The Problem of Artificial Control of Rainfall on the Globe: II. The Coastal Orographic Maxima of Precipitation in Autumn and Winter

Abstract

The coastal maxima of rainfall during autumn and winter cannot be explained alone by the classical theory of orographic precipitation. Three different frictional effects , and one effect leading to orographically conditioned convergence at quasi-isophysic flow, are discussed and illustrated by Scandinavian and Dutch rainfall maps of remarkable special cases of maximum coastal precipitation. The rainfall maps over Holland even hint secondary maxima further inland, at zero-level and parallel to the coastal maximum; they might be explained in connection with standing waves in the sense of Queney and others. With all these effects a low condensation level and an efficient precipitation release , by ice-nuclei or in other ways, seem indispensable to cause intense orographic condensation and bring its results promptly down to the earth. As stated in article I such orographic cloud-systems, if lacking an efficient release, just represent the clouds where artificial seeding might materially increase precipitation, because (1) condensation may for a long time keep intense and stationary, and (2) the clouds may keep releasable but unreleased. This increase, if at all possible, would diminish precipitation inland where it is much more useful. — A better scheme would therefore be trying to overseed coastal cloud-systems in order to prevent their precipitation release, and thereby increasing precipitation inland, provided the overseeding could be avoided there. This overseeding process might also be used for dissipating inland Cunb, but presumably not for dissipating tropical hurricanes. The generally accepted idea of the increase of precipitation with height is often, e.g. in Norway, contradicted by individual cases with a marked decrease from the very coast and upward when moving inland. A rational attack on this problem was hitherto impossible because the complex nature of orographic precipitation (hinted above) was not recognized. - Outside the Tropics excessive precipitation needs low cloud formed by an intense updraft at high temperature (= maximum condensation and minimum evaporation) in continual contact with an upper cloud (part) surpassing the —10 p C-level, or seeded artificially (= efficient precipitation release). These conditions will be fulfilled in certain orographic clouds discussed above, and within some convective and frontal clouds. DOI: 10.1111/j.2153-3490.1949.tb01264.x