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

This study used 20 years of Oklahoma Mesonet data to investigate the changes of near-surface water vapor mixing ratio q(upsilon) during the afternoon to evening transition (AET). Similar to past studies, increases in q(upsilon) are found to occur near sunset. However, the location, magnitude, and timing of the q(upsilon) maximum 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 on q(upsilon). By analyzing time series of q(upsilon) during the AET, it is found that the likelihood of a presunset q(upsilon) maximum is strongly dependent upon vegetation, soil moisture, wind speed, and cloud cover. Analysis also reveals that the increase in q(upsilon) during 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.