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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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Y. Mahrer and M. Segal

Abstract

Model scaling of the characteristics of the sea breeze circulation involved with circular and slab elongated-symmetric midlatitude islands was performed. Larger horizontal and vertical velocities were indicated over the small circular islands as compared to those over corresponding elongated islands. The circulation characteristics of both types of islands become similar when the half-width of the island is about the same as the distance of inland penetration of the sea breeze in a nonisland case. Evaluation of the results through a wale analysis is presented.

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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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