David W. Martin

Enhancement of Cumulus Clouds over Deforested Lands in Amazonia

Abstract A survey of shallow (fair weather) cumulus clouds over part of Amazonia yields evidence of enhanced frequency where the forest had been cleared. The survey covers one dry-season month from 1988. It employs a threshold algorithm to construct an image of cumulus cloud cover from sets of geostationary satellite visible-infrared image pairs. Cumulus images were constructed for two times. The morning image shows no association of the cumulus index with cultural features. However, in the afternoon image a patch of high index values coincides with deforestation along highway BR-364 in the state of Rondonia.

On The Relationship between the QBO and Tropical Deep Convection

Abstract The height and amount of tropical deep convection are examined for a correlation with the stratospheric quasi-biennial oscillation (QBO). A new 23-yr record of outgoing longwave radiation (OLR) and a corrected 17-yr record of the highly reflective cloud (HRC) index are used as measures of convection. When binned by phase of the QBO, zonal means and maps of OLR and HRC carry a QBO signal. The spatial patterns of the maps highlight the QBO signal of OLR and HRC in typically convective regions. Spectral analysis of zonal mean OLR and HRC near the equator reveals significant peaks at QBO frequencies. Rotated empirical orthogonal function (REOF) analysis is used to determine if ENSO variations of convection are aliased into the observed QBO signals. Some analyses are repeated using the OLR record after ENSO REOF modes have been removed, yielding very similar results compared to the original analyses. It appears that the QBO signal is distinct from the ENSO signal, although the relative brevity of the ...

A model for calculating desert aerosol turbidity over the oceans from geostationary satellite data.

Abstract A technique has been developed to infer the optical thickness of Saharan dust from Synchronous Meteorological Satellite (SMS) brightness measurements at visible wavelengths. The scattering model consists of an air layer, a dust layer and a lower boundary of variable albedo. Single-scatter properties of the dust computed from Mie theory were the basis for calculations by plane-parallel theory of radiative transfer in the dust layer. Radiative interactions between air and dust layers and the lower boundary were calculated with an adding version of the doubling scheme. Optical thickness was determined from satellite brightness measurements through a lookup table produced by the adding program. SMS visible sensors were calibrated from the prelaunch calibration measurements and measurements of sun and space. Error analysis and tests indicate a potential accuracy of ∼0.1 unit of optical thickness. The main limits on accuracy are digitizing resolution of the SMS visible signals, and mistaking clouds for...

The Amazon Boundary-Layer Experiment (ABLE 2B): A Meteorological Perspective

Abstract In July and August 1985, and April and May 1987, two atmospheric chemistry field experiments called the Amazon Boundary-layer Experiments, (ABLE 2A and 2B) were conducted from a base near Manaus, Brazil in the central Amazon basin. The experiments were designed to determine sources, sinks, concentrations, and transports of trace gases and aerosols originating from the tropical rain forest soils, wetlands, and vegetation. We describe in this paper the design of these experiments and some of the preliminary results which have emerged. We wish, in particular, to illustrate the complexities of determining the inter-related roles between meteorological scales of motion and the fluxes, transports, and reactions of chemical species and aerosols embedded in the atmospheric fluid. Illustrative results from ABLE 2A and 2B are presented which represent both meteorological findings largely independent of the chemistry and results which stem from the chemical nature of the experiment and might not have been f...

Satellite observations of shallow cumulus coverage over the central United States: An exploration of land use impact on cloud cover

A simple algorithm is presented which identifies cumulus clouds in visible and infrared satellite image pairs. The algorithm is applied to hourly pairs of the Geostationary Operational Environmental Satellite (GOES) to determine the relative frequency of daytime cumulus clouds over the central United States for two consecutive Julys: one drought (1988) and one control (1987). In both Julys, cumulus frequency decreased from southeast to northwest, against the gradient of elevation, but with the gradient of midday average relative humidity. On the scale of uplands and lowlands and even isolated ridges it tended to increase with elevation. During the drought in July, contrasts in vegetative cover were especially large. These contrasts tended to oppose contrasts in surface temperature inferred from satellite radiometers. They also tended to oppose contrasts in the frequency of shallow cumulus. Over central and southern Illinois, a flat to gently undulating, mostly rural plain, during July 1988, shallow cumulus clouds occurred more often over lightly vegetated than heavily vegetated landscapes. These clouds also tended to be relatively abundant near the larger urban centers. During the control July of 1987, fewer cumulus clouds were recorded.

