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Joanne Simpson and Harry J. Cooper

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Harry J. Cooper and Eric A. Smith

The local meteorological events leading up to the launch of the space shuttle Atlantis on 2 August 1991 were captured in full-resolution GOES visible data being archived for the Convection and Precipitation/Electrification Experiment. The postponement of the launch on 1 August, and the successful lift-off on the following day provide a good example of the important role played by nowcasting and short-term forecasting at Cape Canaveral. In this brief article, we discuss the local weather conditions prior to, during, and after the launch and demonstrate the importance of short-term forecasting capabilities around the cape during launch operations.

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

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

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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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Eric A. Smith, Harry J. Cooper, Xuwu Xiang, Alberto Mugnai, and Gregory J. Tripoli

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A cloud-radiation model is used to investigate the relationship between emerging microwave brightness temperatures (T B's) and vertically distributed mixtures of liquid and frozen hydrometeors as a means to establish the framework for a hybrid statistical-physical rainfall retrieval algorithm. The focus in this study is on the microwave characteristics of an intense hailstorm in which cold-rain microphysics dominate the precipitation process. The T B calculations exhibit a high degree of intercorrelation across a wide frequency range (15–128 GHz) due to the pervasive influence of large ice particles on attenuation of upwelling radiation emerging from the rain layers. When the radiative emission source is near blackbody, fluctuations of the mixing ratios of ice particles are wholly responsible for the T B variations, as opposed to fluctuations in the cloud-or raindrop mixing ratios. Supercooled cloud drops, suspended in the graupel layers, can exert influence on the T B's but only at the higher frequencies. Emission by the large ice particles themselves becomes an important radiative source to the emerging T B's at the top of the atmosphere as the graupel-mixing ratios increase and effectively block the radiative sources from within the liquid layers.

Strong relationships are found between the emerging T B's and various rain parameters, but these correlations are misleading in that the T B's are largely controlled by fluctuations in ice-particle mixing ratios, which in turn are highly correlated to fluctuations in liquid-particle mixing ratios. This does not negate the use of empirically based brightness-temperature-rain-rate (T B-RR) algorithms as useful tools for estimating precipitation (i.e., graupel particles ultimately fall out as rain), but it does point to a basic problem that remote-sensing methodology must address. More specifically, the hydrometeor profiles used for T B-RR algorithms must not be specified in an ad hoc fashion. It is argued that a cloud model can overcome the ad hoc assumptions.

It is shown that the lowest SSM/I frequency (19 GHz) is actually a better estimator of columnar ice water content than the highest frequency (85 GHz). This is because both cloud-water emission and multiple scattering by ice particles are more prevalent at 85 GHz than at 19 GHz (which tends to be mostly influenced by single scattering). As a consequence, 85 GHz is much more sensitive to the configurational details of the vertical distribution of large ice particles and to the presence of supercooled cloud drops within the lower ice layers.

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

A simulation of the appearance of an intense hailstorm in the passive microwave spectrum is conducted in order to characterize the vertical sources of radiation that contribute to the top-of-atmosphere microwave brightness temperatures (TB) which can be measured by satellite-borne radiometers. The study focuses on four frequencies corresponding to those used on the USAF Special Sensor Microwave Imager (SSM/I), a recently launched payload flown on the U.S. Air Force DMSP satellites. Computation of the microwave brightness temperatures is based on a vertically, angularly, and spectrally detailed radiative transfer scheme that has been applied to the highly resolved thermodynamical and microphysical output from the three-dimensional Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The RAMS model was used to carry out a 4-h simulation of an intense hailstorm that occurred on 11 July 1986 in the vicinity of Eldridge, Alabama. Initial conditions for the cloud model run were developed from the 1986-COHMEX data archive.

Two types of vertically resolved radiative structure functions referred to as a “generalized weighting function” and an “emission source weighting function” are used to describe the process by which radiation originates and reaches the satellite radiometer. In addition, these weighting functions are subdivided into individual contributions by the various hydrometeor species generated by the cloud model. Along with the surface contribution and cosmic background radiation, these weighting functions provide a normalized description of where radiation originates and how it ultimately reaches the satellite. It is emphasized that this information provides an indepth understanding of how precipitation retrieval algorithms should be designed vis-à-vis the passive microwave problem.

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Eric A. Smith, Harry J. Cooper, William L. Crosson, and Donald D. Delorey

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Recently, there has been growing emphasis on improving surface flux inputs to mesoscale models and general circulation models. Since there is presently no operational network providing this information, we have conducted a feasibility experiment to determine whether the Bowen ratio (and indirectly surface heat and moisture fluxes) can be reasonably and accurately derived from thermodynamic measurements obtained from balloon-launched radiosondes.

The experiment took place during July 1988 at the Regional Airport in Tallahassee, Florida using an Atmospheric Instrumentation Research, Inc. (AIR) airsonde system and a surface radiation and energy budget station (SREBS) developed at Florida State University. The AIR system consists of a balloon-launched airsonde, which measures vertical profiles of atmospheric pressure, temperature, and relative humidity, and an automatic data acquisition system, which receives and records sensor output from the airsonde package. The SREBS is a compact, self-contained, battery-powered system used to measure approximately 100 surface parameters. For this experiment, the system was used to monitor in situ surface energy fluxes at the time of the radiosonde flights. The data recorded from the airsonde launches were used to create mixing-line profiles for each launch. Using the profiles, an objective technique for choosing the appropriate surface-layer mixing lines was developed, and from these the associated Bowen ratios within the surface layer were deduced.

