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E. B. Kraus

Abstract

Analysis of nine weathership records indicates that maritime precipitation is significantly more frequent at night. The effect varies with season and latitude. The diurnal variation is related to the absorption of solar radiation. Non-adiabatic heating may cause a reduction of liquid water production within rising clouds; this factor becomes important when vertical velocities are not too high. At the top of layer clouds, the diurnal rhythm of irradiation can also cause a time-lagged diurnal change of inversion levels and cloud thickness. The last inference is supported by observational data.

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E. B. Kraus

Abstract

The spatial coherence of subtropical rainfall anomalies is documented by variance analysis. Major droughts repeatedly were felt at the same time around the globe along the arid margins of the tropical rainfall belt. The persistence of anomalies becomes apparent in precipitation time series which combine data from relatively large areas and in streamflow records. These can be used to demonstrate autocorrelations and unexpectedly long runs of wet or dry years.

The latest drought episode culminated in 1972 not only in the Sahel and the Sudan, but also along the borders of the Indian desert and in Central America. It is shown to have been accompanied by relatively low temperatures in the southern subtropics and by abnormally high temperatures in the antarctic. The meridional temperature gradient and the meridional slope of the 500 mb surface were correspondingly reduced. It is suggested that this was associated with a reduced demand for energy (and zonal momentum) exports from the tropics and therefore relatively weak direct tropical circulations. As a result, these circulations–which tend to straddle the equator–did not deliver the normal amount of precipitation along their northernmost borders in the monsoonal fringe area.

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E. B. Kraus and E. N. Lorenz

Abstract

Experiments with six different heating fields in a numerical general circulation model are described. Three different vertical heating gradients are each used once with and once without variations on the continental-oceanic scale along parallel circles. The zonal and the meridional heating fields are forced to vary seasonally. Integration has been carried out over a simulated period of one century for one particular configuration, and over periods of five years for each of the five other configurations.

Results which may be represented by an electrical analogue are rather similar to actual general circulation observations. They also show stronger summer westerlies and North-South temperature gradients in the model without schematic oceans and continents. Dynamic lag effects cause differences between the “climates” of spring and fall. In all experiments there was a breakdown in fall of a predominantly zonal circulation, accompanied by the development of “equinoctial storms.”

Lag correlations computed for the mean zonal thermal wind in the 100-year experiment show persistence in summer between successive ten-day means and significant negative values over longer lag periods. No significant lag correlations were found during the winter months.

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