitation and evapotranspiration. As in the case of the flux field, the flux divergence exhibits a pronounced diurnal variation south of 50°N., particularly during the summer. Nevertheless, the results of water balance computation over the United States and southern Canada, using twice-daily observations from the existing aerological network, indicate that reliable mean annual, season, and monthly values of P-E can usually be obtained when averaging over areas of 20×105km.2 or larger. Averages over smaller areas are less reliable, and become quite erratic as the size of the area is reduced to less than 10×105km.2 This deterioration is mainly due to the presence of a systematic error pattern of relatively large scale and amplitude. The mean monthly values of evapotranspiration and storage change, obtained from balance computations over the United States and southern Canada, and over that portion of the area east of the Continental Divide are presented and discussed. A comparison of values of evapotranspiration computed by means of the atmospheric water vapor balance equation, with those computed using Thornthwaite climatic water balance data indicates that over the United States and southern Canada the latter systematically overestimates P-E during the winter, and underestimates it during the summer by a substantial amount. This contributes to a computed seasonal change in surface and subsurface storage which averages more than twice that obtained from an evaluation of the flux divergence. Examination of the relationship between precipitation and storage over eastern North America indicates that for areas of this size, the departure from normal of precipitation by itself serves as a fairly good quantitative indicator of the departure from normal storage change.