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Robert M. Rabin and Timothy J. Schmit

Abstract

In this note, the relationship between the observed daytime rise in surface radiative temperature, derived from the Geostationary Operational Environmental Satellites (GOES) sounder clear-sky data, and modeled soil moisture is explored over the continental United States. The motivation is to provide an infrared (IR) satellite–based index for soil moisture, which has a higher resolution than possible with the microwave satellite data. The daytime temperature rise is negatively correlated with soil moisture in most areas. Anomalies in soil moisture and daytime temperature rise are also negatively correlated on monthly time scales. However, a number of exceptions to this correlation exist, particularly in the western states. In addition to soil moisture, the capacity of vegetation to generate evapotranspiration influences the amount of daytime temperature rise as sensed by the satellite. In general, regions of fair to poor vegetation health correspond to the relatively high temperature rise from the satellite. Regions of favorable vegetation match locations of lower-than-average temperature rise.

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Steven D. Smith and Robert M. Rabin

Abstract

Applications of Doppler weather radar data to the analysis of wind fields are reviewed. Radial velocity measurements from a single radar are used to estimate horizontal wind vectors within small azimuthal sectors using two different models. One assumes a uniform wind, the other a linear wind within the sector. Errors in wind estimates owing to gradients of wind are derived using harmonic analysis. The radar data analysis techniques are tested on complex wind patterns which were reconstructed from dual-Doppler radar measurements.

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Steven D. Smith and Robert M. Rabin

Abstract

An analysis technique to derive wind field parameters from single-Doppler velocity measurements, known as Modified Velocity-Volume Processing (MVVP) is examined from both theoretical and operational perspectives. For this technique, radar data within limited spatial volumes are fit to a model which usually assumes linearity of the Cartesian wind components. The accuracies and limitations of this technique are illustrated with examples from a case study of a severe storm outbreak in central Oklahoma on 17 May 1981. Implications for use of the MVVP in convective storm forecasting are considered.

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Edward A. Brandes and Robert M. Rabin

Abstract

The utility of Doppler radar to study boundary-layer kinematics of a weak nonprecipitating cold front in Oklahoma on 16 October 1987 was examined with measurements from two radars. Diagnosis was impeded by operation at low antenna elevation angles, short radar ranges, and low signal-to-noise ratios. Further, kinematic parameters computed by single-radar velocity-azimuth-display (VAD) technique for meteorological wavelengths <125 km were significantly smoothed (more than 50% attenuated). Meteorological scales ≥5 km were well resolved (less than 50% attenuated) in wind fields synthesized from dual-Doppler radar observations, but derived parameters were particularly sensitive to the vertical extrapolation of radial velocity measurements in the presence of strong vertical wind shear.

Nonetheless, radar-derived wind flows depicted a sequence of events consistent with other instrumentation. In the vicinity of the front, mean-flow divergence, vertical velocity, and deformation, computed from single-radar measurements for an analysis domain of 30-km radius, were −4×10−5 s−1, 3 cm s−1, and 16×10−5 s−1, respectively. Agreement between the radars attested to the accuracy of the measurements. Local peak absolute values of divergence, deformation, and vertical vorticity, determined from dual-Doppler analysis, were 200–300 (×10−5) s−1. Extrema were concentrated along the frontal zone where signals were strong, and had dimensions of ∼10 km.

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Robert M. Rabin and Richard J. Doviak

Abstract

Observations are presented of time-varying radar reflectivity during a partial solar eclipse in Oklahoma. The measurements from a radar of 10-cm wavelength, were obtained in the clear-air boundary layer. The reflectivity changes closely follow the variation in solar radiation associated with the eclipse. Possible mechanisms for the change in reflectivity are reviewed, in particular the effect of surface fluxes of heat and mositure. A formula is derived that relates radar reflectivity to surface fluxes of sensible and latent heat. Other evidence of a relationship between these fluxes and sensible and latent heat. Other evidence of a relationship between these fluxes and radar reflectivity is presented, namely the effect of local cloud cover on reduced insolation and variations in surface wetness.

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William H. Raymond, Robert M. Rabin, and Gary S. Wade

The Mississippi River floodplain in the states of Arkansas, Tennessee, Mississippi, and Louisiana presents a readily discernible feature in weather satellite images. This floodplain appears in the spring and early summer as a daytime warm anomaly at infrared (IR) wavelengths and as a bright reflective area at visible wavelengths. Remnants of this feature can occasionally be identified at nighttime in the IR satellite images. During June the normalized difference vegetation index identifies major contrasts between this intense agricultural region and the surrounding mixed-forest region. This distinction and the homogeneity of the floodplain, with its alluvial soil, contrast with the encircling region, creating an agricultural region containing heat island features. Thirty years of climatological surface station data for the month of June reveal that the surface air temperatures in the floodplain experience less diurnal variation than those in the surrounding regions. This is primarily because nighttime minimums are warmer in the Mississippi River floodplain.

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Alexandre O. Fierro, Stephanie N. Stevenson, and Robert M. Rabin

Abstract

Total lightning data obtained from the Geostationary Lightning Mapper (GLM) were analyzed to present a first glimpse of relationships with intensity variations and convective evolution in Hurricane Maria (2017). The GLM has made it possible, for the first time, to analyze total lightning within a major hurricane for a long period, far from ground-based detection networks. It is hoped that these observations could enlighten some of the complex relationships existing between intensity fluctuations and the distribution of electrified convection in these systems.

