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Y. Mahrer

Abstract

The use of a fine vertical grid resolution near the surface may lead to a numerically inconsistent approximation of the horizontal gradient terms in a terrain-following coordinate system. This occurs when the distance between two vertical grid points is smaller than the elevation difference between two horizontally adjacent (in the terrain-following coordinate system) points. In this paper an improved numerical procedure is proposed which eliminates this inconsistency and significantly increases the accuracy of the numerical approximation. Results are compared with those obtained with the conventional forward and centered schemes.

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J. Neumann and Y. Mahrer

Abstract

In this paper the authors study the lake and land breezes of a mesometearological system consisting of a circular lake and a level land area surrounding it. It is assumed that initially the atmosphere is at rest, the meteorological variables being uniform along horizontal planes. At time t=0, a diurnal temperature wave is imposed on the land surface and the resulting circulation is studied through the application of the non- linear equations of motion and heat conduction for an axially symmetric flow system. In the numerical solution we consider two lake sizes: one of 25 km radius (“small”lake) and the other one of 50 km radius (“large”lake).

In both lake-size cases the landward penetration of the cool lake air takes place along a front. As the lake breezes are divergent horizontally, and those of the small lake are more strongly divergent, and as horizontal divergence of winds tends to dissolve fronts, the front spearheading the breezes of the large lake is more fully developed. For the same reason, the dynamic developments along the lake-breeze front of the large lake are more intense. Thus the evolution of a pressure low, the deformation of the temperature field (and, presumably, of the water-vapor concentration field), and the circulation forming about the front of the large lake are more pronounced than is the case for the small lake. Relatedly, the velocities of the breezes of the large lake are greater than those of the small lake. This feature as well as the problem of the rate of advance of the lake-breeze front is explored on the basis of dynamical equations.

The general pressure field shows some unexpected developments in the afternoon hours. Because of the, strong horizontal divergence of the lake breezes, the pressure falls nearly uniformly by about 1 mb over the whole of our mesometeorological system.

The land breezes are weak over the land but of some strength over the lake. Due to the horizontal convergence of these breezes, a cell of upward motion (weak) develops over the central area of the lake.

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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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J. Neumann and Y. Mahrer

Abstract

Estoque's model for the sea breeze is modified with respect to some essential and other, less essential, details. It is shown among other things that the accelerational terms in the equation for the vertical component of motion are important and should therefore be retained. In addition, the equation of continuity is retained in its original form in order to prevent violation of the mass conservation law. The new model is integrated numerically and the results presented. The results include horizontal and vertical winds, mass conservation, vertical components of vorticity, time hodographs, temperatures (including vertical profiles), vertical transfer of sensible heat, and wind stress at the surface. Particular attention is paid to the land breeze phase of the circulation which so far has received little attention in the published literature. Further, the sea breeze front is discussed in some detail.

The integration is carried out for three daily cycles setting out from an atmosphere at rest. The results for Days 2 and 3 as well as for the second part of Day 1 are in good agreement.

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J. Neumann and Y. Mahrer

Abstract

The axisymmetric sea and land breezes of circular islands are studied. First we show, from the equation of turbulent energy, that the marked horizontal convergence of the sea breeze intensifies the turbulence in the flow and that this conclusion affects the equations that model the turbulence. Next, the equations of motion are integrated numerically for two circular islands: a “large” island of radius 51.25 km and a “small” island of radius 26.25 km. The horizontal grid is 2.5 km and the vertical 100 m beginning from the top of a postulated constant-flux layer 25 m thick.

Large island. The sea-breeze front (SBF) is much better developed than in the case of a straight coast. In the first hours of the afternoon the low-level winds ahead of the front are nearly opposite in direction to those of the sea breeze behind the front. There is a strong horizontal convergence in advance of, and divergence behind, the front up to an altitude of 400–500 m; the reverse distribution is the case aloft, where the winds ahead of the surface position of the front depart from the picture of a countercirculation and blow in the same direction as the sea breeze near the surface. The maximum computed upward velocity occurs about the front and reaches 50 cm sec−1. The most surprising feature is the formation in the afternoon of an “eye” of downward velocities around the island center ahead of the front. By late afternoon, however, the island center becomes the center of upward motion. All the above events are connected with an instability developing about the SBF, including the evolution of a pressure low about the surface position of the front. This low deepens as the front moves inland. The land breezes are horizontally divergent with the largest speeds found over the sea.

Small island. The SBF reaches the center about 1300 local time so that little development can take place along the front. However, the upward velocities reach 150 cm sec−1 over the center.

In conclusion, it is pointed out that the most interesting features of the sea breeze occur well inland, whereas those of the land breeze occur well offshore.

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

Abstract

This paper examines two coordinate representations for slope flow models, one a rotation of the coordinate axes, the other a generalized vertical coordinate transformation. An analytic solution is developed in both representations for a uniform slope to examine the differences due to slightly different forms of a generalised hydrostatic equation. For the first transformation, velocity acclerations in the direction of the generalized vertical coordinate are ignored, while for the second transformation, velocity accelerations perpendicular to the terrain are neglected. Surprisingly, only the period of flow oscillation and not the mean strength of the slope flow was changed in using the first coordinate representation instead of the second. Only for slopes greater than 45° does the difference in periods between the two transformations 30%. Differences which may occur for nonuniform slopes, however, still need to be examined.

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M. Segal, J. L. Song, R. A. Pielke, J. F. W. Purdom, and Y. Mahrer

Abstract

This study presents scale analysis and numerical model evaluations of the impact of cloud shading on 1) the development of sea breeze and thermally induced upslope flows and 2) the generation of mesoscale circulations between cloudy areas adjacent to clear areas. Based on the assumption of modifications of solar and longwave radiation which are typical for some overcast conditions, it was found that the reduction in the first type of circulation is significant and most noticeable in the vertical velocities. In the second case, thermally induced circulations in favorable conditions appear to approach the typical intensity of the sea breeze.

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