Investigation of the Large-Scale Atmospheric Moisture Field over the Midwestern United States in relation to Summer Precipitation. Part I: Relationships between Moisture Budget Components on Different Timescales

Abstract

Atmospheric moisture budget components are evaluated for a large area (1.23 ? 106 km2) in the midwestern United States for all 12-h (1200?0000, 0000?1200 UTC) and 24-h (1200?1200 UTC) periods during the contrasting summers (May?August) of 1975, 1976, 1979, and 1988. The atmospheric moisture flux divergence (MFD, separated into horizontal and vertical advection components, HA and VA) and storage change (dPW) are estimated using a standard finite-difference method applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface?300 hPa) for 0000 and 1200 UTC. Area-averaged precipitation (P) totals are derived from approximately 600 relatively evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as a residual of the moisture budget equation and compares favorably with the few existing observations, especially when totaled for periods of 1 month or longer. Relationships between the budget components are established for the daily, monthly, and seasonal timescales using stratification, correlation, and cross-spectral analyses. On monthly and seasonal timescales, the surface is a net source of water vapor (positive E?P) and the bulk of this surplus is exported from the region, largely through HA. For the daily budget, a threshold P rate (?4 mm day?1) separates surplus E?P budgets from deficit budgets. On all timescales, most of the P variance is reflected in the VA component of MFD, while HA explains ?80% of the variation in dPW. For the monthly and (especially) daily budgets, E has bimodal distributions with P where the minimum E occurs at P ? 2.6 mm day?1 (monthly) and P ? 4?5 mm day?1 (daily). For drier daily P regimes, relatively high E is associated with increased (decreased) dry VA (HA). The correlation of E with P becomes substantially more positive from the daily-to-monthly timescale, confirming the importance of land?atmosphere interactions over longer periods. The above stratification and correlation results are complemented by cross-spectral analyses that identify strong associations between P?HA and P?dPW previously masked by phase differences. The cross-spectral results also prompt the development of a conceptual model that describes the temporal relationships among the budget components for eastward-moving large-scale, ?wavelike? disturbances with 3?10-day timescales. The suggested sequence of interactions?moist HA is accompanied by a pronounced PW increase and then followed by a moist VA maximum; this horizontal and then vertical moisture redistribution is first associated with an E minimum and then culminates in a P maximum; after the P event, atmospheric drying occurs through increased (diminished) dry HA (moist VA), which leads to an E maximum and then P minimum.