Intercomparisons were made between Bowen ratios derived from the airsonde profiles and the Bowen ratios measured directly by the surface radiation and energy budget station. The results show that this technique produces estimates of the Bowen ratios within 9% of measured values, and a sensitivity analysis indicates that estimates of sensible and latent heat fluxes have root-mean-square differences of less than 6 W m−2.

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Eric A. Smith, Mickey M-K. Wai, Harry J. Cooper, Michael T. Rubes, and Ann Hsu

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Surface, aircraft, and satellite observations are analyzed for the 21-day 1989 intensive field campaign of the First ISLSCP Field Experiment (FIFE) to determine the effect of precipitation, vegetation, and soil moisture distributions on the thermal properties of the surface including the heat and moisture fluxes, and the corresponding response in the boundary-layer circulation. Mean and variance properties of the surface variables are first documented at various time and space scales. These calculations are designed to set the stage for Part II, a modeling study that will focus on how time–space dependent rainfall distribution influences the intensity of the feedback between a vegetated surface and the atmospheric boundary layer. Further analysis shows strongly demarked vegetation and soil moisture gradients extending across the FIFE experimental site that were developed and maintained by the antecedent and ongoing spatial distribution of rainfall over the region. These gradients are shown to have a pronounced influence on the thermodynamic properties of the surface. Furthermore, perturbation surface wind analysis suggests for both short-term steady-state conditions and long-term averaged conditions that the gradient pattern maintained a diurnally oscillating local direct circulation with perturbation vertical velocities of the same order as developing cumulus clouds. Dynamical and scaling considerations suggest that the embedded perturbation circulation is driven by surface heating/cooling gradients and terrain effects rather than the manifestation of an inertial oscillation. The implication is that at even relatively small scales <30 km), the differential evolution in vegetation density and soil moisture distribution over a relatively homogenous ecotone can give rise to preferential boundary-layer circulations capable of modifying local-scale horizontal and vertical motions.

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Jiujing Gu, Eric A. Smith, Harry J. Cooper, Andrew Grose, Guosheng Liu, James D. Merritt, Maarten J. Waterloo, Alessandro C. de Araújo, Antonio D. Nobre, Antonio O. Manzi, Jose Marengo, Paulo J. de Oliveira, Celso von Randow, John Norman, and Pedro Silva Dias

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In this first part of a two-part investigation, large-scale Geostationary Operational Environmental Satellite (GOES) analyses over the Amazônia region have been carried out for March and October of 1999 to provide detailed information on surface radiation budget (SRB) and precipitation variability. SRB fluxes and rainfall are the two foremost cloud-modulated control variables that affect land surface processes, and they require specification at space–time resolutions concomitant with the changing cloud field to represent adequately the complex coupling of energy, water, and carbon budgets. These processes ultimately determine the relative variations in carbon sequestration and carbon dioxide release within a forest ecosystem. SRB and precipitation retrieval algorithms using GOES imager measurements are used to retrieve surface downward radiation and surface rain rates at high space–time resolutions for large-scale carbon budget modeling applications in conjunction with the Large-Scale Biosphere–Atmosphere Experiment in Amazônia. To validate the retrieval algorithms, instantaneous estimates of SRB fluxes and rain rates over 8 km × 8 km areas were compared with 30-min-averaged surface measurements obtained from tower sites located near Ji-Paraná and Manaus in the states of Rondônia and Amazonas, respectively. Because of large aerosol concentrations originating from biomass burning during the dry season (i.e., September and October for purposes of this analysis), an aerosol index from the Total Ozone Mapping Spectrometer is used in the solar radiation retrieval algorithm. The validation comparisons indicate that bias errors for incoming total solar, photosynthetically active radiation (PAR), and infrared flux retrievals are under 4%, 6%, and 3% of the mean values, respectively. Precision errors at the analyzed space– time scales are on the order of 20%, 20%, and 5%. The visible and infrared satellite measurements used for precipitation retrieval do not directly detect rainfall processes, and yet they are responsive to the rapidly changing cloud fields, which are directly associated with precipitation life cycles over the Amazon basin. In conducting the validation analysis at high space–time scales, the comparisons indicate systematic bias uncertainties on the order of 25%. These uncertainties are comparable to published values from an independent assessment of bias uncertainties inherent to the current highest-quality satellite retrievals, that is, from the Tropical Rainfall Measuring Mission. Because precipitation is a weak direct control on photosynthesis for the Amazon ecosystem, that is, photosynthesis is dominated by the strong diurnal controls of incoming PAR and ambient air-canopy temperatures, such uncertainties are tolerable. By the same token, precipitation is a strong control on soil thermal properties and carbon respiration through soil moisture, but the latter is a time-integrating variable and thus inhibits introduction of modeling errors caused by random errors in the precipitation forcing. The investigation concludes with analysis of the monthly, daily, and diurnal variations intrinsic to SRB and rainfall processes over the Amazon basin, including explanations of how these variations arise during wet- and dry-season periods.

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