HR: 1340h
AN: A53B-0884    [Abstracts]
TI: Could Atmospheric Storage Play a Significant Role in Regional Precipitation Recycling?
AU: * Dominguez, F
EM: dominguz@uiuc.edu
AF: University of Illinois at Urbana-Champaign, Hydrosystems Lab 205 N. Mathews, Urbana, IL 61801 United States
AU: Kumar, P
EM: kumar1@uiuc.edu
AF: University of Illinois at Urbana-Champaign, Hydrosystems Lab 205 N. Mathews, Urbana, IL 61801 United States
AB: Over the past decade, there has been a growing interest in the hydrologic community on the influence of land surface processes on regional precipitation patterns. Numerous studies have focused their attention on the contribution of local evapotranspiration to local precipitation, or precipitation recycling. The atmospheric moisture budget is generally expressed as the equation of continuity of water mass integrated over an atmospheric column of unit area. The general form of the equation is: $ \frac{\partial \cdot \{\overline{q}\}}{\partial t} + \nabla\cdot \{{\overline {q\textbf{v}}\}} + \nabla\cdot \{{\overline {q'\textbf{v}'}\}}= \overline{E}-\overline{P}$ where $\{{\cdot}\}$ indicates vertical integration with respect to pressure, q is the specific humidity, \textbf{v} is the horizontal wind vector, E is evapotranspiration rate, P is precipitation rate and the overbars indicate temporal averaging while the primes denote deviation from the temporal average. Existing theoretical models of precipitation recycling are based on the above equation of conservation of water vapor mass. However, in these models, the change in storage of atmospheric water vapor is usually neglected, on the assumption that the term is insignificant at monthly or longer timescales. But is this simplification really valid? In order to answer this question we need to quantify the relative magnitude of the neglected term. In this study we use six-hourly Reanalysis II data (from 1979-2003) over North America ($-170^\circ $ to $ -50^\circ$ lon and $20^\circ $ to $ 80^\circ$ lat) to numerically evaluate the relative magnitude of the terms in the moisture budget equation. When analyzing the ratio of the moisture tendency term ($ \frac{\partial \cdot \{\overline{q}\}}{\partial t}$) over the average horizontal transport term ($\frac{\partial \cdot \{\overline{qu}\}}{\partial x}$ , $\frac{\partial \cdot \{\overline{qv}\}}{\partial y}$), we have found that the average ratio over North America is around 0.03 at a seasonal scale, 0.07 at a monthly scale and 1.2 at a daily time scale. During some years, these ratios can be as large as 0.12, 0.5 and 60 for seasonal, monthly and daily timescales respectively. As expected, the term increases in importance at smaller time scales. The relative importance of the storage term is also dependent on the geographical location. Eastern Canada, northeastern US and the drier regions of southwestern US and western Mexico have important contributions of the storage term. In these regions the ratio is on average around 0.1 to 0.2 at a monthly time scale. Based on this simple scale analysis we can see that although the tendency term is smaller than the average transport term, a 10 to 20% contribution is non-negligible. The implication of including the storage term in moisture recycling effects will be explored.
DE: 1833 Hydroclimatology
DE: 1836 Hydrologic budget (1655)
DE: 1854 Precipitation (3354)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting