A Proposed Index for Mesoscale Activity

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  • 1 Department of Geophysics and Planetary Sciences, Tel A Aviv University, Ramat Aviv, Israel
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Abstract

The diurnal and interdiural wind variabilities are defined in terms of the “relative gustiness” −σν/. 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.

Abstract

The diurnal and interdiural wind variabilities are defined in terms of the “relative gustiness” −σν/. 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.

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