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Abstract
A strong thunderstorm produced a flash flood on the evening of 12 July 1996 in Buffalo Creek, Colorado, that caused two deaths and significant property damage. Most of the rain fell in a 1-h time period from 2000 to 2100 MDT. The performance of the WSR-88D rainfall algorithm, Precipitation Processing System, was examined in detail to determine how well it performed. In particular the sensitivity to the algorithm’s rain-rate threshold (hail cap) parameter and the performance of the gauge–radar adjustment subalgorithm on the resulting radar rainfall estimates were examined by comparison with available rain gauge data.
It was determined that the WSR-88D rainfall algorithm overestimated the rainfall in general over the radar scanning domain for this event by about 60% relative to the rain gauges although the radar-derived rainfall for the flood-producing storm cell nearly matched the single gauge that sampled it. The derived rainfall over the radar scanning domain was not very sensitive to the setting of the rain-rate threshold parameter. Lowering it reduced the overestimation in general but did not bring the estimates satisfactorily close to unbiasedness. Other error sources were suggested, including use of an inappropriate Z–R relationship and/or radar reflectivity miscalibration. Relative importance of these sources could not be determined.
The portion of the rainfall algorithm that adjusts the radar estimates using rain gauge data was tested to determine if it could have satisfactorily reduced the observed overestimation. It was found to have performed suboptimally due primarily to the methodology in the algorithm that forms the gauge–radar pairs. A simpler technique was proposed and tested, and the algorithm’s performance was greatly enhanced as a result. Therefore the performance of the gauge–radar adjustment algorithm depends on the gauge–radar rainfall data that are passed to it, and that data are dependent on the method by which the pairs are formed.
Abstract
A strong thunderstorm produced a flash flood on the evening of 12 July 1996 in Buffalo Creek, Colorado, that caused two deaths and significant property damage. Most of the rain fell in a 1-h time period from 2000 to 2100 MDT. The performance of the WSR-88D rainfall algorithm, Precipitation Processing System, was examined in detail to determine how well it performed. In particular the sensitivity to the algorithm’s rain-rate threshold (hail cap) parameter and the performance of the gauge–radar adjustment subalgorithm on the resulting radar rainfall estimates were examined by comparison with available rain gauge data.
It was determined that the WSR-88D rainfall algorithm overestimated the rainfall in general over the radar scanning domain for this event by about 60% relative to the rain gauges although the radar-derived rainfall for the flood-producing storm cell nearly matched the single gauge that sampled it. The derived rainfall over the radar scanning domain was not very sensitive to the setting of the rain-rate threshold parameter. Lowering it reduced the overestimation in general but did not bring the estimates satisfactorily close to unbiasedness. Other error sources were suggested, including use of an inappropriate Z–R relationship and/or radar reflectivity miscalibration. Relative importance of these sources could not be determined.
The portion of the rainfall algorithm that adjusts the radar estimates using rain gauge data was tested to determine if it could have satisfactorily reduced the observed overestimation. It was found to have performed suboptimally due primarily to the methodology in the algorithm that forms the gauge–radar pairs. A simpler technique was proposed and tested, and the algorithm’s performance was greatly enhanced as a result. Therefore the performance of the gauge–radar adjustment algorithm depends on the gauge–radar rainfall data that are passed to it, and that data are dependent on the method by which the pairs are formed.
Abstract
Observations of an isolated group of Oklahoma thunderstorms from NASA's high altitude ER-2 aircraft are presented. These observations include passive radiometric measurements at frequencies in the microwave (92, 183 GHz), infrared (10.7 μm) and visible portion of the spectrum from a perspective above the storm top. Direct measurements of cloud top height were also collected using a pulsed lidar instrument. These remote observations are discussed and compared with coincident radar data from the National Severe Storms Laboratory's two Doppler radars and in situ cloud top particle data from the University of North Dakota's Citation aircraft.
