Interannual variability of summer precipitation in Texas and its implication to summer drought

Since Texas normally receives most of its precipitation in the warm season, precipitation deficits in summertime may bring serious agricultural and hydrological disasters. While the underlying physical processes of summer precipitation deficit and drought are unclear, they can be understood in terms of convective instability. This research is designed to investigate how convective instability influences monthly mean precipitation in Texas in the summertime and to examine the modulation of convective instability and precipitation by upper-level circulations, soil moisture, vertical motion, and low-tropospheric warm air transport using NCEP/NCAR reanalysis data. Statistical approaches including correlation analysis, multiple linear regression analysis and back trajectory analysis were used to reveal the underlying dynamics of their linkage and causality. The results show that warming at 700 mb and surface dryness result in excessive convective inhibition (CIN), leading to precipitation deficits on a monthly time-scale. Temperature at 700 mb (Tlt) and surface dewpoint have little correlation suggesting different processes contribute to warming at 700 mb and surface dryness, respectively. Correlation analysis among the surface variables emphasizes the role of soil moisture on the dewpoint and thermodynamics at the surface. Back trajectory analysis indicates that a significant contributor to warming at 700 mb is the inversion caused by warm air transport from the Rocky Mountains and the Mexican Plateau where the surface potential temperature is greater than 307.5K rather than by subsidence. It was found that downward motion and warm air transport are enhanced in Texas when upper-level anticyclonic circulation develops in the southern US. Upper-level anticyclonic circulations in the southern US strongly affect Texas summertime precipitation by modulating the principal processes as follows. They increase CIN not only by enhancing warm air transport from the high terrain but also by suppressing occurrence of disturbances. The resulting reduced precipitation and dry soil significantly modulate surface conditions, which elevates CIN and decreases precipitation. The aforementioned chain-reaction of upper-level anticyclone influences can be understood in the context of CIN.