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P. Alpert, M. Tsidulko, and U. Stein

Abstract

The contribution of a particular process is shown to be strongly dependent upon the other processes under investigation because of synergistic contributions. In general, as the number of relevant factors being investigated increases, the role of any specific factor diminishes because the synergistic interactions with the new factors are extracted. This is illustrated with the variations of the topographic role in the impressive lee cyclone deepening event on 3–5 March 1982 during the Alpine Experiment. When latent heat release, latent heat flux, and sensitive heat flux enter into our comparative study, the topographic contribution to the surface pressure lee cyclone deepening gradually diminishes down to 50% or more.

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P. Alpert, S. O. Krichak, M. Tsidulko, H. Shafir, and J. H. Joseph

Abstract

A dust prediction system, developed earlier at the University of Athens within the framework of the Mediterranean Dust Experiment (MEDUSE) project, was enhanced at Tel Aviv University to support the Israeli–American Mediterranean Israeli Dust Experiment (MEIDEX) project. These enhancements include development of a dust initialization approach using Total Ozone Mapping Spectrometer (TOMS) aerosol index (AI) data and improved specification of the dust sources. The skill of the model against the TOMS AI measurements was tested during two periods in March and June 2000 using four different scores. It is shown that the TOMS-based initialization has a significant positive impact on all the scores. For instance, the average distance between the predicted and TOMS-observed dust plumes drops from 350–485 to less than 200 km. Verification of model forecasts against surface dust measurements in Tel Aviv shows correlations of up to 0.69 based on 27 predictions, for both 24 and 48 h. One example of a narrow dust plume over Israel, successfully forecast with the current system, is presented. This event occurred in midsummer (4 July) when dust bursts are rare over the Eastern Mediterranean.

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P. Alpert, S. O. Krichak, T. N. Krishnamurti, U. Stein, and M. Tsidulko

Abstract

The contributions of boundary factors, which may be considered to be independent of the physics or the dynamics of the mesoscale model, are explored in a consistent approach for a widely investigated Alpine Experiment (AL-PEX) lee cyclogenesis case. The roles of the lateral boundaries and the initial fields in conjunction with that of the topography, as well as their possible nonlinear interactions in various model settings, are calculated with the aid of the recently developed factor separation method. Focus is given to the influences of the extent of the model domain and of the running period prior to the climax of the lee cyclone development during 3–6 March 1982. It is shown that the initial conditions are dominant in the first 9–15 h, during which time the topography and lateral boundaries play negative roles because of the adjusting processes. The nonlinear interaction BI between lateral boundaries (B) and the initial conditions (I) was found to be the major contributor to the cyclone deepening during the adjustment period. For longer running periods, some equilibrium is reached in which both the BI interaction and the lateral boundary dominate. The topographic contribution to the lee cyclone deepening in this ALPEX case was indeed limited to about 20% only, as already indicated by earlier studies. Testing several distances of the western lateral boundary suggests the existence of an optimal distance for good results. Both too distant and too close lateral boundaries yield worse results. Testing with frozen boundary conditions shows that the update of the lateral boundaries at a specific time of +36 h was crucial to the development. The results are clearly dependent to some extent on the model type and the particular case under investigation, as well as on the boundary conditions, the initialization procedures, and other model characteristics. The current experiments, however, provide a quantitative approach for estimating the relative roles of the aforementioned boundary factors in mesoscale developments with the aid of the Pennsylvania State University-National Center for Atmospheric Research MM4 mesoscale model and The Florida State University regional system.

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