Reflectivity cores are nearly colocated with cold anomalies in the microwave brightness temperature field. Coldest infrared brightness temperatures however, are displaced downshear of the convective region in association with the cirrus anvil. Radar and in situ microphysical comparisons support previous theoretical and numerical modeling results which suggest that microwave frequencies are sensitive to the deeper layer of large ice particles in the storm's convective region. The trailing anvil which is comprised of smaller ice particles is transparent at 92 GHz and nearly transparent at 183 GHz. This observation has relevance to spaceborne passive microwave measurements of rainfall.
Evolution of the thunderstorm complex is also discussed. The trend of the radar volumetric rain rate correlates well with the trends of minimum 92 GHz brightness temperature and area of the cold brightness temperature region at 92 GHz. The correlation at 183 GHz as well as at the infrared wavelength is not nearly as clear.
Abstract
Observations of an isolated group of Oklahoma thunderstorms from NASA's high altitude ER-2 aircraft are presented. These observations include passive radiometric measurements at frequencies in the microwave (92, 183 GHz), infrared (10.7 μm) and visible portion of the spectrum from a perspective above the storm top. Direct measurements of cloud top height were also collected using a pulsed lidar instrument. These remote observations are discussed and compared with coincident radar data from the National Severe Storms Laboratory's two Doppler radars and in situ cloud top particle data from the University of North Dakota's Citation aircraft.
Reflectivity cores are nearly colocated with cold anomalies in the microwave brightness temperature field. Coldest infrared brightness temperatures however, are displaced downshear of the convective region in association with the cirrus anvil. Radar and in situ microphysical comparisons support previous theoretical and numerical modeling results which suggest that microwave frequencies are sensitive to the deeper layer of large ice particles in the storm's convective region. The trailing anvil which is comprised of smaller ice particles is transparent at 92 GHz and nearly transparent at 183 GHz. This observation has relevance to spaceborne passive microwave measurements of rainfall.
Evolution of the thunderstorm complex is also discussed. The trend of the radar volumetric rain rate correlates well with the trends of minimum 92 GHz brightness temperature and area of the cold brightness temperature region at 92 GHz. The correlation at 183 GHz as well as at the infrared wavelength is not nearly as clear.
Abstract
The use of passive remote microwave radiance measurements above cloud tops for rainrate estimation is complicated by the complex nature of cloud microphysics. The knowledge of the microphysical structure of clouds, specifically the hydrometeor types, shapes, sizes, and their vertical distribution, is important because radiative emission and scattering effects are dependent upon the hydrometeor distribution. This paper has two purposes: first, to document the structure and evolution of two strong thunderstorms in Alabama using radar multiparameter data; and second, to relate the inferred microphysics to the resulting upwelling microwave radiance observed concurrently by high altitude aircraft. These measurements were collected during the COHMEX field program in the summer of 1986. The radar analysis includes a description of the parameters reflectivity Z, differential reflectivity ZDR , linear depolarization ratio LDR, and hail signal HS for two thunderstorm cases on 11 July 1986. The simultaneous aircraft data includes passive microwave brightness temperature (TB ) measurements at four frequencies ranging from 18 to 183 GHz as well as visible and infrared data.
The remote radar observations reveal the existence of large ice particles within the storms which is likely to have caused the observed low microwave brightness temperatures. By relating the evolution of the radar measureables to the microwave TB 's it has been found that knowledge of the storm microphysics and its evolution is important to adequately understand the microwave TB 's.
