Abstract

The diurnal harmonic in longwave emission in the tropical belt (30°N–30°S) is estimated from nine years of NOAA polar-orbiting satellite data. The results are compared successfully with Nimbus-7 ERB scanner data and with GOES-West geosynchronous satellite data. An interesting and consistent diurnal variation in longwave emission is found over the regions of intense oceanic convection such as the ITCZ and SPCZ regions with a peak-to-peak variation of 6–8 W m⁻² and a maximum in the morning (0600–1200 LST). Histogram analysis indicates that this variation is associated with a diurnal variation in convective cloud (∼400 mb). Over regions of very intense convection, a diurnal variation of very high clouds (∼100 mb), which is out of phase with the variations at lower levels in the atmosphere, reduces the magnitude of the diurnal harmonic in longwave emission.

It is interesting that histograms based on data averaged over 8-km and 250-km boxes give the same qualitative information about cloud and emission variability.

Abstract

A compositing technique is used to obtain the average structure of 18 disturbances which traversed an area in the equatorial western Pacific during the wet season (July–September) of 1967. Principal emphasis is placed on the wave properties in the triangular area described by Ponape, Kwajalein and Eniwetok within which it was possible to measure divergence and vertical motion and to compute moisture and heat budgets.

Meridional wind maxima of nearly opposite phase occurred in the lower and upper troposphere. Negative temperature deviations were found in the vicinity of the wave trough at low and high levels; positive deviations were observed at intermediate levels. Highest relative humidities occurred in the trough region. This was also the region of strongest upward motion and greatest rainfall and cloud amount. The maximum upward velocity of 2.5 cm sec⁻¹ was found at 300 mb. Convergence was strongest in the sub-cloud layer; divergence was concentrated near 175 mb. The maximum anticyclonic vorticity was also observed at that level.

The wave structure changed in a systematic fashion across the network. The change is attributed to the variation with longitude of the shear of the basic current.

The rainfall computed from the observed wind and moisture fields agreed well with observed amounts which varied from about 2 cm day⁻¹ in the vicinity of the trough axis to about 0.5 cm day⁻¹ near the ridge axis. The diabatic heating difference between trough and ridge regions was largest at 400 mb where it was estimated to be nearly 10C day⁻¹.

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.

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.

Abstract

Heat and moisture budgets are used to compute net condensation rates in the GATE B-scale network for four hours of the day: 0000, 0600, 1200 and 1800 GMT. Budgets are presented for all phases combined, for selected periods of enhanced convection and for selected periods of suppressed convection. Computations are based on fitted values of the meteorological sounding data for the center of the B-scale ship array, on surface heat flux and evaporation measurements for seven ships in the array and on Cox and Griffith's (1979) radiation measurements for Phase III. Results are also presented for the diurnal variation of the basic variables.

Main findings are as follows:

1. 1) Temperature variations are small, of the order of a few tenths of a degree, with a daytime maximum and nighttime minimum. There is evidence of possible radiation error above 400 mb where the amplitude is largest.

2. 2) The atmosphere appears to be relatively dry during the day, when convection is most active, and relatively moist at night. It is suggested that this behavior may be caused by instrumental error.

3. 3) The divergence and vertical velocity undergo characteristic cycles that were repeated in all three phases. It is hypothesized from their behavior that the convection evolves in three distinct stages: a stage of shallow convection during the early night, a stage of vigorously growing, primarily moderate-depth convection in the late night and morning and a stage of predominantly deep convection in the afternoon.

4. 4) The vertical advection terms dominated both budgets. Variations of heat and moisture storage and of radiation were also important. Surface evaporation and sensible heat flux were essentially constant throughout the day.

5. 5) Rainfall diagnosed from the heat budget was less than the observed precipitation and rainfall diagnosed from the moisture budget was greater than the observed precipitation in all three phases.

6. 6) Maximum diagnosed condensation preceded maximum observed precipitation by ∼6 h. Differences between condensation and precipitation rates are attributed in part to storage of condensed water, though errors of measurement undoubtedly contributed to them as well.

7. 7) The diurnal cycles of precipitation and low-level vertical motion were much larger in the disturbed (trough) region of easterly waves than in the suppressed (ridge) region. During highly suppressed periods the precipitation was uniformly distributed throughout the day while the vertical motion still appeared to show a variation.

Abstract

Data from a dense network of ship observations are used to study the structure and properties of westward-moving wave disturbances observed in the eastern Atlantic Intertropical Convergence Zone (ITCZ) during Phase III of the GAPP Atlantic Tropical Experiment (GATE). Comparisons are made with similar disturbances found in the ITCZ of the western Pacific. Wave fields are determined by fitting low-order polynomials to the ship data with use of the method of least squares.

The wave structures in the two regions are found to be similar in many respects, the principal difference being in the divergence field and associated vertical motion. Unlike in the Pacific a multi-layer divergence pattern exists in the eastern Atlantic, leading us to hypothesize the existence of three main cloud populations with outflow levels near 800, 500 and 250 mb. The soundings for the Atlantic exhibit lesser parcel instability then the Pacific soundings in agreement with the reduced vigor of the convective cells and the greater tendency for multiple cloud layers. The strongest upward motion (∼150 mb day⁻¹) occurs in and somewhat ahead of the wave trough, as in the Pacific, but at a much lower level (800–700 mb). A secondary maximum appears near 350 mb, where the primary maximum appears in the Pacific. The maximum precipitation rate of 22 mm day⁻¹ is observed in the region of strongest upward motion. The rate decreases to 4 mm day⁻¹ in the region of suppressed convection near the wave ridge. Vertical eddy flux of total heat is largest at the 800 mb level in the wave trough (225 W m⁻²) and produces cumulus heating and cooling of about 5°C day⁻¹ above and below the maximum, respectively.

A nearly balanced moisture budget for the inner ship array or B-scale area was obtained from the fitted fields when data from both outer and inner ships were employed in the fitting. In particular, two individual waves and the composite or average wave yielded sufficiently accurate budgets to encourage their use in quantitative studies of interactions between synoptic-scale and convective-scale systems. The residual in the heat budget suggests a radiational cooling rate of 0.9°C day⁻¹. The surface energy budget indicates a net radiative flux at the surface of 129 W m⁻² of which 106 W m⁻² was used for evaporation and 12 W m⁻² for sensible heat flux to the atmosphere, leaving 11 W m⁻² for heating of the ocean mixed layer. The heat exchange between ocean and atmosphere underwent a pronounced variation with passage of the synoptic disturbances, causing sea surface temperatures to be 0.3°C warmer ahead of the wave troughs than behind. Precipitation rates employed in the budgets were based on radar measurements; surface sensible and latent heat fluxes were computed by the bulk aerodynamic method with use of temperatures, humidities and winds from the booms of four B-scale ships; and net radiation at the surface was obtained from measurements made aboard the same four ships.

The kinetic energy of the waves was provided by the barotropic conversion process (conversion from zonal kinetic energy), the baroclinic conversion being negative and thus a sink for the eddy kinetic energy. Likewise, the generation of eddy available potential energy was negative, implying that latent heat release opposed, rather than contributed, to the wave growth. The described conditions are quite unlike those in the western Pacific ITCZ where condensation heating provides the source for the wave energy and the barotropic conversion constitutes a weak sink.

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.