Relative importance of local recycling versus external advection for CLASIC rainfall and clouds

 
Poster PDF

Authors

Peter J. Lamb — University of Oklahoma
Diane Portis — CIMMS/University of Oklahoma
Abraham Zangvil — Ben-Gurion University

Category

Field Campaigns

Description

Mean values of the daily atmospheric moisture budget (top two rows) when the recycling ratio (P<sub>E</sub>/P) was in one of four categories shown. The abbreviations for these budget terms are defined in the text. The daily values from all eight study months are considered. The last row includes the stratifications by recycling ratio for its two components (E and IF/A), precipitation (P), and a solar radiation (SR) anomaly that is based on the mean daily SR value over the eight study months. The number of cases in each category from lowest to highest recycling ratio is 81, 87, 45, and 27. Except for dimensionless P<sub>E</sub>/P and SR anomaly in MJ m<sup>-2</sup> day<sup>-1</sup>, all of the variables have units of mm day<sup>-1</sup>.
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). Using a recycling methodology that was developed during an earlier investigation of the atmospheric moisture budget for the Corn Belt in the upper Midwest (Zangvil et al. 2004), estimates were made of the contributions of locally evaporated moisture (i.e., recycled from within the region) versus externally advected water vapor for the precipitation (and cloud development) on a range of timescales (daily, monthly, bimonthly). The figure (top two rows) depicts the mean values of the daily atmospheric moisture budget when the recycling ratio (PE/P) was in one of four categories shown. Within the moisture budget, the surplus/deficit of evaporation (E) over precipitation (P) is balanced by the moisture flux divergence (MFD) and atmospheric storage change (dPW). The two MFD components are horizontal water vapor advection (HA) and horizontal velocity divergence in the presence of water vapor (HD). The last row includes the stratifications by recycling ratio for its two components (E and the inflow of water vapor into the region [IF/A]), precipitation (P), and a solar radiation (SR) anomaly that is based on the mean daily SR value over the eight study months. The largest recycling ratios occur with large E-P surpluses and large MFD, and not when E-P ~ 0, which is a condition that some have interpreted as all P has its origins in E (recycled moisture). Variations in the recycling ratio above 0.20 are reflected more in the externally advected water vapor (IF/A) than locally evaporated moisture (E). There is a striking correlation that is also supported by linear correlations between the externally advected water vapor with MFD and its components HD and HA. The largest negative SR anomaly (most clouds) occurs in the largest recycling category.