A Proposed Index for Mesoscale Activity

Abstract

The diurnal and interdiural wind variabilities are defined in terms of the ?relative gustiness? ?σ?/V?. The proportion α between the diurnal and the interdiurnal variabilities thus defined is suggested as a useful index in mesoscale studies. The α index is calculated in more than 30 stations in the Israel area during summer and winter, and is shown as a simple tool for the identification of climatic-geographic regions in which: The variability due to the changes of the large-scale pressure gradients may or may not be ignored in mesoscale models. An α index larger than 1 indicates the dominance of the diurnal processes over the interdiumal processes in producing wind variability, in which case large-scale effects are relatively small. Hodograph representation for the average diurnal wind variation is or is not a faithful picture of the actual wind observations on a particular day. An α index larger than 1 suggests a more faithful representation of the wind hodograph. As expected, the α index is larger than 1 during summer in nearly all stations in Israel while during winter it is less than 1. The values of α during summer are very high. They are generally between 1.5 and 2.5 but in the Jordan Valley, Arava and the Negev desert α indices reach 3 or even 4?5, indicating both vigorous mesoscale forcing due to channeling and/or differential heating and the reduced cited of the migrating cyclones. In the winter only few southern stations get α-values slightly larger than 1, indicating the general dominance of the large-scale (synoptic) variability relative to the locally induced mesoscale variability.