Abstract
The use of passive remote microwave radiance measurements above cloud tops for rainrate estimation is complicated by the complex nature of cloud microphysics. The knowledge of the microphysical structure of clouds, specifically the hydrometeor types, shapes, sizes, and their vertical distribution, is important because radiative emission and scattering effects are dependent upon the hydrometeor distribution. This paper has two purposes: first, to document the structure and evolution of two strong thunderstorms in Alabama using radar multiparameter data; and second, to relate the inferred microphysics to the resulting upwelling microwave radiance observed concurrently by high altitude aircraft. These measurements were collected during the COHMEX field program in the summer of 1986. The radar analysis includes a description of the parameters reflectivity Z, differential reflectivity ZDR , linear depolarization ratio LDR, and hail signal HS for two thunderstorm cases on 11 July 1986. The simultaneous aircraft data includes passive microwave brightness temperature (TB ) measurements at four frequencies ranging from 18 to 183 GHz as well as visible and infrared data.
The remote radar observations reveal the existence of large ice particles within the storms which is likely to have caused the observed low microwave brightness temperatures. By relating the evolution of the radar measureables to the microwave TB 's it has been found that knowledge of the storm microphysics and its evolution is important to adequately understand the microwave TB 's.
Abstract
Passive microwave observations using the Special Sensor Microwave/Imager are presented for severe tornadic storms in the lower midwestern United States on 16 November 1987. These measurements are compared with Geostationary Operational Environmental Satellite infrared (IR) measurements for the same case. The IR observations had a classic “V” cold feature commonly associated with severe Midwest thunderstorms. The minimum microwave brightness temperatures at 86 GHz, which primarily respond to ice scattering by larger ice particles, were located in the convective region and the warm interior region of the anvil top, between the arms of the IR V feature. The interior warm region was the only portion of the entire anvil region that had high 86-GHz polarization difference temperatures. Microphysical implications of these multispectral observations are discussed. The observations suggest that there are large variations of ice microphysical characteristics spatially and vertically in the anvil region. These observations are discussed in the context of previous dynamical and microphysical hypotheses on the IR V feature.
Abstract
Passive microwave observations using the Special Sensor Microwave/Imager are presented for severe tornadic storms in the lower midwestern United States on 16 November 1987. These measurements are compared with Geostationary Operational Environmental Satellite infrared (IR) measurements for the same case. The IR observations had a classic “V” cold feature commonly associated with severe Midwest thunderstorms. The minimum microwave brightness temperatures at 86 GHz, which primarily respond to ice scattering by larger ice particles, were located in the convective region and the warm interior region of the anvil top, between the arms of the IR V feature. The interior warm region was the only portion of the entire anvil region that had high 86-GHz polarization difference temperatures. Microphysical implications of these multispectral observations are discussed. The observations suggest that there are large variations of ice microphysical characteristics spatially and vertically in the anvil region. These observations are discussed in the context of previous dynamical and microphysical hypotheses on the IR V feature.
Abstract
A radar polarimetric method for areal rainfall estimation is examined. In contrast to the polarimetric algorithm based on specific differential phase K DP, the proposed method does not require rain-rate estimation from K DP inside the area of interest, but it utilizes only values of total differential phase ΦDP on the areal contour. Even if the radar reflectivity and differential phase data inside the area are corrupted by ground clutter, anomalous propagation, biological scatterers, or hail contamination, reliable areal rainfall estimate is still possible, provided that correct ΦDP estimates are available at a relatively small number of range locations in or at the periphery of the contour of this area.
This concept of areal rainfall estimation has been tested on the Little Washita River watershed area in Oklahoma that contains 42 densely located rain gauges. The areal rainfall estimates obtained from the polarimetric data collected with the 10-cm Cimarron radar are in good agreement with the gauge data, with the standard error of about 18%. This accuracy is better than that obtained with the algorithm utilizing areal averaging of pointwise estimates of K DP inside the watershed area.
Abstract
A radar polarimetric method for areal rainfall estimation is examined. In contrast to the polarimetric algorithm based on specific differential phase K DP, the proposed method does not require rain-rate estimation from K DP inside the area of interest, but it utilizes only values of total differential phase ΦDP on the areal contour. Even if the radar reflectivity and differential phase data inside the area are corrupted by ground clutter, anomalous propagation, biological scatterers, or hail contamination, reliable areal rainfall estimate is still possible, provided that correct ΦDP estimates are available at a relatively small number of range locations in or at the periphery of the contour of this area.
