A PC-based interactive image processing system has been developed for aiding the analysis of Indian Geosynchronous Satellite (INSAT) data for Asian monsoon studies. In view of its diminutive stature, the system has been given the name “MIDGET,” for a Micro-based Image Display and Graphics Enhancement Tool. Various analysis procedures involving INSAT data and other monsoon datasets are described in conjunction with the MIDGET system. These include the use of the system for monitoring monsoon evolution and the behavior of organized tropical storms, analysis of low-frequency intraseasonal oscillations, diagnostic studies of cloudiness, retrieval of monsoon precipitation and its relationship to satellite cloudiness, and statistical prediction of monsoon cloud bands associated with low-frequency intraseasonal fluctuations of monsoon rainfall.

The system is designed as a workstation terminal in a supercomputer environment. The basic virtue of this system is that it is an inexpensive approach for generating a high-resolution video time-lapse of large satellite datasets in an interactive environment familiar to PC users. The motivation for developing this system derives from a new source of Indian satellite data that has been made available to United States scientists through archive holdings at the National Center for Atmospheric Research.

Abstract

An investigation of the transition between spring and summer seasons of the surface energy budget in the Gobi desert is presented. The motivation behind this study is to determine eventually the degree to which changes in a desert system can be monitored over a short-term climate time scale (decadel) by remote means. A seasonal transition is used to evaluate the control factors involved in a variational process. The measurements incorporated in the analysis were obtained in 1984 from a specialized surface energy budget monitoring system deployed at a site in the western Gobi desert, just north of the northeastern edge of the Tibet Plateau in western Gansu province, P.R.C. The data were collected during the spring and summer periods in 1984 by a joint team of United States and Chinese scientists.

Results of the analysis reveal an interesting feature of the seasonal transition which had not been expected of a midlatitude desert. That is, although radiative forcing at the surface is altered between spring and summer through the diurnal net radiation heating function, the total radiative energy integral available for heating is largely unchanged. In some sense, the partitioning of the radiative heat supply at the surface can be viewed as a principal ingredient in defining the seasonal cycle. In terms of the Gobi desert, it may well be the only important ingredient.

Both similarities and differences in the spring and summer surface energy budgets arise from differences imparted to the system by an increase in the summertime atmospheric moisture content. Changes in the near-surface mixing ratio are shown to alter the effectiveness of the desert surface in absorbing radiative energy and redistributing it to the lower atmosphere through sensible and latent heat exchange.

Abstract

Abstract

Analysis of surface latent heat flux measurements taken within the sea-breeze front of the coast of Florida during active thunderstorm periods demonstrates an important effect of the timing of coastal storms on the seasonal surface water budget. Historical records document a systematic cross-peninsula water runoff gradient across Florida, with total runoff greater on the east coast (Atlantic side) than on the west coast (gulf side). This situation persists even though convective rainfall tends to be greater in the summertime on the gulf side. In this paper, the authors examine the effect of the time of day that summer thunderstorms occur at a given location on poststorm evaporation of rainfall and place these effects into the context of the annual runoff at the coasts and seasonal rainfall in order to assess their possible significance.

A surface water exchange analysis, based on datasets obtained during the 1991 summertime Convection and Precipitation Electrification Experiment, finds that part of the runoff gradient can be explained by an indirect atmospheric mechanism. Results indicate that differences in the diurnal timing of thunderstorms between the two coasts and the associated differences in postthunderstorm evapotranspiration can account for a significant portion of the annual differential in runoff. During the summer months, gulf coast storms often occur earlier in the day than Atlantic coast storms because of the combined effects of the mesoscale sea-breeze convergence and synoptic-scale flow around the Bermuda high. Under these conditions, once the later-day east coast thunderstorms dissipate, there is no longer any net solar radiation source to drive evapotranspiration, so that rainwater not taken up by ground filtration tends to go into runoff. On the west coast, when thunderstorms occur earlier and dissipate in midafternoon, there is still enough net surface radiation to drive significant rates of evapotranspiration, which reduces the amount of water available for runoff.

The difference in available rainfall that results from the increased evaporation after the earlier storms is found to be about 2 mm, which over the summer season amounts to some 50 mm of water not made available for runoff on the west coast. This is significant when compared to the annual cross-peninsula runoff gradient of 250 mm. It is also found that it takes 4.5 days of clear-sky latent heat fluxes to reevaporate average storm rainfall back into the atmosphere. In addition, areas that are not close to the center of storm outflows tend to be neutral in terms of daily surface water exchange, evaporating as much as they receive, while cloudy areas with no rain evaporate at rates close to 90% of the clear-sky rates on a daily basis. This paper addresses the details of these processes and quantifies the surface water exchange in south Florida as a function of the proximity to the summertime thunderstorm outflows.

