No abstract

Enhanced surface and upper-air observations from the field campaign of the Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) project are used to partition rainfall amounts over the West African Sudanian zone during the 2002 summer monsoon season into several characteristic types and subtypes of precipitating systems. The most prominent rainfall subtype was fast-moving, long-lived, and extensive cloud clusters that often developed far upstream over the central Nigerian highlands in the afternoon hours and arrived at the Upper Ouémé Valley (UOV) after midnight. These organized convective systems (advective OCSs, subtype Ia) accounted for 50% of the total rain amount in the UOV catchment in Benin. Subtypes Ia and IIa (i.e., locally developing OCSs) were found to pass by or organize when a highly sheared environment with deep and dry midtropospheric layers was present over the UOV. These systems were most frequent outside the peak of the monsoon season. The second major type of organized convection, termed mesoscale convective systems (subtypes Ib, IIb, and IIIb) in the present study, contributed 26% to the annual UOV precipitation. They occurred in a lesssheared and moister tropospheric environment mainly around the height of the rainy season. A third distinct class of rainfall events occurred during an unusual synoptic situation in which a cyclonic vortex to the north of the UOV led to deep westerly flow. During these periods, the African easterly jet was lacking. The so-called vortex-type rainfalls (subtypes IIIa, IIIb, and IIIc) contributed about 9% to the annual rainfall totals.

An air-sea interaction model that includes turbulent transport due to capillary waves (surface ripples) is developed. The model differs from others in that the physical premises are applicable to low wind speeds (10-m wind speed, U 10 Ͻ 5 m s Ϫ1) as well as higher wind speeds. Another new feature of the model is an anisotropic roughness length, which allows a crosswind component of the stress to be modeled. The influence of the angle between the mean wind direction and the mean direction of wave propagation is included in the anisotropic roughness length. Most models are not accurate at low wind speeds and tend to underestimate fluxes in low wind speed regions such as the tropical oceans. Improvements over previous models are incorporated in the momentum roughness length parameterization. In addition, the dimensionless constant in the relationship between the capillary wave component of momentum roughness length and friction velocity is reevaluated using both wave tank data and field data. The new value is found to be 0.06, a factor of 3 smaller than the original estimate of 0.18. Modeling the influence of capillary waves is shown to improve the accuracy of modeled surface fluxes and drag coefficients. Several sets of tropical observations are used to examine mean increases in modeled fluxes due to capillary waves. The changes in latent heat fluxes are compared to proposed increases due to convective overturning (sometimes called gustiness) and are found to be larger by a factor of 4. For U 10 Ͻ 7 m s Ϫ1 , the mean estimates for tropical fluxes of momentum and latent heat are found to increase by 0.004 N m Ϫ2 and 6 W m Ϫ2. * Deceased.

No abstract

Data from 298 stations between $5' " and 30"s have bzen processed for the period July 1957 to December 1964. The mean wind and temperature fields are presented together with the momentum and heat fluxes for two three month seasons, December-February and June-August. The tropospheric results were obtained from objective analysis of the long term station means and the stratospheric results from latitude band means giving equal weight to data fromodd and even years so that the biennial component has been virtually eliminated.The mean temperature cross-sections show little seasonal variation in the tropical troposphere and gradients are small. The maximum temperature is in the vicinity of 5"s during the southern summer but moves to 20"N during June-August as a consequence of stronger heating over the continents. In the tropical stratosphere the seasonal variation is from lower temperatures in December-February to higher temperatures in June-August. The mean zonal wind pattern shows the tropical easterlies to be stronger and to occupy a greater range of latitudes in the upper troposphere during June-August. The Hadley circulation obtained appears quite reasonable with the winter hemisphere cell predominating and extending over the Equator.Although all three components of the momentum flux were evaluated for the troposphere, only the transient eddy contribution could be obtained for the stratosphere. A strong flux is observed in the upper equatorial troposphere directed into the summer hemisphere. The sensible heat-flux in the middle troposphere is equatorward in the Tropics of both hemispheres and a small transport into the summer hemisphere is noted. This equatorward flux is countergradient in both hemispheres except during the northern summer. The mean motion transport of total energy is about an order of magnitude larger than the eddy transport at the Equator and supplies energy to the winter hemisphere.The zonal and eddy forms of kinetic energy and available potential energy have been evaluated for the tropical troposphere together with all conversions between them except for that by transient eddies between available potential energy and kinetic energy. On the average, kinetic energy contents are about twice as large as available potential energy contents and the momentum flux conversions are larger than those involving heat fluxes. The conversion by the mean motions, however, is largest of all over the range 24ON-24"S and leads to a rejuvenation time of some 3-5 days for the zonal kinetic energy. This conversion appears to be sufficiently large to offset the destruction in middle latitudes.Taken together, the results presented here give a consistent pattern for the tropical circulation. The direct Hadley cell is seen as a source of both heat and momentum which are distributed by the eddy fluxes. The energy supply for the mean cell is thought to be the latent heat of condensation in its ascending branch.Ibid., 11, pp. 481-483. 140-150.