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  • Author or Editor: Eric A. Smith x
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David A. Faysash
and
Eric A. Smith

Abstract

GOES-8 thermal infrared split window measurements have been used with a simultaneous land surface temperature (LST)–spectral emissivity retrieval algorithm to examine the potential of a combined retrieval methodology cast into a variational solution for temperatures at multiple but short-term 6- to 24-h time intervals and emissivities at multiple spectral bands assumed to be invariant over the selected time intervals. Retrieved LST and emissivity quantities under differing atmospheric conditions over an annual cycle are validated and analyzed in regard to their underlying diurnal and seasonal variations over the Department of Energy’s Atmospheric Radiation Measurement–Cloud and Radiation Test Bed (ARM–CART) site in Kansas and Oklahoma.

It is shown that the accuracy of the retrieval algorithm depends primarily on GOES infrared channel detector noise and uncertainties in columnar water vapor path, in which retrieval accuracy increases as pathlength decreases. A detailed analysis is given of the characteristic temporal–spatial gradient structures of LSTs and emissivities over the ARM–CART domain at point to area space scales and diurnally to seasonally varying timescales. Emphasis is given to explaining the relationship of heterogeneous features in the retrievals in conjunction with physical attributes of the landscape, that is, ecotones and phenology, and the effects of prior cloudiness on subsequent LSTs.

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Mitchell Weiss
and
Eric A. Smith

Abstract

A quantitative investigation of the relationship between satellite-derived cloud-top temperature parameters and the detection of intense convective rainfall is described. The area of study is that of the Cooperative Convective Precipitation Experiment (CCOPE), which was held near Miles City, Montana during the summer of 1981. Cloud-top temperatures, derived from the GOES-West operational satellite, were used to calculate a variety of parameters for objectively quantifying the convective intensity of a storm. A dense network of rainfall provided verification of surface rainfall. The cloud-top temperature field and surface rainfall data were processed into equally sized grid domains in order to best depict the individual samples of instantaneous precipitation.

The technique of statistical discriminant analysis was used to determine which combinations of cloud-top temperature parameters best classify rain versus no-rain occurrence using three different rain-rate cutoffs: 1, 4, and 10 mm h−1. Time lags within the 30 min rainfall verification were tested to determine the optimum time delay associated with rainfall reaching the ground.

A total of six storm cases were used to develop and test the statistical models. Discrimination of rain events was found to be most accurate when using a 10 mm h−1 rain-rate cutoff. Use parameters designated as coldest cloud-top temperature, the spatial mean of coldest cloud-top temperature, and change over time of mean coldest cloud-top temperature were found to be the best classifiers of rainfall in this study. Combining both a 10-min time lag (in terms of surface verification) with a 10 mm h−1 rain-rate threshold resulted in classifying over 60% of all rain and no-rain cases correctly.

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Alberto Mugnai
and
Eric A. Smith

Abstract

In a two-part study we investigate the impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation at several frequencies across the EHF and lower SHF portions of the microwave spectrum in order to explore the feasibility of using multichannel passive-microwave retrieval techniques for the estimation of precipitation from space-based platforms.

A series of numerical sensitivity experiments have been conducted that were designed to quantify the impact of an evolving cumulus cloud in conjunction with a superimposed rain layer on the transfer to space of microwave radiation emitted and scattered from the cloud layers, rain layer and the underlying surface. The specification of cloud microphysics has been based on the results of a time-dependent two-dimensional numerical cumulus model developed by Hall (1980). An assortment of vertically homogeneous rain layers, described by the Marshall-Palmer rain drop distribution, has been inserted in the model atmosphere to simulate the evolution of rainfall in a precipitating cumulus cell. The effects of ice hydrometeors on upwelling brightness temperatures have been studied by placing several types of ice canopies over the cloud and rain layers. Both rough ocean and land backgrounds have been considered. The top-of-atmosphere brightness temperatures have been computed by means of a vertically and angularly detailed plane-parallel radiative transfer model for unpolarized microwave radiation.

