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Byung-Ju Sohn
and
Eric A. Smith

Abstract

The source and forcing mechanisms of radiation budget variability were examined over tropical latitudes by separating the variations into cloud- and surface-forced components. A zonal harmonic analysis of emitted longwave radiation emphasizes that these variations are largely controlled at the planetary wave scale. Positive total and cloud-forced longwave (LW) anomalies embedded within this planetary-scale structure show eastward movement from the Indian Ocean toward the eastern Pacific together with the easterly displacement of negative anomalies from the western Pacific toward Africa during the period prior to and after the active phase of the 1982–83 ENSO. The overall effect leads to an approximately 50° per year propagation phase speed that is considerably slower than the oceanic Kelvin wave capable of driving east-west LW anomalies through sea surface temperature (SST) feedback. The oceanic Kelvin wave speed is about 60° per month over the Pacific basin in the course of an ENSO cycle. This suggests there are longer time scales of climatic signals in the tropical radiation budget.

The examination of time-dependent radiative energetics over the tropics reveals that the aforementioned anomaly LW propagation is mainly due to cloud forcing associated with east-west circulation changes, although surface forcing contributes within the Pacific basin. Since cloud amount changes are directly linked to variations in latent heat release, diabatic heating associated with coupled ocean-atmosphere feedback appears to be largely responsible for the LW anomaly propagation. An examination of the complete radiation budget over the maritime continent and equatorial central Pacific during the 1982–83 ENSO event demonstrates that radiative forcing produces positive feedbacks in conjunction with the sea surface temperature anomalies that develop in both regions. Furthermore, surface forcing is found to be an important control on net radiation variability within this teleconnection. An examination of two additional tropical cast-west teleconnections shows that surface forcing is even more important than cloud forcing in controlling variations in the east-west net radiation gradients.

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Byung-Ju Sohn
and
Eric A. Smith

Abstract

Required global energy transports determined from Nimbus-7 satellite net radiation measurements have been separated into atmospheric and oceanic components by applying a maximum entropy production principle to the atmospheric system. Strong poleward fluxes by the oceans in the Northern Hemisphere exhibit a maximum of 2.4 1015W at 18°N, whereas maximum atmospheric transports are found at 37°N with a magnitude of 4.5 1015W. These results are in good agreement with other published results. In the Southern Hemisphere, atmospheric transports are found to be considerably stronger than oceanic transports, and this finding corroborates findings based on other published direct estimates. Maximum atmospheric energy transports are found at 37°S with a magnitude of 4.7 × 1015 W; two local oceanic transport maxima are shown at 1 8°S and 45°S with magnitudes of 1.3 × 1O15 W and 1.1 × 1015 W, respectively. There is also evidence of net cross-equatorial) transport in which the Southern Hemisphere oceans give rise to a net transfer of beat northward across the equator that exceeds a net transfer from Northern to Southern Hemisphere by the atmosphere. Since Southern Hemisphere results in this study should have the same degree of accuracy as in the Northern Hemisphere, these findings suggest that Southern Ocean transports are weaker than previously reported. A main implication of the study is that a maximum entropy production principle can serve as a governing rule on macroscale global climate, and in conjunction with conventional satellite measurements of the net radiation balance, provides a means to decompose atmosphere and ocean transports from the total transport field. Furthermore, the modeling methodology provides a possible means to partition the transports in a two-dimensional framework; this approach is tested on the separate ocean basins with qualified success.

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Fong-Chiau Chang
and
Eric A. Smith

Abstract

A drought pattern and its time evolution over the U.S. Great Plains are investigated from time series of climate divisional monthly mean surface air temperature and total precipitation anomalies. The spatial pattern consists of correlated occurrences of high (low) surface air temperature and deficit (excess) rainfall. The center of maximum amplitude in rain fluctuation is around Kansas City; that of temperature is over South Dakota. Internal consistency between temperature and precipitation variability is the salient feature of the drought pattern. A drought index is used to quantify drought severity for the period 1895–1996. The 12 severest drought months (in order) during this period are June 1933, June 1988, July 1936, August 1983, July 1934, July 1901, June 1931, August 1947, July 1930, June 1936, July 1954, and August 1936. Hydrological conditions are examined using National Centers for Environmental Prediction (NCEP) reanalysis precipitable water (PW) and monthly surface observations from Kansas City, Missouri, and Bismarck, North Dakota, near the drought centers. This analysis explains why droughts exhibit negative surface relative humidity anomalies accompanied by larger than normal monthly mean daily temperature ranges and why maximum PWs are confined to a strip of about 10° longitude from New Mexico and Arizona into the Dakotas and Minnesota.

Dynamical conditions are examined using NCEP reanalysis sea level pressures and 500- and 200-mb geopotential heights. The analysis indicates a midtroposphere wave train with positive centers situated over the North Pacific, North America, and the North Atlantic, with negative centers in the southeastern Gulf of Alaska and Davis Strait. Above-normal sea level pressures over New Mexico, the North Atlantic, and the subtropical Pacific along with below-normal sea level pressures over the Gulf of Alaska eastward to Canada, Davis Strait, and Greenland are present during drought periods. The most prominent feature is the strong anticyclone over central North America.

