Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: Bernhard Lettau x
  • Refine by Access: All Content x
Clear All Modify Search
Bernhard Lettau

Abstract

Seasonally persistent westerly winds at the surface in the vicinity of the equator from 40E to 170E longitude are shown to be the generally predictable result of a geostrophically appropriate pressure gradient at latitudes higher than about 5° north or south, and a negative value of the second derivative of the pressure with latitude within about five degrees latitude of the equator. Existing empirical equatorial wind-pressure relationships are shown to be specific cases of these general relationships. The initiating mechanism for equatorial westerlies seems to be a movement of the equatorial trough more than five degrees of latitude from the equator.

Full access
BERNHARD LETTAU

Abstract

Pilot balloon wind profiles obtained by the first and second Byrd Antarctic Expeditions are analyzed to show that the mean observed wind shear between the surface and 1,000 m. can be resolved into a frictional component which produces a normal boundary layer wind spiral, and a thermal component resulting from the temperature gradient at the ice edge, which deforms the normal wind spiral. Values of surface stress, surface Rossby number, geostrophic drag coefficient, energy dissipation, and roughness length derived from the wind profiles are collectively sufficiently different from values obtained over land or water surfaces, to suggest that the ice surface produces its own characteristic wind distribution.

Full access
LANCE F. BOSART, VITO PAGNOTTI, and BERNHARD LETTAU

Abstract

Results of a study on back-door cold fronts for the months April through October of 1964–71 are presented. Results include information on frequency, associated air-mass duration, precipitation, temperature and dew-point temperature changes, sky cover, and rate of frontal movement. Composite 850- and 500-mb height contours are also constructed.

Our findings include: (1) for the sample period, southward penetration of back-door cold fronts is a maximum in June as is frontal frequency, (2) orography plays a prominent role in funneling shallow cold air pools southward east of the Appalachians, (3) more than half of all frontal passages are associated with trace amounts or less of precipitation, (4) heaviest precipitation tends to occur in conjunction with the advancing cold fronts at the more southern and western locations and with the returning warm fronts at northern and eastern locations, (5) temperature changes following frontal passage decrease from north to south and from the coast inland (dew-point temperature changes follow a similar pattern but not so clearly), and (6) cloudiness increases following frontal passage, especially at southern and inland locations.

The composite study reveals a short-wave trough at both the 850- and 500-mb level just east of Hudson Bay preceding the initial movement of back-door cold fronts southward. This short wave intensifies east-southeastward toward the Canadian Maritime Provinces while anticyclogenesis takes place upstream.

Full access