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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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Sandra E. Yuter
,
Robert A. Houze Jr.
,
Eric A. Smith
,
Thomas T. Wilheit
, and
Edward Zipser

Abstract

The Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) was designed to obtain an empirical physical characterization of precipitating convective clouds over the tropical ocean. Coordinated datasets were collected by three aircraft, one ship, five upper-air sounding sites, and a variety of continuously recording remote and in situ surface-based sensors, including scanning Doppler radars, profilers, disdrometers, and rain gauges. This paper describes the physical characterization of the Kwajalein cloud population that has emerged from analyses of datasets that were obtained during KWAJEX and combined with long-term TRMM ground validation site observations encompassing three rainy seasons. The spatial and temporal dimensions of the precipitation entities exhibit a lognormal probability distribution, as has been observed over other parts of the tropical ocean. The diurnal cycle of the convection is also generally similar to that seen over other tropical oceans. The largest precipitating cloud elements—those with rain areas exceeding 14 000 km2—have the most pronounced diurnal cycle, with a maximum frequency of occurrence before dawn; the smallest rain areas are most frequent in the afternoon. The large systems exhibited stratiform rain areas juxtaposed with convective regions. Frequency distributions of dual-Doppler radar data showed narrow versus broad spectra of divergence in the stratiform and convective regions, respectively, as expected because strong up- and downdrafts are absent in the stratiform regions. The dual-Doppler profiles consistently showed low-level convergence and upper-level divergence in convective regions and midlevel convergence sandwiched between lower- and upper-level divergence in stratiform regions. However, the magnitudes of divergence are sensitive to assumptions made in classifying the radar echoes as convective or stratiform. This sensitivity implies that heating profiles derived from satellite radar data will be sensitive to the details of the scheme used to separate convective and stratiform rain areas. Comparison of airborne passive microwave data with ground-based radar data indicates that the pattern of scattering of 85-GHz radiance by ice particles in the upper portions of KWAJEX precipitating clouds is poorly correlated with the precipitation pattern at lower levels while the emission channels (10 and 19 GHz) have brightness temperature patterns that closely correspond to the lower-level precipitation structure. In situ ice particle imagery obtained by aircraft at upper levels (∼11 km) shows that the concentrations of ice particles of all densities are greater in the upper portions of active convective rain regions and lower in the upper portions of stratiform regions, probably because the active updrafts convey the particles to upper levels, whereas in the stratiform regions sedimentation removes the larger ice particles over time. Low-level aircraft flying in the rain layer show similar total drop concentrations in and out of convective cells, but they also show a sudden jump in the concentration of larger raindrops at the boundaries of the cells, indicating a discontinuity in growth processes such as coalescence at the cell boundary.

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

Abstract

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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Ziad S. Haddad
,
Jonathan P. Meagher
,
Stephen L. Durden
,
Eric A. Smith
, and
Eastwood Im

Abstract

The threat of flooding from landfalling tropical cyclones is a function of the local variation in rain rate and rain accumulation. To date, these have been inferred from single-frequency radar reflectivity measurements. However, the Tropical Rainfall Measuring Mission experience has confirmed that one of the main difficulties in retrieving rain profiles using a single-frequency radar is the unknown raindrop size distribution (DSD). A dual-frequency radar such as the one planned for the upcoming Global Precipitation Measurement (GPM) core satellite is expected to help sort out at least part of this DSD-induced ambiguity. However, the signature of precipitation at 14 GHz does not differ greatly from its signature at 35 GHz (the GPM radar frequencies). To determine the extent of the vertical variability of the DSD in tropical systems and to quantify the effectiveness of a dual-frequency radar in resolving this ambiguity, several different models of DSD shape are considered and used to estimate the rain-rate and mean-diameter profiles from the measurements made by Jet Propulsion Laboratory’s (JPL’s) airborne second generation precipitation radar (PR-2) over Hurricanes Gabrielle and Humberto during the Fourth Convection and Moisture Experiment (CAMEX-4) in September 2001. It turns out that the vertical structures of the rain profiles retrieved from the same measurements under different DSD assumptions are similar, but the profiles themselves are quantitatively significantly different.

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Ralph R. Ferraro
,
Eric A. Smith
,
Wesley Berg
, and
George J. Huffman

Abstract

The success of any passive microwave precipitation retrieval algorithm relies on the proper identification of rain areas and the elimination of surface areas that produce a signature similar to that of precipitation. A discussion on the impact of and on methods that identify areas of rain, snow cover, deserts, and semiarid conditions over land, and rain, sea ice, strong surface winds, and clear, calm conditions over ocean, are presented. Additional artifacts caused by coastlines and Special Sensor Microwave/Imager data errors are also discussed, and methods to alleviate their impact are presented. The strengths and weaknesses of the “screening” techniques are examined through application on various case studies used in the WetNet PIP-2. Finally, a methodology to develop a set of screens for use as a common rainfall indicator for the intercomparison of the wide variety of algorithms submitted to PIP-2 is described.

