Planning the Israeli-4 Experiment: Using daily precipitation from high resolution WRF model as the formal controls for evaluating cloud seeding effect on orographic precipitation in northern Israel
The Israeli Water Authority is planning to conduct a new randomized cloud seeding experiment in northern Israel (Israel-4) having the Lake of Galilee drainage basin (2,830 km2) as the target area. Lake Kinneret watershed, located in the central part of the Jordan Rift Valley, is the most important surface water resource in Israel, providing approximately 35% of the annual drinking water to a population of almost 7.5 million inhabitants which is constantly growing.
Glaciogenic seeding will be carried out by both aircrafts and ground based generators. The experiment will include microphysical and statistical components. The statistical analysis for the randomized seeding will be done by using a double ratio calculation with respect to control rainfall. The target-control correlation between daily precipitation in the upper Kinneret basin and the northern coastal areas in Israel is not very high (R=0.70). On the other hand, recent work showed that the correlation between calculated to actual daily precipitation in the target area (the Golan heights and the Hermon mountain) using the NCAR Weather Research and Forecasting (WRF) model running at 1.3km resolution is much higher (R=0.90).
The WRF model was initially used to provide semi-operational precipitation forecasts during the 2008-09 and 2009-10 winter (wet) seasons in Israel and surrounding regions where complex terrain dominates. The daily WRF model precipitation forecasts were verified against the measurements of a dense rain gauge network in Israel and the results indicated a good reliability. Numerical experiments were carried out to test the impact of various WRF model resolutions and different WRF microphysical schemes to determine an optimal model configuration for this application. It was found that, because of the strong Air-Sea interactions and orographic forcing in the region, it is necessary to run WRF at 4 – 1.3 km grid sizes along with a sophisticated microphysical scheme (mp-6) that consider liquid water, ice, snow, and graupel in order to produce accurate predictions of precipitation. Because the model takes into account only the natural processes in the clouds it can be used as the primary control for the actually measured precipitation values in seeded days during the Israeli-4 experiment. Significant positive differences between the actual values and the model predictions will be ascribed to the seeing effect. A possible systematic model bias will be corrected by the double ration calculation.