On a regional scale, midtropospheric westerly winds are weakened (or become easterly) south of a thermal heat low centered in South Dakota during drought episodes because of the north–south temperature reversal perturbation. The associated westward displaced Bermuda high leads to enhanced low-level warm flow into the Dakotas, thus helping to maintain the reversal in the meridional temperature gradient and the concomitant thermal wind reversal. Enhanced moisture transport from the Gulf of California into the western plains (part of the Great Basin monsoon process) results from the large-scale perturbation pressure pattern. Middle-upper level convergence maintains the water vapor strip east of the Rocky Mountains, while the Mississippi valley undergoes moisture cutoff from both this process and the westward shift in the Bermuda high. The strip of maximum PW then undergoes enhanced solar and infrared absorption that feeds back on the thermal heat low. Surface air temperatures warm while sinking motion balances middle-upper level radiative cooling around the Kansas City area. This is the dynamical coupling that leads to reduced surface relative humidities. The centers of high surface air temperature and deficit rainfall are dynamically consistent with patterns in geopotential heights, vertical velocities, and water vapor amounts.

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Song Yang
and
Eric A. Smith

Abstract

Time–space distributions of mean monthly latent heating estimated from Special Sensor Microwave/Imager (SSM/I) passive microwave satellite measurements using the Florida State University precipitation profile retrieval algorithm over ocean regions are investigated for the 1992 annual cycle. The space domain is considered in both horizontal and vertical coordinates, with vertical retrieval made possible by the profiling design of the rain algorithm and the underlying relationship between the vertical derivatives of equivalent liquid water mass fluxes and latent heat release.

Comparisons of the retrieved mean monthly rainfall and rain frequency to climatological datasets and atoll rain gauge measurements indicate reasonable agreement except at latitudes above 40° where the satellite values are low biased relative to the climatologies. The horizontal distributions of mean monthly latent heating show that the locations of maximum heating lie in the vicinity and along the axes of well-documented large-scale convergence areas, particularly the intertropical convergence zone (ITCZ) and its transient offshoots, the South Pacific convergence zone (SPCZ), the tropical monsoon systems, and the middle-latitude storm tracks. The vertical distributions show that maximum heating rates of 5°C day−1 are located near the 5-km height level with positive heating extending to the top of the troposphere in the Tropics. Convection shifts associated with the 1992 El Niño–Southern Oscillation (ENSO) episode are well represented in the latent heating field. The seasonal variations of the ITCZ, SPCZ, and monsoon systems are clearly evident. The intraseasonal oscillation of latent heating during the northward propagation of the summer Indian monsoon is also a well-defined feature. Finally, the evolution of the Walker circulation is clearly depicted for both active and inactive ENSO conditions throughout 1992.

Emphasis is given to comparison and contrast of the SSM/I-derived heating fields to results given in the published literature. Many of the stationary and transient features appearing in the retrievals are consistent with previous studies concerning cloudiness, convection, and rainfall over low latitudes, with the exceptions stemming from specific features of the 1992 ENSO event. Therefore, the study provides a framework for using SSM/I measurements as a means to estimate the four-dimensional structure of latent heating over the tropical–subtropical oceans. Since the details of these structures are of considerable importance to the earth’s weather–climate system both in terms of forcing and response, and by virtue of the design of a rain profiling algorithm, these results are presented as a necessary first step in seeking to use satellite measurements to obtain the most important component of the diabatic heating field.

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Mickey M-K. Wai
and
Eric A. Smith

Abstract

Land–atmosphere interactions are examined for three different synoptic situations during a 21-day period in the course of the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment 1989 to better understand the relationship between biophysical feedback processes, boundary layer structure, and circulations in the boundary layer. The objective is to understand how the secondary circulation discussed in Part I of this paper was able to sustain itself throughout the duration of the 1989 intensive field campaign. The study is based on diagnostic analysis of measurements obtained from a network of surface meteorology and energy budget stations, augmented with high vertical resolution radiosonde measurements. Shallow convection associated with an undisturbed boundary layer situation and rainfall occurring during two different disturbed boundary layer situations—one associated with a surface trough, the other with the passage of a cold front—led to markedly different impacts on the surface layer and the boundary layer recovery timescale. In the undisturbed case, the growth of a cloud layer produced a negative feedback on the boundary layer by stabilizing the surface layer, and cutting off the turbulence transport of heat and moisture into the subcloud layer. The deficits in heat and moisture then led to cloud dissipation. During the surface trough development and cold front passage events, rainfall reaching the surface led to the collapse of the surface layer, decrease of surface and subsurface soil temperatures, depressed sensible heating, and a slow reduction and even temporary termination of evapotranspiration. After the rains subsided, the boundary layer recovery process began with vigorous evapotranspiration rates drying the upper soil layers on a timescale of 1–2 days. During this period, 55%–65% of the net surface available heating was used for evapotranspiration, whereas only 30%–35% went directly into boundary layer heating. As the near-surface soil moisture dropped, surface sensible heating became more important in influencing boundary layer energetics. The boundary layer required approximately two days to recover to its initial temperature in the case of the surface trough. After passage of the cold front, both the soil and boundary layer cooled and dried due to cold temperature advection. Evapotranspiration rates remained relatively large for about two days after the frontal passage. The boundary layer had not completely recovered by the end of the intensive data collection period after the frontal passage, so recovery time was at least a week. The analysis shows that with the exception of three days during the surface trough event, and two or three days during the frontal passage event, the surface-driven secondary circulation persisted.

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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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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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Alberto Mugnai
,
Eric A. Smith
, and
Gregory J. Tripoli

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 TB 's. This involves two distinct types of vertical weighting functions for the TB '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 TB 's are examined in detail. The analysis emphasizes how microwave TB 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 TB '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.

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Michael R. Farrar
,
Eric A. Smith
, and
Xuwu Xiang

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.

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