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Paul M. Tag and Steven W. Payne

Abstract

Cloud top entrainment instability, as a mechanism for the breakup of marine stratus, is examined with a three-dimensional, planetary boundary layer (PBL) model. Specifically, we examine the criterion developed by Randall and Deardorff; this criterion states that stratus will break up if the equivalent potential temperature gradient at cloud top becomes less than a critical value. To examine this hypothesis, we simulate a horizontally uniform stratus layer which is excited from above by small random temperature perturbations. The buoyancy instability ratio (BIR), defined as Δθe(Δθe)crit and computed at cloud top, is calculated locally across the domain and also averaged to define a mean value. Six cases, involving different wind speeds and above-cloud soundings, produce different initial BIRs and different breakup sequences. In general, we find that a mean BIR greater that one is a necessary condition for stratus breakup; however, we also find that the timing of breakup following achievement of the critical ratio is different from run to run. The low wind speed cases, initially most stable at cloud top, are the first to break up, while the higher wind speed (most unstable) cases require longer time to break up. We conclude that an additional mechanism is necessary to stimulate vertical motion in order to take advantage of the cloud-top entrainment instability. In our simulations, that additional stimulation comes from vertical motions generated by Rayleigh-type instability in the PBL.

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Steven W. Payne and Mary M. McGarry

Abstract

SMS-1 infrared brightness was estimated subjectively for each 1° square for the area from the equator to 20°N latitude and from 10°E to 30°W longitude for Phase III of GATE and the previous interphase period at 6 h intervals. Each grid square was assigned one of the following brightness categories: 0, no clouds or clouds with low tops; 1, clouds with tops at middle heights; or 2, clouds with coldest tops—presumably of convective origin. The percentage of area covered by category 2 clouds was then computed for squares 3° an a side. The data were filtered with respect to time to isolate 3–4 day period wave-related oscillations. In addition, over 160 individual cloud clusters, including a special type of “squall” cluster, were identified in the IR images and tracked on a 3 h basis.

A progressive pattern of 3–4 day period fluctuations in convective activity was observed. A comparison of this pattern with time-filtered 700 mb wind data indicates a correspondence of convective activity to wave trough and ridge positions inferred from the wind data. Histograms of the wave phase location of maximum filtered convective cloud coverage indicate that convection was most enhanced at and ahead of the trough axis. However, convection was distributed to some extent throughout the wave. Similarly, convection was shown to be most suppressed at and ahead of the ridge axis. Greatest fluctuations of wave-related convective activity occurred along the path of the center of the 700 mb disturbance.

Most large cloud clusters were found to he located ahead of the trough. These clusters tended to be the longest lived and to move along with the wave at slightly less than wave phase speed. Squall clusters were also most prevalent ahead of the trough. They were found to move at about twice wave phase speed, most moving forward through the wave to a position just behind the ridge before termination.

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Robert M. Thompson Jr., Steven W. Payne, Ernest E. Recker, and Richard J. Reed

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.

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