Long-term precipitation trend as a function of isentropic variability

Analysis of trends in climatological isentropes provides a means of evaluating seasonal precipitation trends. Past research has indicated that isentropic patterns are linked to moisture influx and adiabatic uplift. We calculate trends across the summer 315 K isentropic surface for the conterminous United States over the last fifty years. We identify positive correlations between time and the pressure levels of the 315 K isentropic surface in the southern Plains states and the New England area. Such correlations suggest long-term rising of pressures (lowering heights) for the isentropic surface (and therefore enhanced moisture inflow) into those regions. Weaker correlations with time and the isentropic surface are found in the Southwest and in the Northwest. Using two well-accepted precipitation databases, we compare the map pattern correlations between the trends in isentropic surfaces and the trends in both precipitation daily totals and in hourly durations. A strong correlation (+0.67) exists between the trends in the isentropic surface and the daily precipitation and indicates that the precipitation increases are likely influenced by a stronger moisture influx. A weaker positive correlation (+0.44) is evident between the trends in daily precipitation intensity and the trends in the 315 K isentropic surface. However, the spatial relationship between the trends in the isentropic surface and the trends in the hourly precipitation duration is only -0.07 indicating no significant accounting of trends in duration by corresponding isentrope surface trends. Continued study of the relatively neglected analysis of climatological isentropes will add to our understanding of the physical changes in precipitation patterns across the United States.