This concept of areal rainfall estimation has been tested on the Little Washita River watershed area in Oklahoma that contains 42 densely located rain gauges. The areal rainfall estimates obtained from the polarimetric data collected with the 10-cm Cimarron radar are in good agreement with the gauge data, with the standard error of about 18%. This accuracy is better than that obtained with the algorithm utilizing areal averaging of pointwise estimates of K DP inside the watershed area.
Abstract
This paper describes the characteristics and evolving nature of a vigorous thunderstorm density current very early in the morning of 9 May 1981 in Oklahoma. Because the ambient lower atmosphere was stratified, interesting interactions between the outflow current and the ambient environment resulted. The leading portion of the current was modulated by at least three gravity wavelike perturbations of horizontal spacing 12 km which initially coexisted with it. However, as the current evolved, it initiated an undular borelike disturbance which propagated ahead into the stable boundary layer, carrying cold outflow air in large amplitude rolls. Eventually the wave family left the decelerating outflow air in its wake. This borelike disturbance resembles the Australian “morning glory” phenomenon and appears to represent an early stage in the development of a solitary wave family.
The observations resemble other reported morning glories and solitary waves as well their laboratory and numerically simulated counterparts. Comparisons are discussed. This case study is unique not only because it combines Doppler radar, tall tower, and surface mesonet observations, but especially because the period of observation captures the disturbance in its formative stage when it is still very near the density current.
Abstract
This paper describes the characteristics and evolving nature of a vigorous thunderstorm density current very early in the morning of 9 May 1981 in Oklahoma. Because the ambient lower atmosphere was stratified, interesting interactions between the outflow current and the ambient environment resulted. The leading portion of the current was modulated by at least three gravity wavelike perturbations of horizontal spacing 12 km which initially coexisted with it. However, as the current evolved, it initiated an undular borelike disturbance which propagated ahead into the stable boundary layer, carrying cold outflow air in large amplitude rolls. Eventually the wave family left the decelerating outflow air in its wake. This borelike disturbance resembles the Australian “morning glory” phenomenon and appears to represent an early stage in the development of a solitary wave family.
The observations resemble other reported morning glories and solitary waves as well their laboratory and numerically simulated counterparts. Comparisons are discussed. This case study is unique not only because it combines Doppler radar, tall tower, and surface mesonet observations, but especially because the period of observation captures the disturbance in its formative stage when it is still very near the density current.
Abstract
The instrumented NASA ER-2 aircraft overflew severe convection with infrared (IR) V features for the first time in the Midwest United States during May 1984. Measurements taken by the ER-2 were: visible and IR imagery, high-frequency passive microwave (92, 183 GHz) imagery, nadir lidar backscattered return, and flight altitude information. The 7 May and 13 May 1984 cases are analyzed in detail and the various data sources are combined and compared with GOES imagery. Topics addressed in the paper are 1) relation of thermal couplets and V features in aircraft IR measurements to previous findings from GOES data, 2) examination of the cloud radiative hypothesis for the V feature, and 3) stratospheric perturbations above severe thunderstorms and mesoscale convective systems.
The high resolution aircraft IR imagery shows that thermal couplets are considerably more pronounced than in GOES imagery. In one of the cases (7 May 1984) the minimum cloud-top IR temperature was located upshear of the overshooting cloud top in the lidar height field. This was suggested in previous papers to result from cloud top mixing with the stratospheric environment and subsidence. The IR temperatures in the downshear anvils were as much as 5°C warmer than the ambient air temperatures, implying that the upwelling IR radiance comes from about 0.5–1.0 km below the cloud top. Finally, the in situ ER-2 measurements of temperature and air velocity 3–4 km above the overshooting tops showed very intense temperature and vertical velocity perturbations. These perturbations are suggestive of 1) lee waves generated by the overshooting tops, or 2) a cold dome above the squall line possibly due to tropopause lifting by the storms.