Part I describes the modeling framework. In addition, it provides a detailed description of the single-scattering properties of the hydrometeors (model-cloud water drops, ice crystals and rain drops) in order to evaluate each component's role in influencing the upwelling radiation to space. We demonstrate that cloud water can have a major impact on the upwelling microwave radiation originating from both the surface and a rain layer placed below cloud base. The radiative properties of the model cloud are shown to be significantly different from those of an equivalent Marshall-Palmer treatment. It is the appearance of the large-drop mode (r> 100 μm) of the cumulus cloud drop distribution function that denotes the point at which cloud drops begin to attenuate the microwave signals, even at the lower frequencies, which are normally considered to be mostly unaffected by purely cloud processes. It is shown that at the early stages of cloud evolution, the model cloud acts mainly through absorption/emission processes. As the cloud develops, however, scattering plays an ever-increasing role. It is also demonstrated that the relative contribution by the small drop mode (r<100 μm) of the cloud to absorption/emission is always significant. It is concluded that the vertical variation of the microphysical structure of the rain-cloud plays an important role in the interpretation of passive microwave rainfall signatures and thus should be considered in precipitation retrieval algorithms.

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Eric A. Smith
and
Alberto Mugnai

Abstract

The time-dependent role of cloud liquid water in conjunction with its vertical heterogeneities on top-of-atmosphere (TOA) passive microwave brightness temperatures is investigated. A cloud simulation is used to specify the microphysical structure of an evolving cumulus cloud growing toward the rain stage. A one-dimensional multistream solution to the radiative transfer equation is used to study the upwelling radiation at the top of the atmosphere arising from the combined effect of cloud, rain, and ice hydrometeors. Calculations are provided at six window frequencies and one H2O resonance band within the EHF/SHF microwave spectrum. Vertically detailed transmission functions are used to help delineate the principal radiative interactions that control TOA brightness temperatures. Brightness temperatures are then associated with a selection of microphysical situations that reveal how an evolving cloud medium attenuates rainfall and surface radiation. The investigation is primarily designed to study the impact of cloud microphysics on space-based measurements of passive microwave signals, specifically as they pertain to the retrieval of precipitation over water and land backgrounds.

Results demonstrate the large degree to which the relationship between microwave brightness temperature (BT) and rainrate (RR) can be altered purely by cloud water processes. The relative roles of the cloud and rain drop spectra in emissive contributions to the upwelling radiation are assessed with a normalized absorption index, which removes effects due purely to differences in the magnitudes of the cloud and rain liquid water contents. This index is used to help explain why the amplitudes of the BT-RR functions decrease with respect to cloud evolution time and why below-cloud precipitation is virtually masked from detection at the TOA.

Although cloud water tends to obscure BT-RR relationships, it does so in a differential manner with respect to frequency, suggesting that the overall impact of cloud water is not necessarily debilitating to precipitation retrieval schemes. Furthermore, it is shown how a “surface” of “probability” can be defined, which contains an optimal time-dependent BT-RR function associated with an evolving cloud at a given frequency and removes ambiguities within the BT-RR functions at the critical retrieval frequencies. The influence of a land surface having varying emissivity characteristics is also examined in the context of an evolving cloud to show how the time-dependent cloud microphysics modulates the sign and magnitude of brightness temperature differences between various frequencies.

Model results are assessed in conjunction with a Nimbus-7 SMMR case study of precipitation within an intense tropical Pacific storm. It is concluded that in order to obtain a realistic estimation and distribution of rainrates, the effects of cloud liquid water content must be considered.