Review of Satellite Rainfall Estimation Methods

This paper grew out of two ad hoc conferences held last year to discuss the problem of estimating precipitation from satellite data. Although developments in an area as new as this are rapid, there was sufficient interest in the problem, progress towards a solution, and ignorance of the work of others to warrant drawing the conference summaries together in a review paper of wider circulation and general interest. Our purpose is to examine, from the perspective of GATE rainfall requirements, methodologies in use and under development for the estimation of rainfall from satellite visible and infrared images. As an introduction to critical descriptions of each method, we touch upon the uses of rainfall observations in GATE, the advantages of satellite monitoring, and the purposes of the two conferences held to date. A discussion of calibration and accuracy requirements, including ground truth and accuracies for GATE, is followed by descriptions of two field programs—one completed, the other planned—which hav...

Over-Ocean Validation of the Global Convective Diagnostic

The global convective diagnostic (GCD) is a bispectral (infrared and water vapor), day–night scheme for operationally mapping deep convection by means of geostationary satellite images. This article describes a test of GCD performance over tropical and subtropical waters near North America. The test consists of six cases, each involving a convective cloud complex. A seventh case treats convection over land. For each case, a map of deep convection was constructed from image pairs from Geostationary Operational Environmental Satellite-12 (GOES-12). Case by case and for all maritime cases together, the GCD map was compared with a convective parameter derived from the radar on the Tropical Rainfall Measuring Mission (TRMM), a polar-orbiting satellite. In general, each GCD map showed a bloblike feature. In each case, the radar convective pixels typically fell within the GCD blob. However, (except for the land case) the GCD predicted far too many convective pixels. In the maritime cases overprediction was reduced (without correspondingly impairing other measures of performance) by lowering the nominal GCD threshold. With this adjustment in place, for the six maritime cases taken individually, the GCD tended to yield more consistent results than did a monospectral (infrared) convective scheme. With the cases combined, at the lower threshold the GCD performed somewhat better than one of the more stable versions of the infrared scheme. Comparison with lightning events (also observed by TRMM) suggests the possibility of future improvement to the GCD through the incorporation of geostationary satellite observations of lightning.

Pilot Analysis of Hourly Rainfall in Central and Eastern Amazonia

Abstract This paper presents results of a pilot study of rainfall along the part of the Amazon River that flows through Brazil. Rain was measured at three stations, one for each of three regimes: coastal, interior bottomland, and interior upland. For each station the record from 1 January 1988 through 31 December 1990 was parsed into accumulation periods of 1 h. Storms on the coast tended to be more showery than those in the interior and storms in the interior upland tended to be more showery than those in the interior lowland. The diurnal cycle varied with distance from the Amazon River as well as with distance from the Atlantic coast.

Is there a quasi‐biennial oscillation in tropical deep convection?

We investigate the possibility that the stratospheric Quasi-Biennial Oscillation (QBO) modulates deep convection in the tropics. Interannual variations of outgoing longwave radiation (OLR) in the tropics during 1975–87 are compared with stratospheric zonal winds at Singapore (a measure of the QBO), and with the Tahiti-Darwin sea level pressure difference (the Southern Oscillation Index, or SOI). A monthly time series of anomalous OLR was constructed for regions of consistently low OLR, thus targeting areas of chronic deep convection. This “chronic cold” index and the SOI correlate at −0.6 for zero lag. The “chronic cold” index correlates with 30 hPa Singapore winds at +0.3 and with 50 hPa-70 hPa wind differences at +0.4, both near zero lag. These results are not inconsistent with the hypothesis that deep convection may be enhanced in chronically cold areas when QBO westward shear exists in the lower stratosphere, and diminished during eastward shear.

A Satellite Study of Cloud Clusters over the Tropical North Atlantic Ocean

Signal enhancement is advanced as a method for isolating selected features of ATS images. The discussion covers enhancement theory, limitations, and one important application, the isolation of deep convection within tropical cloud clusters. Comparisons of brightness contoured ATS images with radar images from BOMEX test the validity of associating deep convection with very bright clouds. The enhancement technique is then applied in a census of Atlantic cloud clusters, and in case studies of individual clusters. It is shown that in spite of difficulties involving control of the ATS signal, enhancement is an effective, precise tool for isolating selected features of ATS images. Comparisons of ATS and radar images establish a high correlation of bright areas on ATS with large radar echoes; therefore, enhanced ATS pictures emphasizing the upper levels of the brightness range effectively isolate deep convection. The brightness structure of convective clouds is such that they can be studied over a three-to four...