Abstract
The instrumented NASA ER-2 aircraft overflew severe convection with infrared (IR) V features for the first time in the Midwest United States during May 1984. Measurements taken by the ER-2 were: visible and IR imagery, high-frequency passive microwave (92, 183 GHz) imagery, nadir lidar backscattered return, and flight altitude information. The 7 May and 13 May 1984 cases are analyzed in detail and the various data sources are combined and compared with GOES imagery. Topics addressed in the paper are 1) relation of thermal couplets and V features in aircraft IR measurements to previous findings from GOES data, 2) examination of the cloud radiative hypothesis for the V feature, and 3) stratospheric perturbations above severe thunderstorms and mesoscale convective systems.
The high resolution aircraft IR imagery shows that thermal couplets are considerably more pronounced than in GOES imagery. In one of the cases (7 May 1984) the minimum cloud-top IR temperature was located upshear of the overshooting cloud top in the lidar height field. This was suggested in previous papers to result from cloud top mixing with the stratospheric environment and subsidence. The IR temperatures in the downshear anvils were as much as 5°C warmer than the ambient air temperatures, implying that the upwelling IR radiance comes from about 0.5–1.0 km below the cloud top. Finally, the in situ ER-2 measurements of temperature and air velocity 3–4 km above the overshooting tops showed very intense temperature and vertical velocity perturbations. These perturbations are suggestive of 1) lee waves generated by the overshooting tops, or 2) a cold dome above the squall line possibly due to tropopause lifting by the storms.
Abstract
A procedure for real-time adjustment of range-dependent biases in Weather Surveillance Radar-1988 Doppler version (WSR-88D) rainfall estimates due to nonuniform vertical profile of reflectivity is proposed. Using volume-scan measurements of effective reflectivity, the procedure estimates conditional mean and variance profiles of point reflectivity in the vertical and calculates, as a function of elevation angle and slant range, a multiplicative adjustment factor to be applied to the apparent radar rain rate. As a by-product, the maximum effective coverage of radar is also delineated, outside of which radar rainfall estimates are subject to beam overshooting. To evaluate the procedure, unadjusted and adjusted radar rainfall estimates from a Pacific Northwest winter storm are examined and compared with rain gauge data.
Abstract
A procedure for real-time adjustment of range-dependent biases in Weather Surveillance Radar-1988 Doppler version (WSR-88D) rainfall estimates due to nonuniform vertical profile of reflectivity is proposed. Using volume-scan measurements of effective reflectivity, the procedure estimates conditional mean and variance profiles of point reflectivity in the vertical and calculates, as a function of elevation angle and slant range, a multiplicative adjustment factor to be applied to the apparent radar rain rate. As a by-product, the maximum effective coverage of radar is also delineated, outside of which radar rainfall estimates are subject to beam overshooting. To evaluate the procedure, unadjusted and adjusted radar rainfall estimates from a Pacific Northwest winter storm are examined and compared with rain gauge data.
Abstract
A detailed description of the operational WSR-88D rainfall estimation algorithm is presented. This algorithm, called the Precipitation Processing System, produces radar-derived rainfall products in real time for forecasters in support of the National Weather Service’s warning and forecast missions. It transforms reflectivity factor measurements into rainfall accumulations and incorporates rain gauge data to improve the radar estimates. The products are used as guidance to issue flood watches and warnings to the public and as input into numerical hydrologic and atmospheric models. The processing steps to quality control and compute the rainfall estimates are described, and the current deficiencies and future plans for improvement are discussed.