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Steven T. Fiorino
and
Eric A. Smith

Abstract

The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager precipitation profile retrieval algorithm (2a12) assumes cloud model–derived vertically distributed microphysics as part of the radiative transfer–controlled inversion process to generate rain-rate estimates. Although this algorithm has been extensively evaluated, none of the evaluation approaches has explicitly examined the underlying microphysical assumptions through a direct intercomparison of the assumed cloud-model microphysics with in situ, three-dimensional microphysical observations. The main scientific objective of this study is to identify and overcome the foremost model-generated microphysical weaknesses in the TRMM 2a12 algorithm through analysis of (a) in situ aircraft microphysical observations; (b) aircraft- and satellite-based passive microwave measurements; (c) ground-, aircraft-, and satellite-based radar measurements; (d) synthesized satellite brightness temperatures and radar reflectivities; (e) radiometer-only profile algorithm retrievals; and (f) radar-only profile or volume algorithm retrievals. Results indicate the assumed 2a12 microphysics differs most from aircraft-observed microphysics where either ground or aircraft radar–derived rain rates exhibit the greatest differences with 2a12-retrieved rain rates. An emission–scattering coordinate system highlights the 2a12 algorithm's tendency to match high-emission/high-scattering observed profiles to high-emission/low-scattering database profiles. This is due to a lack of mixed-phase-layer ice hydrometeor scatterers in the cloud model–generated profiles as compared with observed profiles. Direct comparisons between aircraft-measured and model-generated 2a12 microphysics suggest that, on average, the radiometer algorithm's microphysics database retrieves liquid and ice water contents that are approximately 1/3 the size of those observed at levels below 10 km. Also, the 2a12 rain-rate retrievals are shown to be strongly influenced by the 2a12's convective fraction specification. A proposed modification of this factor would improve 2a12 rain-rate retrievals; however, fundamental changes to the cloud radiation model's ice parameterization are necessary to overcome the algorithm's tendency to produce mixed-phase-layer ice hydrometeor deficits.

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Song Yang
and
Eric A. Smith

Abstract

Datasets of daily high-resolution upper-air soundings and Special Sensor Microwave/Imager (SSM/I) passive microwave measurements from the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive operation period are used for large-scale diagnostic budget calculations of the apparent heat source (Q 1), the apparent moisture sink (Q 2), and latent heating to investigate the mechanisms of diabatic heating and moistening processes within the TOGA COARE Intensive Flux Array (IFA). Latent-heating retrievals are obtained from The Florida State University SSM/I-based precipitation profile retrieval algorithm. The estimates are correlated well with heating calculations from the soundings in which approximately 70% of the total heating arises from latent heat release. Moisture-budget processes also have a strong relationship with the large-scale environment, in which drying from condensation is mainly balanced by large-scale horizontal convergence of moisture flux. It is found that there may be more convective activity in summer than in winter over the tropical region of the western Pacific Ocean. Results also show that Q 1 and Q 2 exhibit a 20–30-day oscillation, in which active periods are associated with strong convection.

Comparisons of the Q 1Q 2 calculations over IFA are made with a number of previously published results to help to establish the similarities and differences of Q 1Q 2 between the warm pool and other regions of the Tropics. The Q 1Q 2 budget analyses over IFA then are used to study quantitatively the detailed vertical heating structures. Cumulus-scale heating–moistening processes are obtained by using published radiative divergence (Q R ) data, retrieved latent heating, and the Q 1Q 2 calculations. These results show that cumulus-scale turbulent transport is an important mechanism in both heat and moisture budgets. Although daily estimates of eddy vertical moisture flux divergence are noisy, by averaging over 7-day periods and vertically integrating to obtain surface latent heat flux, good agreement with measured surface evaporation is found. This agreement demonstrates the feasibility of estimating averaged eddy heat–moisture flux profiles by combining satellite-derived rain profile retrievals with large-scale sounding and Q R data, a methodology that helps to shed light on the role of cumulus convection in atmospheric heating and moistening.

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Eric A. Smith
,
Elmar R. Reiter
, and
Youxi Gao

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.

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Dennis R. Phillips
,
Eric A. Smith
, and
Verner E. Suomi

Abstract

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Harry J. Cooper
,
Eric A. Smith
, and
Michael T. Rubes

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.

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Harry J. Cooper
,
Eric A. Smith
, and
J. David Martsolf

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.

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