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  • Author or Editor: Ernest E. Recker x
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Donald C. Norquist, Ernest E. Recker, and Richard J. Reed

Abstract

Fields of the meteorological variables in composite wave disturbance constructed for the region from IOOE to 31°W and 1°S to 26°N and for land and ocean subregions are used to diagnose energy transformations in African waves. The composites are based on data contained in the GATE Quick Look Data Set for the period 23 August to 19 September, 1974. The measurements indicate that for the region as a whole the kinetic energy of the waves is maintained almost equally by conversions from zonal kinetic energy and eddy available potential energy. Eddy available potential energy is supplied by the zonal available potential energy at a comparable rate. From the measured conversion rates it is estimated that in the absence of friction the kinetic energy of the waves would double in about 3 days.

Measurements for the subregions show that the conversion from zonal to eddy kinetic energy is stronger over the limited oceanic region considered than over the land, while conversely, the conversion of eddy available potential energy to eddy kinetic energy is stronger over the land than over the ocean. The conversion of zonal to eddy available potential energy differs little between the two regions. From these findings, and budgetary considerations, it is inferred that latent beat release in organized convection plays an important role in the wave growth and maintenance in west Africa but not over the adjacent ocean. This conclusion, however, must be regarded as tentative.

The distributions of the various energy conversion processes in meridional cross section are considered. The conversions of zonal kinetic and available potential energies to their corresponding eddy energies are characterized by concentrated regions of high values closely associated with the mid-tropospheric easterly jet stream. The conversion of eddy available potential energy to eddy kinetic energy exhibits a complex pattern in which the net conversion is a, small residual. Consequently this conversion cannot be regarded as being determined with the same high degree of reliability as the other two. However, major features of the pattern can be explained on physical grounds.

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Richard J. Reed, Donald C. Norquist, and Ernest E. Recker

Abstract

A compositing method is used to determine the average structure and properties of eight wave disturbances observed over west Africa and the eastern Atlantic during the period 23 August-19 September, 1974, a period marked by well-developed and regular wave activity. The disturbance centers propagated westward in the zone of cyclonic shear to the south of the 700 mb easterly jet, located at 16–17°N. The mean wave- length was about 25M km and the mean period 3.5 days. The mean zonal current satisfied the necessary condition for barotropic instability.

The composite disturbance was most intense at 650 mb, being cold core below and warm core above. Two circulation centers were evident at the surface, one located below the upper center and the other displaced 10° to the north at about the latitude of the monsoon trough. When separate composites were constructed for land and ocean stations, the dual centers were found to be primarily a land phenomenon. Distinctive features of the high-level (200 inb) circulation were a strong region of divergence located just ahead of the disturbance center and pronounced regions of anticyclonic and cyclonic vorticity situated several hundred kilometers to the north and south, respectively. Maximum low-level convergence and upward vertical motion were found in the region ahead and slightly south of the center. This was also the region of greatest convective cloud cover and largest precipitation amount.

Some minor differences are noted between wave behavior over land and sea. Over the ocean wavelengths were shorter, vorticities were greater at all levels, especially at the surface, and the horizontal wave axis was more tilted at levels close to the core of the mid-tropospheric jet stream. In association with the greater tilt, the northward momentum flux and transformation of zonal kinetic energy to eddy kinetic energy were stronger.

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Mark D. Albright, Ernest E. Recker, Richard J. Reed, and Renqing Dang

Abstract

Infrared satellite data are used to determine the diurnal variation of deep convection and inferred rainfall in the central tropical Pacific during January-February 1979. The parameter employed to characterize the convection is the percent coverage of 1.5° latitude-longitude squares by clouds with tops colder than various specified equivalent blackbody temperatures. Rainfall estimates are based on an empirical relationship between precipitation rate and fractional coverage by cold clouds derived from measurements taken during the GARP Atlantic Tropical Experiment (GATE). In addition, the diurnal variation of vertical motion, determined kinematically from level III-b gridded wind analyses of the European Centre for Medium Range Weather Forecasts, is examined. Principal conclusions are:

  1. A pronounced diurnal variation of deep convection occurred throughout the region of study. At certain hours fractional coverage by clouds colder than −36°C deviated by as much as 40% from the daily mean. The variation was especially large in the South Pacific Convergence Zone (SPCZ).

  2. The character of the diurnal cycle varied considerably within the region of study, distinctly different convective regimes being found in five subregions that were examined.

  3. The diurnal variation of inferred precipitation also differed from one subregion to another. The SPCZ exhibited a cycle similar to that observed in the GATE B-scale area where a prominent afternoon maximum occurred. Morning maxima, however, prevailed in the Intertropical Convergence Zone (ITCZ) and in tropical cloud intrusions that occurred frequently south of Hawaii.

  4. Vertical motions were upward relative to the mean at 2000 LST in the belt between 10 and 20°N and downward in the SPCZ and ITCZ. The opposite behavior occurred at 0800 LST. No obvious relationship existed between the cloud and vertical motion variations.

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