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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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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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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 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

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

Abstract

Spatial and temporal changes of atmospheric water vapor and surface wind speeds are investigated for a period following an intrusion of cold continental air over the Gulf of Mexico, during the Gulf of Mexico Experiment (GUFMEX) in March 1988. Microwave and infrared satellite measurements from the Special Sensor Microwave/Imager (SSM/I) instrument aboard the Defense Meteorological Satellite Project (DMSP) F8 satellite and from the GOES VISSR Atmospheric Sounder (VAS) are used to augment the sparse coverage of rawinsonde sites and surface reports in the vicinity of the Gulf of Mexico. Total precipitable water is derived from both instruments and from rawinsonde measurements at coastal locations and auxiliary sites on ships and platforms over the Gulf. Accuracies of the precipitable water derived from SSM/I and GOES are comparable, though microwave data provide more uniform coverage, when they are available, than VAS since they are relatively free from contamination by most clouds. Also, the moisture fields derived from microwave data appear to be less noisy than those derived from the infrared. To illustrate possible use of satellite data in the forecast office, moisture fields from both SSM/I and VAS are blended together into imagery, which are compared to analyses from an operational model. Surface wind speeds are also obtained from the microwave data and are compared to the surface observations. Analyses from satellite data appear to add considerable information to the moisture and wind analysis over the Gulf of Mexico and should help in forecasting moisture changes, particularly moisture return near the surrounding coastal areas.

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

Abstract

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations.

The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four “steps” along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels.

The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north–south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.

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