Abstract

Observations taken by two surface radiation and energy budget stations deployed in the University of Florida/Institute for Food and Agricultural Service experimental citrus orchard in Gainesville, Florida, have been analyzed to identify the effects of sprayer irrigation on thermal stability and circulation processes within the orchard during three 1992 winter freeze episodes. Lapse rates of temperature observed from a micrometeorological tower near the center of the orchard were also recorded during periods of irrigation for incorporation into the analysis. Comparisons of the near-surface temperature lapse rates observed with the two energy budget stations show consistency between the two sites and with the tower-based lapse rates taken over a vertical layer from 1.5 to 15 m above ground level. A theoretical framework was developed that demonstrates that turbulent-scale processes originating within the canopy, driven by latent heat release associated with condensation and freezing processes from water vapor and liquid water released from sprayer nozzles, can destabilize lapse rates and promote warm air mixing above the orchard canopy. The orchard data were then analyzed in the context of the theory for evidence of local overturning and displacement of surface-layer air, with warmer air from aloft driven by locally buoyant plumes generated by water vapor injected into the orchard during the irrigation periods. It was found that surface-layer lapse rates were lower during irrigation periods than under similar conditions when irrigation was not occurring, indicating a greater degree of vertical mixing of surface-layer air with air from above treetops, as a result of local convective overturning induced by the condensation heating of water vapor released at the nozzles of the sprinklers. This provides an additional explanation to the well-accepted heat of fusion release effect, of how undertree irrigation of a citrus orchard during a freeze period helps protect crops against frost damage.

Abstract

We present the second part of a study on the development of a framework for precipitation retrieval from space-based passive microwave measurements using a three-dimensional time-dependent cloud model to establish the microphysical setting. We first develop the theory needed to interpret the vertically distributed radiative sources and the emission-absorption-scattering processes responsible for the behavior of frequency-dependent top-of-atmosphere brightness temperatures T_B 's. This involves two distinct types of vertical weighting functions for the T_B 's: an emission-source weighing function describing the origin of emitted radiation that eventually reaches a satellite radiometer, and a generalized weighting function describing emitted-scattered radiation undergoing no further interactions prior to interception by the radiometer. The weighting-function framework is used for an analysis of land-based precipitation processes within a hail-storm simulation originally described in Part I. The individual roles of cloud drops, rain drops, graupel particles, ice crystals, and snow aggregates—as well as absorbing gases, the earth's surface, and cosmic background—on generating and modulating the frequency-dependent T_B 's are examined in detail. The analysis emphasizes how microwave T_B measurements are highly regulated by mixtures of hydrometeors, with particular emphasis on the importance of the vertical profile structures. We demonstrate how scattering produces sequential, frequency-dependent, vertical “break aways” of the peak amplitudes in the generalized weighting functions, thus explaining how a multichannel radiometer can be used to depth probe a precipitating cloud. We also seek to explain the extent to which 19-, 37-, and 85-GHz T_B 's are responding to separate and distinct processes in precipitating cells in an unambiguous fashion, helping to elucidate the two key aspects of these standard satellite frequencies. That is, 1) they are best suited to decipher certain microphysical profile features above the main rain layers and near cloud top, and 2) they are ill suited for directly sensing precipitation intensity information within the main rain layers, particularly the surface rain rates. Finally, a summary of the various components of a hybrid statistical-physical rainfall algorithm used to produce liquid-ice profile information, as well as surface rain rates, is given. The algorithm employs the cloud model to provide a consistent and objectively generated source of detailed microphysical information as the underpinnings to an inversion-based perturbative retrieval scheme.