Prior to rapidly intensifying from a category 1 to category 5 storm, Maria produced few inner-core flashes. Increases in total lightning in the inner core (r ≤ 100 km) occurred during both the beginning and end of an intensification cycle, while lightning increases in the outer region (100 < r ≤ 500 km) occurred earlier in the intensification cycle and during weakening. Throughout the analysis period, the largest lightning rates in the outer region were consistently located in the southeastern quadrant, a pattern consistent with modeling studies of electrification within hurricanes. Lightning in the inner core was generally tightly clustered within a 50-km radius from the center and most often found in the southeastern portion of the eyewall, which is atypical. Bootstrapped correlation statistics revealed that the most robust and systematic relationship with storm intensity was obtained for inner-core lightning and maximum surface wind speed. A brief comparison between flash rates from GLM and a very low-frequency ground-based network revealed that not all lightning peaks are seen equally, with hourly flash-rate ratios between both systems sometimes exceeding two orders of magnitude.

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Carl E. Hane, Robert M. Rabin, Todd M. Crawford, Howard B. Bluestein, and Michael E. Baldwin

Abstract

A dryline that occurred on 16 May 1991 within a synoptically active environment is examined in detail using research aircraft, radar, surface, satellite, and upper air observations. The work focuses on multiple boundaries in the dryline environment and initiation of tornadic storms in two along-line areas.

Aircraft measurements in the boundary layer reveal that both the east–west extent of moisture gradients and the number of regions containing large moisture gradients vary in the along-dryline direction. Aircraft penetrations of thinlines observed in clear air return from radar reveal that all thinlines are associated with convergence and a moisture gradient, and that more distinct thinlines are associated with stronger convergence. However, significant moisture gradients are not always associated with either thinlines or convergent signatures.

Convective clouds on this day formed at the dryline rather than significantly east of the dryline. The three thunderstorm cells that occurred in east-central Oklahoma developed along a 20-km section of the dryline south of a dryline bulge and within a 30-min period. The storms appear to have developed in this location owing to enhanced convergence resulting from backed winds in the moist air in response to lowered pressure in the warm air to the northwest. Aircraft measurements in the boundary layer and satellite-sensed surface temperature both indicate localized warming in this area to the northwest.

Farther north there was a 70–100-km segment along the dryline where few convective clouds formed during the afternoon. This coincided with a swath of cooler boundary layer air that resulted from reduced surface heating over an area that received significant thunderstorm rainfall during the previous night.

A severe thunderstorm complex that developed along the Kansas–Oklahoma border was initiated at the intersection of the dryline and a cloud line that extended into the dry air. An aircraft penetration of the cloud line about 12 km from its intersection with the dryline shows convergence and deepened low-level moisture at the cloud line. The cloud field that evolved into the cloud line over a period of several hours developed over the area that had received the heaviest rainfall during the previous night.

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Robert M. Rabin, Lynn A. McMurdie, Christopher M. Hayden, and Gary S. Wade

Abstract

The atmospheric water budget is examined for a 12-day period following an intense cold-air outbreak over the Gulf of Mexico. Budget terms are compared using analyses from the U.S. National Meteorological Center's operational Nested Grid Model (NGM) and using precipitable water and surface wind speed estimated from the Special Sensor Microwave/Imager (SSM/I) instrument aboard the defense meteorological satellite F8. The atmospheric-storage term, determined from the areal-averaged total precipitable water, does not differ significantly between that obtained from the NGM and that obtained from SSM/I data. The storage increases by a factor of more than 3 during the initial five days following the passage of the surface high over the Gulf. Horizontal flux divergence of water vapor computed from the full vertical structure in the NGM output is well approximated by the substitution of the surface-700-mb mean wind and the total precipitable water for the vertical profiles along the boundaries of the atmospheric volume. Evaporation from the sea surface is determined using GOES surface temperatures and NGM surface air conditions. The impact of satellite-derived surface winds on the areal-average evaporation is determined by replacing NGM wind speeds with those estimated from the SSM/I data. The relative importance of precipitation on the water budget is assessed from model estimates. During the onset of airmass modification, evaporation appears to be the dominant mechanism in producing the observed atmospheric moistening. As evaporation diminishes after one to two days, evaporation and flux convergence are of similar magnitude. Together, these terms underestimate the amount of moistening observed during the first five days.

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Robert M. Rabin, Lynn A. McMurdie, Christopher M. Hayden, and Gary S. Wade

Abstract

Spatial and temporal changes in the vertical distribution of atmospheric water vapor are investigated during a period following the intrusion of cold continental air over the Gulf of Mexico, during the Gulf of Mexico Experiment (GUFMEX) in February-March 1988. Infrared satellite measurements from the GOES (Geostationary Operational Environmental Satellite) VISSR (Visible-Infrared Spin Scan Radiometer) Atmospheric Sounder (VAS) are used to augment the sparse coverage of rawinsonde sites in the vicinity of the Gulf of Mexico. Precipitable water from two vertical layers, surface-850 and 850–250 mb, are estimated from the VAS and compared to those from rawinsonde observations. The accuracy of precipitable-water estimates in each vertical layer is less than that for the total precipitable water. However, improvements in the estimate of precipitable water for each layer are observed with respect to the profiles used in initializing the retrieval process. A consistent horizontal and temporal pattern of the vertical partition of water vapor between the lower and middle to upper troposphere is obtained from the analysis in both layers. A band of moist air that develops with return to southerly flow is common to both layers; however, the width of the band is more extensive in the lower layer. Drying to the rear of the band predominates in the upper layer while the lower layer remains quite moist.

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