The Effect of Variations in Surface Moisture on Mesoscale Circulation

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

A two-dimensional numerical Model with moist physics is used to simulate circulations induced by horizontal variations in surface-moisture availability. The model contains prognostic equations for water vapor, cloud water, and rain water, with a simple parameterization of cloud microphysical processes. Four geometric variations of surface-moisture availability are examined: 1) an edge geometry which includes a land-water contrast (classic sea breeze) and moist land adjacent to dry land (inland sea breeze), 2) a single strip of moist land surrounded by dry land 3) alternating bands of moist and dry land, and 4) a single strip of dry land surrounded by moist land.

For convectively unstable initial conditions with a relative humidity of 50%, lifting associated with the sea-breeze front induces a precipitation system which propagates inland from the coast. The sea-breeze circulation associated with dry land is considerably stronger than that produced by moist land; however, the evaporation over land in the sea-breeze simulation with moist land results in increased rainfall in spite of the weaker circulation. When moist land is located adjacent to dry land, an “inland sea breeze” is generated which is almost as strong as the dry-land sea breeze, and significant precipitation is produced.

In the simulations with a single moist strip surrounded by dry land, two inland sea breezes form and move outward over the dry land. For strips of width 24 and 48 km, the relatively weak circulations fail to produce clouds or precipitation. As the width of the strip increases, however, the increased strength of the inland sea-breeze circulations, together with increased evaporation, results in the formation of precipitation systems, with the amount of precipitation increasing with increasing width of the moist strip.

With alternating bands of dry and moist land, two inland sea-breeze fronts converge toward the center of the dry bands and produce vigorous rainstorms for bandwidths of 96 km and greater. For a given width of moist Land, the bands are more efficient at generating rainfall than a single strip, because of greater evaporation and a constructive interference of the inland see-breeze circulations in the band simulations.

A single strip of dry land of width 144 km and surrounded by moist land produces greater rainfall than either the 144-km moist strip or the 144-km bands, because of the greater total evaporation. The maximum 24-h gridpoint value (6-km average) rainfall in this simulation is 7.93 cm.

The results indicate that inhomogeneities in land moisture on a horizontal scale of 100–200 km can, in a convectively unstable environment with weak environmental flow and sufficient moisture, initiate convective rainfall. They support Anthes' hypothesis that planting bands of vegetation with widths of order 100 km in semiarid regions could under favorable large-scale conditions, produce increases in convective precipitation.

Abstract

A two-dimensional numerical Model with moist physics is used to simulate circulations induced by horizontal variations in surface-moisture availability. The model contains prognostic equations for water vapor, cloud water, and rain water, with a simple parameterization of cloud microphysical processes. Four geometric variations of surface-moisture availability are examined: 1) an edge geometry which includes a land-water contrast (classic sea breeze) and moist land adjacent to dry land (inland sea breeze), 2) a single strip of moist land surrounded by dry land 3) alternating bands of moist and dry land, and 4) a single strip of dry land surrounded by moist land.

For convectively unstable initial conditions with a relative humidity of 50%, lifting associated with the sea-breeze front induces a precipitation system which propagates inland from the coast. The sea-breeze circulation associated with dry land is considerably stronger than that produced by moist land; however, the evaporation over land in the sea-breeze simulation with moist land results in increased rainfall in spite of the weaker circulation. When moist land is located adjacent to dry land, an “inland sea breeze” is generated which is almost as strong as the dry-land sea breeze, and significant precipitation is produced.

In the simulations with a single moist strip surrounded by dry land, two inland sea breezes form and move outward over the dry land. For strips of width 24 and 48 km, the relatively weak circulations fail to produce clouds or precipitation. As the width of the strip increases, however, the increased strength of the inland sea-breeze circulations, together with increased evaporation, results in the formation of precipitation systems, with the amount of precipitation increasing with increasing width of the moist strip.

With alternating bands of dry and moist land, two inland sea-breeze fronts converge toward the center of the dry bands and produce vigorous rainstorms for bandwidths of 96 km and greater. For a given width of moist Land, the bands are more efficient at generating rainfall than a single strip, because of greater evaporation and a constructive interference of the inland see-breeze circulations in the band simulations.

A single strip of dry land of width 144 km and surrounded by moist land produces greater rainfall than either the 144-km moist strip or the 144-km bands, because of the greater total evaporation. The maximum 24-h gridpoint value (6-km average) rainfall in this simulation is 7.93 cm.

The results indicate that inhomogeneities in land moisture on a horizontal scale of 100–200 km can, in a convectively unstable environment with weak environmental flow and sufficient moisture, initiate convective rainfall. They support Anthes' hypothesis that planting bands of vegetation with widths of order 100 km in semiarid regions could under favorable large-scale conditions, produce increases in convective precipitation.

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