Lanterna Rally

Author: Eugene M. Rasmusson

Publisher:

Published: 1966

Total Pages: 170

ISBN-13:

The atmospheric water vapor flux and certain aspects of the water balance over the North American Sector are investigated for the period May 1, 1961 - April 30, 1963. The vertical variation of the flux, as well as the total vertically integrated flux, are investigated from mean monthly data. The flux exhibits important diurnal variations, particularly during the summer south of 50 0N. These variations are primarily the result of diurnal variations in the mean wind, rather than in the moisture, and are particularly well organized over eastern North America, the Gulf of Mexico, and the Caribbean Sea. Significant interannual changes in the flux are also observed. The relationship of these changes to the interannual changes in flux divergence and precipitation are discussed. The mean vertical distribution of flux divergence is computed for the United States, for the months of January and July. Strong flux convergence in the lowest 100 mb, and divergence in the remainder of the troposphere, was found in July. Flux convergence was found throughout the troposphere in the east in January, with a maximum between 900 and 950 mb, while in the west convergence (with no particularly pronounced maximum) was found above 800 mb, with weak divergence below. Corresponding features of the profiles were found at higher elevations over the west, where the flux divergence above 500 mb is quite significant. Particular emphasis is placed on computations of the vertically integrated vapor flux divergence, and its use in estimating E-P , the mean difference between evaporation and precipitation. Water balance studies, using twice daily observations from the existing aerological network, indicate that reliable mean annual, seasonal, and monthly values of E can usually be obtained for areas of 20 x 105 km2 or larger. The results usually deteriorate rapidly as the size of the area is reduced to less than 10 x 105 km2 . This deterioration is primarily the result of a systematic error pattern, which is tentatively ascribed to the effect of diurnal flux variations, small scale features in the mean flux field, and local station peculiarities. The annual and seasonal values of E-P are computed for the Caribbean Sea and the Gulf of Mexico and are in excellent agreement with independent estimates. Mean values of E-P are computed for North America north of the United States-Mexican border, and individually for the major watersheds of the continent. Latitudinally averaged values show a minimum between 55°N and 65°N. More comprehensive balance studies were made over the United States and southern Canada. Of particular interest is the computation of mean monthly surface and subsurface storage changes directly from measured streamflow and vapor flux data. Consistent and reasonable storage changes are computed for the area as a whole, which indicate an average seasonal variation of around 8 cm. Little net storage change was computed during the two year period for the whole area, but substantial changes were indicated over the western part of the region during the first year, and over the eastern part during the second year. These changes appear to be in qualitative agreement with independent indicators. Rough computations of mean monthly evapotranspiration are made for the United States and southern Canada, using precipitation and flux divergence data. Values exhibit the expected seasonal variations, with a maximum of around 8 cm/mo in summer and a minimum of 1-2 cm in winter. Computations for the larger subdivisions of this area give values which appear, for the most part, to be reasonable.