Abstract

The impact of spatial resolution enhancement on estimates of tropical typhoon rainfall based on SSM/1 (Special Sensor Microwave/Imager) measurements is evaluated with six different microwave precipitation retrieval algorithms. Passive microwave estimates of rainfall are susceptible to errors from nonhomogeneous beam filling. The SSMIX ground footprints for the 19-, 22-, and 37-GHz channels have considerable overlap, and thus deconvolution techniques can be applied to enhance spatial resolution of measurements at those frequencies. The authors utilize a Backus-Gilbert matrix transform approach to accomplish the deconvolution so as to minimize noise amplification, as suggested by Stogryn. The deconvolution scheme is evaluated in terms of its impact on rain rates throughout the life cycles of seven tropical cyclones that occurred during the 1987 hurricane and typhoon season. The evaluation was performed on a single-frequency emission-based algorithm, a single- frequency scattering-based algorithm, two multiple-frequency statistical regression algorithms, and two physical inversion-based profile algorithms. While rainfall patterns detected by all algorithms were qualitatively enhanced by accentuating rainfall gradients and other smaller-wale features, quantitative responses to the deconvolution process were quite different for each algorithm. Furthermore, each of the algorithms, which uses its own distinct scientific approach, exhibits its own distinct properties in retrieving the rainfall patterns and in recovering the storm domain-averaged rain rates. The rain rates derived from the single-frequency emission algorithm were consistently increased by application of the deconvolution procedure. Time-and space-averaged rain rates were elevated by approximately 5%–6% due to the nonlinear relationship of rain rate to brightness temperature. On the other hand, rain rates from the single-frequency scattering algorithm were consistently reduced, with the time-space-averaged reduction between 10% and 20%. This effect is not algorithm related but is due to alteration of noise properties of the two polarized 37-GHz channels introduced during the deconvolution process. The multiple-frequency algorithms have more complex responses to deconvolution. Although instantaneous rain rates can be changed quite significantly by these methods, differences between deconvolved and raw time-space-averaged rain rates are small compared to the single-channel algorithms because the pixel-scale differences tend to be of a more random nature (positive and negative changes instead of consistent bias). However, it appears that the profile methods can undergo the greatest improvement to instantaneous rain rates after deconvolution is applied because they use perturbative inversion procedures rather than fixed brightness temperature-rain rate relationships.

Abstract

Passive microwave brightness temperatures (T_B 's) at 92 and 183 GHz from an aircraft thunderstorm overflight are compared with values calculated from radar-derived hydrometer profiles and a modified proximity sounding. Two methods for modeling particles in the ice canopy are contrasted. The fist is a “traditional” approach employing Marshall–Palmer ice spheres. The second, or “alternative,” method partitions 20% of the ice water content into a Marshall–Palmer component for graupel and hail, and 80% into a modified gamma spherical particle size distribution function representing ice crystals.

Results from the alternative approach are superior to those from the traditional method in the anvil and mature convective core. In the decaying convective region, the traditional approach yields better agreement with observed magnitude. Neither method, however, matches the geometry of the observed T_B depression associated with the decaying convective core. This is likely due to the presence of graupel, which is not detected as a special signature in radar reflectivity, but does diminish T_B 's through scattering. Brightness temperatures at the relatively high microwave frequencies considered are shown to be very sensitive to the ice-particle size distribution.

Abstract

We present a modification to the parameterization scheme of Stephens which improves on the estimation of shortwave absorption by cloud. In particular, the variation of cloud absorption with solar elevation angle is improved with the modified scheme.

Abstract

The objective of this paper is to establish a computationally efficient algorithm making use of the combination of Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and precipitation radar (PR) observations. To set up the TMI algorithm, the retrieval databases developed in Part I served as input for different inversion techniques: multistage regressions and neural networks as well as Bayesian estimators. It was found that both Bayesian and neural network techniques performed equally well against PR estimates if all TMI channels were used. However, not using the 85.5-GHz channels produced consistently better results. This confirms the conclusions from Part I. Generally, regressions performed worse; thus they seem less suited for general application due to the insufficient representation of the nonlinearities of the TB–rain rate relation. It is concluded that the databases represent the most sensitive part of rainfall algorithm development.

Sensor combination was carried out by gridding PR estimates of rain liquid water content to 27 km × 44 km horizontal resolution at the center of gravity of the TMI 10.65-GHz channel weighting function. A liquid water dependent database collects common samples over the narrow swath covered by both TMI and PR. Average calibration functions are calculated, dynamically updated along the satellite track, and applied to the full TMI swath. The behavior of the calibration function was relatively stable. The TMI estimates showed a slight underestimation of rainfall at low rain liquid water contents (<0.1 g m⁻³) as well as at very high rainfall intensities (>0.8 g m⁻³) and excellent agreement in between. The biases were found to not depend on beam filling with a strong correlation to rain liquid water for stratiform clouds that may point to melting layer effects.

The remaining standard deviations between instantaneous TMI and PR estimates after calibration may be treated as a total retrieval error, assuming the PR estimates are unbiased. The error characteristics showed a rather constant absolute error of <0.05 g m⁻³ for rain liquid water contents <0.1 g m⁻³. Above, the error increases to 0.6 g m⁻³ for amounts up to 1 g m⁻³. In terms of relative errors, this corresponds to a sharp decrease from >100% to 35% between 0.05 and 0.5 g m⁻³. The database ambiguity, that is, the standard deviation of near-surface rain liquid water contents with the same radiometric signature, provides a means to estimate the contribution from the simulations to this error. In the range where brightness temperatures respond most sensitively to rainwater contents, almost the entire error originates from the ambiguity of signatures. At very low and very high rain rates (<0.05 and >0.7 g m⁻³) at least half of the total error is explained by the inversion process.