The Observed Mean Annual Cycle of Moisture Budgets over the Central United States (1973–92)

Abstract

The mean annual cycle of the atmospheric and terrestrial water balance over the central United States is examined through an analysis of observational data over the 20-yr period 1973?92. The mean quantities from this study are expected to serve as a climatology for empirical investigations and a benchmark for numerical model-based water balance computations for the central United States. Monthly means and statistics of atmospheric water balance quantities were computed from twice daily radiosonde data. These data form a monthly?climatology? and 240 month time series of the major water budget components, including the vertically integrated vapor flux divergence, the rate of change of precipitable water, and precipitation minus evapotranspiration, P ? E. The mean annual cycle of evaporation given estimates of precipitation over the same area is also computed. Through comparison with observed river discharge, estimates are formed of the mean annual cycle of surface and subsurface storage and its interannual variability (as a residual). The mean observed and residual quantities of the historical water budget components are in general agreement with earlier studies based on shorter time series. The 20-yr mean water budget shows a maximum of P ? E in March?April with a secondary maximum in November?December. In this analysis, mean evaporation exceeds mean precipitation during the June?September period with largest evaporation values in July and August. Thus, the heartland of the United States acts as mean net moisture source during the summer months. Individual monthly estimates of evaporation, given the gauge-estimated precipitation over the region, show negative evaporation estimates during some cold season months over the 1973?92 period. This suggests that gauge-measured precipitation is underestimated, at least during the cold months, in agreement with several rain gauge intercomparison studies. The sonde-based budgets also confirm previous studies in showing that the rate of change of precipitable water is a small contributor to the atmospheric water budget through most of the mean annual cycle. However, the relative importance of this term increases during the transition seasons (late spring and early fall) when the magnitude of the vapor flux divergence term in the atmospheric water balance is also quite small. The mean P ? E estimates computed from the vertically integrated atmospheric moisture flux were found to average 0.4 mm day?1 low in comparison to the observed total net river discharge. When the mean atmospheric P ? E is adjusted to the net discharge, the annual cycle of storage shows an amplitude of 14 cm yr?1, consistent with local measurements of soil moisture in Illinois (Hollinger and Isard) and also in agreement with earlier studies. The 20-yr time series shows multiyear variations in the storage term with magnitudes of near 45 cm, far in excess of the mean annual cycle. This low-frequency variability in storage is generally consistent with the accumulated precipitation anomaly, an independently estimated quantity, for most of the analysis period.