Diane Portis — CIMMS/University of Oklahoma
Peter J. Lamb — University of Oklahoma
Abraham Zangvil — Ben-Gurion University
Category
Atmospheric State & Surface
Description
Figure 1. Mean daily values of HA-HD (top), HD (middle), and HA (bottom) when the mean daily solar radiation (SR) was in one of five categories shown. The abbreviations for these variables are defined in the text. The daily values are areal averages over our study region from all eight study months. The numbers on the top of the bars indicate the number of days that were included in that SR category. SR is in MJ m-2 day-1 with all other variables having units of mm day-1.
The ARM Cloud and Land Surface Interaction Campaign (CLASIC) was conducted over the ARM SGP site during June 2007. A primary goal of CLASIC is to understand the interactive roles of horizontal moisture advection and land surface processes in the evolution of cumulus convection. Our study is providing the larger-scale (Oklahoma-Texas) atmospheric moisture budget background for the interpretation of results derived from CLASIC observational platforms. Extremely wet conditions prevailed during CLASIC, when Oklahoma experienced its wettest June since records began in 1895. Three other contrasting May–June periods were chosen for analysis: 2006 (very dry), 2002 (intermediate wetness), and 1998 (very dry, especially upstream in Texas). The conventional form of the moisture budget equation with negligible atmospheric storage is E-P = MFD = HA+HD where E is evapotranspiration, P is precipitation, and MFD is moisture flux divergence with components HA (horizontal moisture advection) and HD (horizontal velocity divergence in the presence of moisture). If MFD ~0, one might assume that all P is derived from E. We demonstrated in an earlier study that the fraction of P derived from E is provided by the inflow (IF/A) across the region’s boundary that encloses area A, i.e., E/(IF/A + E). Through application of Green’s theorem along the region’s boundaries, MFD also is the net flux at these boundaries (OF/A – IF/A) where OF/A denotes the outflow. In our present work, we examine a new variable, HA-HD, and mathematically show that it is equal to the non-flux (maybe non-linear) terms over the region for the one-dimensional case. Cross-spectral analysis with daily data reveals that this variable has a close association with P with a periodicity of ~ 7 days. It also has a connection with cloud cover as illustrated by Figure 1, which is a plot of daily HA-HD, HA, and HD with respect to solar radiation (SR) over the study region for all eight study months. Except for moderately low SR (mostly cloudy), the opposing contributions of HA and HD tend to maximize the HA-HD variable over the SGP. For 2007 (CLASIC), both HA and HD were of negative sign for the most cloudy category. These opposing roles of HA and HD were not evident in our earlier moisture budget study over the Midwestern U.S. and therefore may indicate some uniqueness of the cloud-generating environment of the SGP site. Results will include further analysis of HA-HD associations, including modeled cloud estimates.
