Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: R. Avissar x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
R. Avissar and R. A. Pielke

Abstract

Natural land surfaces are usually heterogeneous over the resolvable scales considered in atmospheric numerical models. Therefore, model surface parameterizations that assume surface homogeneity may fail to represent the surface forcing accurately.

In this paper, a parameterization of the subgrid-scale forcing of heterogeneous land surfaces for atmospheric numerical models is suggested. In each surface grid element of the numerical model similar homogeneous land patches located at different places within the element are regrouped into subgrid classes. Then, for each one of the subgrid classes, a sophisticated micrometeorological model of the soil-plant-atmosphere system is applied to assess the surface temperature, humidity, and fluxes to the atmosphere. The global fluxes of energy between the grid and the atmosphere are obtained by averaging according to the distribution of the subgrid classes. In addition to the surface forcing, detailed micrometeorological conditions of the patches are assessed for the domain simulated by the atmospheric model.

This parameterization was incorporated into a mesoscale numerical model to test the impact of subgrid-scale land surface heterogeneities on the development of local circulations. Where strong contrasts in total sensible heat flux are generated by land surface heterogeneities, circulations as strong as sea breezes may develop.

Full access
N. Hasler, R. Avissar, and G. E. Liston

Abstract

Running regional climate models at a high resolution may improve their ability to simulate regional precipitation patterns, making them suitable for studying the impact of human-induced land-cover changes on hydrometeorology. The performance of the Regional Atmospheric Modeling System (RAMS) run in the high-resolution climate mode (4-km grid mesh) has been tested over a small domain in a semiarid region in central Spain. Three 1-yr simulations representing dry, intermediate, and wet conditions were compared to observations collected in 35 rain gauges. The model captured general spatiotemporal features of precipitation, such as the timing of precipitation events and approximate location of storms. A high correlation (0.82) between monthly domain-averaged observed and modeled precipitation was obtained. However, the model had a systematic dry bias, averaging −0.29 mm day−1, equivalent to 26% of annual rainfall. The small domain size, chosen because of computational limits, induced strong lateral boundary forcing, which, combined with uncertainty in NCEP relative humidity fields, was a likely cause for this dry bias.

Full access
M. Segal, R. Avissar, M. C. McCumber, and R. A. Pielke

Abstract

The purpose of the present study is to evaluate (i) the effect of vegetated surfaces on modifying sea breeze and daytime thermally induced upslope flows, and (ii) the generation of thermally induced flow by vegetated areas contrasted by bare soil area. In order to address these objectives, the following tasks were carried out: 1) previous documented studies with implication for (i) and (ii) are reviewed; 2) the main features of the thermal balance of vegetated surfaces are outlined qualitatively; 3) a quantitative evaluation of the various components in the thermal balance based on documented observational studies is provided; and 4) scale analyses and numerical model simulations are used to provide quantitative evaluations of the circulations involved with (i) and (ii) for several illustrative cases.

The study suggests that the impact of vegetated surfaces in those cases is highly dependent on the environmental conditions as well as vegetation characteristics. For ideal environmental conditions resulting in high evapotranspiration rates over extended dense vegetated areas, it is shown that the circulation types listed in (i) are substantially reduced. For the situation described by (ii), circulations with an intensity close to that of a sea breeze can develop when the vegetation is very dense, and covers an extended area, and under favorable environmental conditions. The reduction in these impacts for more frequent real world situations involved with less favorable environmental conditions as well as with relatively sparse vegetated areas is also evaluated.

Full access