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Abstract
An extensive application of a rain profiling algorithm (ZPHI) employing a C-band polarimetric radar (the C-POL radar of the Australian Bureau of Meteorology Research Centre in Darwin) is presented. ZPHI belongs to the class of rain profiling algorithms that have been developed for spaceborne or airborne radars operating at attenuating frequencies. By nature, these algorithms are nonlocal: the full profile of the measured radar reflectivity is inverted to derive a retrieved profile of the rainfall rate. The retrieval accuracy lays in the imposition of an “external constraint” in the inversion procedure. In this case, that is supplied by the differential phase shift ΦDP. The primary products of ZPHI are the profile along the beam of the specific attenuation A, and the “normalized” intercept parameter
To provide high quality rain-rate retrieval, ZPHI requires careful radar calibration. Two techniques of calibration checking are investigated; both provide a calibration estimate to within 0.1 and 0.2 dB. One is based upon the climatological stability of the
Comparisons of rain rate, including an extensive dataset in the month of January 1998, show a remarkable agreement between rain gauge data and the ZPHI estimate, whereas the “classical” estimate (standard Z–R relation applied without consideration of the attenuation) appears severely biased with respect to the rain gauges. In these comparisons, evidence for the crucial role of an
Abstract
An extensive application of a rain profiling algorithm (ZPHI) employing a C-band polarimetric radar (the C-POL radar of the Australian Bureau of Meteorology Research Centre in Darwin) is presented. ZPHI belongs to the class of rain profiling algorithms that have been developed for spaceborne or airborne radars operating at attenuating frequencies. By nature, these algorithms are nonlocal: the full profile of the measured radar reflectivity is inverted to derive a retrieved profile of the rainfall rate. The retrieval accuracy lays in the imposition of an “external constraint” in the inversion procedure. In this case, that is supplied by the differential phase shift ΦDP. The primary products of ZPHI are the profile along the beam of the specific attenuation A, and the “normalized” intercept parameter
To provide high quality rain-rate retrieval, ZPHI requires careful radar calibration. Two techniques of calibration checking are investigated; both provide a calibration estimate to within 0.1 and 0.2 dB. One is based upon the climatological stability of the
Comparisons of rain rate, including an extensive dataset in the month of January 1998, show a remarkable agreement between rain gauge data and the ZPHI estimate, whereas the “classical” estimate (standard Z–R relation applied without consideration of the attenuation) appears severely biased with respect to the rain gauges. In these comparisons, evidence for the crucial role of an
Abstract
Disdrometer data measured during the passage of tropical continental squall lines in Darwin, Australia, are analyzed to study characteristics of raindrop size distribution (DSD). Fifteen continental squall lines were selected for the DSD analysis. An observed squall line was partitioned into three regions based on radar reflectivity pattern, namely, convective line, stratiform, and reflectivity trough. A convective line was further partitioned into the convective center, leading edge, and trailing edge using a threshold rain rate of 20 mm h−1. Statistics of modified gamma DSD parameters obtained by a least squares fitting method show distinct differences between the convective-center and the stratiform regions; the shape of DSD for the convective center is convex upward, but it is more exponential for the stratiform region; the intercept parameter N 0 of the modified gamma function for the convective center and the reflectivity trough tends to be larger than that for the stratiform region, also. The observed drop size distributions are normalized to remove the effect of differences in rainfall rate. Gamma distributions then are least squares fitted to the normalized DSD data to show distinct differences between the convective-center and the stratiform regions; the characteristics of the trailing-edge and reflectivity-trough regions are equivalent to those of the convective center. DSD changes associated with the rainwater content variations are calculated using the obtained normalized gamma DSD function and the observed D 0–M relationship. The simulation demonstrates that the stratiform region is characterized by a larger drop spectrum (i.e., the maximum drop diameter and the median volume diameter are larger for the stratiform region than the convective center and the reflectivity trough for DSD with the same rainwater content). The Waldvogel “N 0 jump” is clearly shown, and the large drop spectrum for the stratiform region suggests the importance of the aggregation mechanism above the melting level in the stratiform region. The difference in the DSD for the convective-center and the stratiform regions causes systematic differences in Z–R relationships (Z = AR b ). A larger value for coefficient A in the stratiform region is found, but values of A and b change case by case; an inverse relationship between A and b (A = 103.22 b −6.25) is found for rainfall in the convective-center and the trailing-edge regions.
Abstract
Disdrometer data measured during the passage of tropical continental squall lines in Darwin, Australia, are analyzed to study characteristics of raindrop size distribution (DSD). Fifteen continental squall lines were selected for the DSD analysis. An observed squall line was partitioned into three regions based on radar reflectivity pattern, namely, convective line, stratiform, and reflectivity trough. A convective line was further partitioned into the convective center, leading edge, and trailing edge using a threshold rain rate of 20 mm h−1. Statistics of modified gamma DSD parameters obtained by a least squares fitting method show distinct differences between the convective-center and the stratiform regions; the shape of DSD for the convective center is convex upward, but it is more exponential for the stratiform region; the intercept parameter N 0 of the modified gamma function for the convective center and the reflectivity trough tends to be larger than that for the stratiform region, also. The observed drop size distributions are normalized to remove the effect of differences in rainfall rate. Gamma distributions then are least squares fitted to the normalized DSD data to show distinct differences between the convective-center and the stratiform regions; the characteristics of the trailing-edge and reflectivity-trough regions are equivalent to those of the convective center. DSD changes associated with the rainwater content variations are calculated using the obtained normalized gamma DSD function and the observed D 0–M relationship. The simulation demonstrates that the stratiform region is characterized by a larger drop spectrum (i.e., the maximum drop diameter and the median volume diameter are larger for the stratiform region than the convective center and the reflectivity trough for DSD with the same rainwater content). The Waldvogel “N 0 jump” is clearly shown, and the large drop spectrum for the stratiform region suggests the importance of the aggregation mechanism above the melting level in the stratiform region. The difference in the DSD for the convective-center and the stratiform regions causes systematic differences in Z–R relationships (Z = AR b ). A larger value for coefficient A in the stratiform region is found, but values of A and b change case by case; an inverse relationship between A and b (A = 103.22 b −6.25) is found for rainfall in the convective-center and the trailing-edge regions.
