An Analysis of the Processes Affecting Rapid Near-Surface Water Vapor Increases during the Afternoon to Evening Transition in Oklahoma

Abstract This study used 20 years of Oklahoma Mesonet data to investigate the changes of near-surface water vapor mixing ratioqυduring the afternoon to evening transition (AET). Similar to past studies, increases inqυare found to occur near sunset. However, the location, magnitude, and timing of theqυmaximum occurring during the AET are shown to be dependent on the seasonal growth and harvest of vegetation across Oklahoma in the spring and summer months. Particularly, the late spring harvest of winter wheat grown in Oklahoma appears to modify the relative contribution of local and nonlocal processes onqυ. By analyzing time series ofqυduring the AET, it is found that the likelihood of a presunsetqυmaximum is strongly dependent upon vegetation, soil moisture, wind speed, and cloud cover. Analysis also reveals that the increase inqυduring the AET can increase the parcel conditional instability despite the surface cooling produced by loss of insolation. Next to known changes in low-level wind shear, these changes in instability and moisture demonstrate new ways the AET can modify the presence of the key ingredients relevant to explaining the climatological increase in severe convective storm hazards around sunset.