Nocturnal Relative Humidity Maxima above the Boundary Layer in the American Midwest: A Diagnostic for the Mountain-Plains Solenoidal Circulation
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Commercial aircraft measurements from the ACARS [Aircraft Communications Addressing and Reporting System] dataset were used to obtain the vertical variation of lower tropospheric relative humidity over the diurnal cycle. Relative humidity maxima were observed during the summer between 2 km and 3 km overnight at six airports in the American Midwest from roughly 3 LST to 9 LST. The nocturnal relative humidity maxima coincide with both positive anomalies in speci c humidity and negative anomalies in temperature, as would be expected if the maxima were generated by upward motion. Spatial, diurnal and vertical variations of vertical winds from the MERRA-2 [Modern-Era Retrospective analysis for Research and Applications, Version 2] reanalysis dataset show that during the daytime in the lower troposphere, there is strong upward motion over the Rocky Mountains and downward motion over the Midwest. At night, the circulation reverses with upward motion over the Midwest that is strongest near 2 km in altitude, therefore coincident with the height and timing of the nocturnal relative humidity maxima. Several studies have indicated that this diurnal variation in vertical motion is induced by baroclinicity over sloping terrain, and is referred to as the mountain-plains solenoidal circulation. The relative humidity maxima are strongest during the summer and also show a high degree of interannual variability. The relative humidity maxima may be used as a diagnostic for the strength of the solenoidal circulation in climate models. The nocturnal Low Level Jet (LLJ) was also studied in relation to the relative humidity maxima. However, the LLJ was too low in altitude to be a main contributor to the maxima.