Abstract
A detailed description of the operational WSR-88D rainfall estimation algorithm is presented. This algorithm, called the Precipitation Processing System, produces radar-derived rainfall products in real time for forecasters in support of the National Weather Service’s warning and forecast missions. It transforms reflectivity factor measurements into rainfall accumulations and incorporates rain gauge data to improve the radar estimates. The products are used as guidance to issue flood watches and warnings to the public and as input into numerical hydrologic and atmospheric models. The processing steps to quality control and compute the rainfall estimates are described, and the current deficiencies and future plans for improvement are discussed.
Abstract
Hurricane eyewalls are often observed to be nearly circular structures, but they are occasionally observed to take on distinctly polygonal shapes. The shapes range from triangles to hexagons and, while they are often incomplete, straight line segments can be identified. Other observations implicate the existence of intense mesovortices within or near the eye region. Is there a relation between polygonal eyewalls and hurricane mesovortices? Are these phenomena just curiosities of the hurricane’s inner-core circulation, or are they snapshots of an intrinsic mixing process within or near the eye that serves to determine the circulation and thermal structure of the eye?
As a first step toward understanding the asymmetric vorticity dynamics of the hurricane’s eye and eyewall region, these issues are examined within the framework of an unforced barotropic nondivergent model. Polygonal eyewalls are shown to form as a result of barotropic instability near the radius of maximum winds. After reviewing linear theory, simulations with a high-resolution pseudospectral numerical model are presented to follow the instabilities into their nonlinear regime. When the instabilities grow to finite amplitude, the vorticity of the eyewall region pools into discrete areas, creating the appearance of polygonal eyewalls. The circulations associated with these pools of vorticity suggest a connection to hurricane mesovortices. At later times the vorticity is ultimately rearranged into a nearly monopolar circular vortex. While the evolution of the finescale vorticity field is sensitive to the initial condition, the macroscopic end-states are found to be similar. In fact, the gross characteristics of the numerically simulated end-states are predicted analytically using a generalization of the minimum enstrophy hypothesis. In an effort to remove some of the weaknesses of the minimum enstrophy approach, a maximum entropy argument developed previously for rectilinear shear flows is extended to the vortex problem, and end-state solutions in the limiting case of tertiary mixing are obtained.
Implications of these ideas for real hurricanes are discussed.
Abstract
Hurricane eyewalls are often observed to be nearly circular structures, but they are occasionally observed to take on distinctly polygonal shapes. The shapes range from triangles to hexagons and, while they are often incomplete, straight line segments can be identified. Other observations implicate the existence of intense mesovortices within or near the eye region. Is there a relation between polygonal eyewalls and hurricane mesovortices? Are these phenomena just curiosities of the hurricane’s inner-core circulation, or are they snapshots of an intrinsic mixing process within or near the eye that serves to determine the circulation and thermal structure of the eye?
As a first step toward understanding the asymmetric vorticity dynamics of the hurricane’s eye and eyewall region, these issues are examined within the framework of an unforced barotropic nondivergent model. Polygonal eyewalls are shown to form as a result of barotropic instability near the radius of maximum winds. After reviewing linear theory, simulations with a high-resolution pseudospectral numerical model are presented to follow the instabilities into their nonlinear regime. When the instabilities grow to finite amplitude, the vorticity of the eyewall region pools into discrete areas, creating the appearance of polygonal eyewalls. The circulations associated with these pools of vorticity suggest a connection to hurricane mesovortices. At later times the vorticity is ultimately rearranged into a nearly monopolar circular vortex. While the evolution of the finescale vorticity field is sensitive to the initial condition, the macroscopic end-states are found to be similar. In fact, the gross characteristics of the numerically simulated end-states are predicted analytically using a generalization of the minimum enstrophy hypothesis. In an effort to remove some of the weaknesses of the minimum enstrophy approach, a maximum entropy argument developed previously for rectilinear shear flows is extended to the vortex problem, and end-state solutions in the limiting case of tertiary mixing are obtained.
Implications of these ideas for real hurricanes are discussed.
