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

• Published in: Journal of climate Vol. 14; no. 4; pp. 582 - 597
• Main Authors: Zangvil, Abraham, Portis, Diane H., Lamb, Peter J.
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 15.02.2001
• Subjects: Advection; Atmospheric moisture; Atmospherics; Budget surpluses; Earth, ocean, space; Exact sciences and technology; External geophysics; Financial budgets; Meteorology; Periodicity; Precipitation; Soil water; USA; Water balance; Water in the atmosphere (humidity, clouds, evaporation, precipitation); Water vapor; Canada; United States; North America; America; USA, Midwest; Central U.S; Vertical component; Horizontal component; Vertical surface; Time scale; Summer; Rawinsonde; Water balance; Water vapor; Pluviometry; Spectral analysis; Rain; Correlation analysis; Evapotranspiration; Models; Atmospheric humidity; Finite difference method
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/1520-0442(2001)014<0582:IOTLSA>2.0.CO;2

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 (positiveE–P) and the bulk of this surplus is exported from the region, largely through HA. For the daily budget, a thresholdPrate (∼4 mm day−1) separates surplusE–Pbudgets from deficit budgets. On all timescales, most of thePvariance is reflected in the VA component of MFD, while HA explains ∼80% of the variation in dPW. For the monthly and (especially) daily budgets,Ehas bimodal distributions withPwhere the minimumEoccurs atP∼ 2.6 mm day−1(monthly) andP∼ 4–5 mm day−1(daily). For drier dailyPregimes, relatively highEis associated with increased (decreased) dry VA (HA). The correlation ofEwithPbecomes 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 betweenP–HA andP–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 anEminimum and then culminates in aPmaximum; after thePevent, atmospheric drying occurs through increased (diminished) dry HA (moist VA), which leads to anEmaximum and thenPminimum.