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Kwo-Sen Kuo
,
Eric A. Smith
,
Ziad Haddad
,
Eastwood Im
,
Toshio Iguchi
, and
Alberto Mugnai

Abstract

In developing the upcoming Global Precipitation Measurement (GPM) mission, a dual-frequency Ku–Ka-band radar system will be used to measure rainfall in such a fashion that the reflectivity ratio intrinsic to the measurement will be sensitive to underlying variations in the drop size distribution (DSD) of rain. This will enable improved techniques for retrieving rain rates, which are dependent upon several key properties of the DSD. This study examines this problem by considering a three-parameter set defined by liquid water content (W), DSD effective radius (r e ), and DSD effective variance (υ e ). Using radiative transfer simulations, this parameter set is shown to be related to a radar reflectivity factor and specific attenuation in such a fashion that details of the DSDs are immaterial under constant W, and thus effectively represent important variations in DSD that affect rain rate but with a minimal number of parameters. The analysis also examines the effectiveness of including some measure of mean Doppler fall velocity of raindrops ( υ ), given that the fundamental properties of a given precipitation situation are uniquely defined by a combination of a drop mass spectrum and drop vertical velocity spectrum. The results of this study have bearing on how future dual-frequency precipitation retrieval algorithms could be formulated to optimize the sensitivity to underlying DSD variability, a problem that has greatly upheld past progress in radar rain retrieval.

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Simone Tanelli
,
Eastwood Im
,
Stephen L. Durden
,
Luca Facheris
,
Dino Giuli
, and
Eric A. Smith

Abstract

For vertical Doppler velocity measurements of a homogeneous rain field, the standard spectral moment estimation techniques commonly used by ground-based and airborne Doppler rain radars can be readily extended for spaceborne application, provided that the radar antenna size is chosen to adequately reduce the satellite motion-induced Doppler spectral broadening. When encountering an inhomogeneous rain field, on the other hand, the nonuniform beam filling (NUBF) causes additional biases on Doppler velocity estimates, which (i) often reach several meters per second, (ii) cannot be corrected with standard spectral moment techniques, and (iii) are strongly dependent on the along-track reflectivity profile within the radar footprint. One approach to overcome this difficulty is to further increase the antenna size such that the radar's horizontal resolution would be sufficiently small to resolve the inhomogeneity in rain cells. Unfortunately, this approach is very challenging in terms of antenna technology and spacecraft resources and accommodation.

In this paper, an alternate data processing approach is presented to overcome the NUBF difficulty. This combined frequency–time (CFT) processing technique is used to process a series of Doppler spectra collected over measurement volumes that are partially overlapping in the along-track direction. Its expected performance is evaluated through a spaceborne simulation study using three case studies from high-resolution 3D rainfall datasets acquired by the NASA JPL airborne rain mapping radar. In each of these cases, each representing a different rain regime with a different degree of spatial variability, the CFT technique can effectively remove the NUBF-induced bias such that the mean Doppler velocity estimates achieve the desired accuracy of 1 m s−1 for a signal-to-noise ratio greater than 10 dB.

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William E. Lewis
,
Eastwood Im
,
Simone Tanelli
,
Ziad Haddad
,
Gregory J. Tripoli
, and
Eric A. Smith

Abstract

The potential usefulness of spaceborne Doppler radar as a tropical cyclone observing tool is assessed by conducting a high-resolution simulation of an intense hurricane and generating synthetic observations of reflectivity and radial velocity. The ground-based velocity track display (GBVTD) technique is used to process the radial velocity observations and generate retrievals of meteorologically relevant metrics such as the maximum wind (MW), radius of maximum wind (RMW), and radius of 64-kt wind (R64). Results indicate that the performance of the retrieved metrics compares favorably with the current state-of-the-art satellite methods for intensity estimation and somewhat better than current methods for structure (i.e., wind radii).

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Byung-Ju Sohn
,
Eric A. Smith
,
Franklin R. Robertson
, and
Seong-Chan Park

Abstract

A methodology is developed for deriving atmospheric water vapor transports over the World Oceans from satellite-retrieved precipitation (P) and evaporation (E) datasets. The motivation for developing the method is to understand climatically varying properties of transports, that is, year-to-year changes of the seasonally averaged divergent transport distribution fields, over regions where conventional data, in particular, winds, are sparse. Ultimately, the method is intended to take advantage of the relatively complete and consistent coverage, as well as continuity in sampling, associated with EP datasets obtained from satellite measurements. Separate P and E retrievals from Special Sensor Microwave Imager (SSM/I) measurements, along with P retrievals from Tropical Rainfall Measuring Mission (TRMM) measurements, are used to obtain the transport solutions.

In this opening study, a 7-yr climatological normal is derived for the January–February–March (JFM) period for years 1988–94, providing the basis for comparing vapor transport anomalies from the 1997/98 El Niño and 1999/2000 La Niña events. These are derived from JFM-averaged transport solutions for 1998 and 1999, respectively. These two periods correspond to times when the Multivariate ENSO Index (MEI) provided by the NOAA Climatic Data Center (CDC) was first at a relative maximum and then at a relative minimum in conjunction with back-to-back west Pacific warm and cold events. Because the El Niño–La Niña events produce such highly contrasting behavior in the transports, shifting from a largely meridionally oriented solution to a largely zonally oriented solution, focusing on this pairing, helps to explain why the methodology is reliable and effective in capturing important details embedded in full-coverage EP fields.

The analysis includes a sensitivity study of the transport solution technique based on 20 combinations of four precipitation datasets (two satellite based and two model based) and five evaporation datasets (two satellite based, one in situ observation based, and two model based), which helps to explain the reliability of the method. The analysis also includes a comparison to water vapor transports derived with the same method, but applied to EP datasets obtained from global analysis products prepared by the National Centers of Environmental Prediction (NCEP), again to help explain the reliability of the method. The study concludes by first showing how the anomaly JFM 1998 El Niño solution behaves in close correspondence to associated SST anomalies and is generally more realistic in comparison to the corresponding NCEP solution. Finally, its reliability is discussed in terms of the implications of the vapor transport features for the El Niño–La Niña transition, vis-à-vis north–south and east–west circulations and their accompanying impact on the atmospheric hydrological cycle.

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