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  • Author or Editor: Y. Mahrer x
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R. Avissar and Y. Mahrer

Abstract

A three-dimensional numerical model was developed to predict the microclimate near the ground surface of local-scale domains during radiative frost events. Its performances are compared with an observational topo-climatological survey of minimum temperatures at a height of 0.5 m above the soil surface which was carried out, during radiative float events, in the Hefer Valley, Israel. Considering only topography and soil type in the numerical simulation, relatively good agreement is obtained between predicted and observed minimum temperature. A more realistic picture is given when vegetation is incorporated in the model although larger discrepancies with observations are obtained. This is mainly explained by the fact that measurements were always carried out above bare surfaces, even when dense vegetation was present and, therefore do not provide a representative minimum temperature of many areas. This assumption is validated by field measurements of nighttime temperatures in an orchard and above a bare soil in its immediate vicinity.

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R. Avissar and Y. Mahrer

Abstract

Radiative frost is one of the most severe weather conditions that affects agricultural activities in many parts of the world. Since various protective methods to reduce frost impact are available, refinements of frost forecasting methodologies should provide economical benefits.

In the present study, a three-dimensional numerical local-scale model for the simulation of the microclimate near the ground surface of nonhomogeneous regions during radiative frost events was developed. The model is based on the equations of motion, heat, humidity and continuity in the atmosphere and the equations of heat and moisture diffusion in the soil. Emphasis was given in establishing a refined formulation of energy budget equations for soil surface and plant canopy Additionally, an improved finite difference scheme procedure for approximating horizontal derivatives in a terrain-following coordinate system was introduced.

The sensitivity of the model to various parameters that way affect the nocturnal minimum temperature near ground surface during radiative frost events was tested by using one- and two-dimensional versions of the model. This temperature was found to be sensitive to topography, plant cover, soil moisture content, air specific humidity and wind velocity.

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S. Assouline and Y. Mahrer

Abstract

Microclimate characteristics and latent and sensible heat fluxes were measured continuously and simultaneously over Lake Kinneret, Israel, during two consecutive summers at the eastern (Ein Gev) and western shores (Sapir) of the lake. The data were used to characterize the variability in basic meteorological variables (air temperature and humidity, water surface temperature, and wind velocity) and in evaporation rates. Analysis of the data on an hourly basis reveals the combined effect of local physical process occurring during airflow over water surfaces and the diurnal regional phenomena of the inland penetration of the Mediterranean sea breeze downslope into the area during the afternoon hours. The resulting strong, hot and dry westerly winds at the western coast become weaker, cooler, and more humid as they reach the eastern shore after a delay of 1–2 h. Consequently, the maximum evaporation rate at Sapir was occasionally twice the corresponding rate at Ein Gev. The data on a daily basis depicted the influence of synoptic systems on the regional climate. Commonly, the mean evaporation rate from the entire lake is assumed to be equal to that evaluated at a specific site. Considering the observed variability, this assumption might lead to errors as large as 100% on the daily basis and of 15% on the seasonal basis.

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M. Segal, Y. Mahrer, and R. A. Pielke

Abstract

A three-dimensional numerical mesoscale model has been applied over the irregular terrain of northern Israel in order to simulate the local surface climate pattern associated with typical July stagnate synoptic meteorological conditions. Comparison of model-evaluated surface flow and temperature fields against observed data illustrate the role in which mesoscale models can be used in order to provide additional insight into the analyses of persistent regional climatological patterns.

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