A Factor Separation Study of the Effect of Synoptic-Scale Wind, Atmospheric Moisture and of Their Synergy on the Diurnal Temperature Range During the Israeli Summer

The trend of the decrease in the global average diurnal temperature range (DTR) is one of the clearest signals in twentieth century climate data. Regionally, DTR trends exhibit large variations. A factor separation analysis was conducted to investigate the impact of atmospheric moisture, synoptic-scale winds, and their synergy on the DTR during the Israeli summer. The Weather Research and Forecasting (WRF) Single Column Model was run for summer representative days, at four locations in Israel. In almost all cases, the contribution of the factors and of their synergy to the DTR was dominated by their contribution to the diurnal minimum temperature, Tmin. The largest contribution resulted from atmospheric moisture, reducing the DTR. The contribution of synoptic-scale winds showed more variability, with significant differences in both magnitude and sign on different days. The sign of the effect on Tmin depended on the relative direction and magnitude of the nocturnal synoptic-scale wind with respect to the local wind, which in turn determined the effect on the low-level jet (LLJ) and vertical mixing. The contribution from synergy between the two factors depended on the effect of moisture on the LLJ and on the effect of the synoptic-scale winds on moisture advection in or out of the atmospheric column. All cases were classified into groups depending on the sign of contributions of the single factors and of their synergy when analyzing the DTR observed trends. These results highlight the importance of the synoptic wind, of its synergy with atmospheric moisture and of the feedback mechanisms.