The evaporation of precipitation and its geographical distribution

Abstract

In order to estimate the evaporation of precipitation in different climate regions, as a first attempt, climate data from four meteorological stations have been used for mean daily precipitation, mean seasonal surface temperature and surface humidity and combined with mean seasonal vertical temperature profiles. Since the profiles are not representative for days with precipitation the results are preliminary and have to be interpreted with some caution. Nevertheless the following conclusions seem to be justified. The least evaporation - as well in absolute as in relative terms compared to the amount of precipitation produced in clouds - takes place in the humid tropics, where high humidity and low clouds prevail and also the most intense daily rain rate at the ground is observed. The calculations indicate that here the highest production of annual precipitation takes place above the cloud base. Evaporation of rain is limited to the levels just above the ground. The highest mean rain rate at the cloud base, however, seems to occur in the subtropics, where storms are rare but intense. Very dry environmental low-level air is responsible for the greatest average height of the cloud base and the most intense evaporation rate, which takes place at the highest possible level halfway between cloud base and ground. The high mean rain intensity computed at the cloud base is drastically reduced by evaporation, thus the observed mean daily rain rate on the ground ranks only on second place. In the midlatitudes precipitation is almost as frequent as in the tropics but the rates are lower, therefore the total annual observed precipitation is also much lower but still ranks on second place. The precipitation rate at the cloud base and the average cloud level do not indicate a great seasonal variation. The total annual precipitation produced in clouds is found to rank on the last place, with about 50% more rain being produced in the summer than snow in the winter, as also the number of days with precipitation is somewhat more frequent in the summer. The results suggest that probably about 60% or more of the precipitation produced in clouds also arrives at the ground. In contrast to the midlatitudes the subarctic shows a significant seasonal variation. Here the greatest number of days with precipitation is observed. The highest frequency of precipitation occurs during winter, when frequent cyclonic activity causes long lasting continuous precipitation. Both seasons contribute about equal amounts to the total annual precipitation, despite the longer duration of the winter season, as the summerly rain rate at the ground exceeds the intensity of snow falls. At the cloud base, however, the precipitation on a winter day seems to exceed the daily production of rain in the summer by almost 100%. Our results suggest a significant difference in the height of the cloud base between summer and winter. During the winter season the cloud base is found at a rather high level. The humidity profile, which should be fairly reliable because of the high frequency of days with precipitation, indicates a high reaching moist but unsaturated layer in which considerable evaporation of snow seems to take place. Actually this result was expected and it is suggested that in arctic and subarctic regions the reduction of precipitation due to evaporation in the subcloud layer is substantial, at least in a relative sense, as during winter the fraction of precipitation generated in clouds, which evaporates, is comparable with the evaporation of rain in the subtropics. As a consequence in the subarctic probably not much less precipitation is produced in clouds than in the tropics, with more than three times as much snow leaving the cloud base as rain, mainly because of prevailing cyclonic conditions during winter time.