Abstract
A propagation correction algorithm utilizing the differential propagation phase (ϕ dp) was developed and tested on C-band polarimetric radar observations of tropical convection obtained during the Maritime Continent Thunderstorm Experiment. An empirical procedure was refined to estimate the mean coefficient of proportionality a (b) in the linear relationship between ϕ dp and the horizontal (differential) attenuation throughout each radar volume. The empirical estimates of these coefficients were a factor of 1.5–2 times larger than predicted by prior scattering simulations. This discrepancy was attributed to the routine presence of large drops [e.g., differential reflectivity Z dr ≥ 3 dB] within the tropical convection that were not included in prior theoretical studies.
Scattering simulations demonstrated that the coefficients a and b are nearly constant for small to moderate sized drops (e.g., 0.5 ≤ Z dr ≤ 2 dB; 1 ≤ diameter D 0 < 2.5 mm) but actually increase with the differential reflectivity for drop size distributions characterized by Z dr > 2 dB. As a result, large drops 1) bias the mean coefficients upward and 2) increase the standard error associated with the mean empirical coefficients down range of convective cores that contain large drops. To reduce this error, the authors implemented a “large drop correction” that utilizes enhanced coefficients a* and b* in large drop cores.
Validation of the propagation correction algorithm was accomplished with cumulative rain gauge data and internal consistency among the polarimetric variables. The bias and standard error of the cumulative radar rainfall estimator R(Z h ) [R(K dp,Z dr)], where Z h is horizontal reflectivity and K dp is specific differential phase, were substantially reduced after the application of the attenuation (differential attenuation) correction procedure utilizing ϕ dp. Similarly, scatterplots of uncorrected Z h (Z dr) versus K dp substantially underestimated theoretical expectations. After application of the propagation correction algorithm, the bias present in observations of both Z h (K dp) and Z dr(K dp) was removed and the standard errors relative to scattering simulation results were significantly reduced.
Abstract
A propagation correction algorithm utilizing the differential propagation phase (ϕ dp) was developed and tested on C-band polarimetric radar observations of tropical convection obtained during the Maritime Continent Thunderstorm Experiment. An empirical procedure was refined to estimate the mean coefficient of proportionality a (b) in the linear relationship between ϕ dp and the horizontal (differential) attenuation throughout each radar volume. The empirical estimates of these coefficients were a factor of 1.5–2 times larger than predicted by prior scattering simulations. This discrepancy was attributed to the routine presence of large drops [e.g., differential reflectivity Z dr ≥ 3 dB] within the tropical convection that were not included in prior theoretical studies.
Scattering simulations demonstrated that the coefficients a and b are nearly constant for small to moderate sized drops (e.g., 0.5 ≤ Z dr ≤ 2 dB; 1 ≤ diameter D 0 < 2.5 mm) but actually increase with the differential reflectivity for drop size distributions characterized by Z dr > 2 dB. As a result, large drops 1) bias the mean coefficients upward and 2) increase the standard error associated with the mean empirical coefficients down range of convective cores that contain large drops. To reduce this error, the authors implemented a “large drop correction” that utilizes enhanced coefficients a* and b* in large drop cores.
Validation of the propagation correction algorithm was accomplished with cumulative rain gauge data and internal consistency among the polarimetric variables. The bias and standard error of the cumulative radar rainfall estimator R(Z h ) [R(K dp,Z dr)], where Z h is horizontal reflectivity and K dp is specific differential phase, were substantially reduced after the application of the attenuation (differential attenuation) correction procedure utilizing ϕ dp. Similarly, scatterplots of uncorrected Z h (Z dr) versus K dp substantially underestimated theoretical expectations. After application of the propagation correction algorithm, the bias present in observations of both Z h (K dp) and Z dr(K dp) was removed and the standard errors relative to scattering simulation results were significantly reduced.
The Island Thunderstorm Experiment (ITEX) is a field and modeling study of the tropical thunderstorms that form regularly over Bathurst and Melville Islands north of Darwin, Northern Territory, Australia, during the transition season and breaks in the summer monsoon season. Such thunderstorms are of widespread occurrence in the tropics and they play an important role in tropical dynamics. ITEX is a joint project of the Bureau of Meteorology Research Centre and Monash University's Centre for Dynamical Meteorology. Preliminary studies have been used to plan an intensive period of observations that was carried out from 20 November to 10 December 1988. The resulting data will provide the basis for a series of analytical and numerical studies of tropical island thunderstorms.
The Island Thunderstorm Experiment (ITEX) is a field and modeling study of the tropical thunderstorms that form regularly over Bathurst and Melville Islands north of Darwin, Northern Territory, Australia, during the transition season and breaks in the summer monsoon season. Such thunderstorms are of widespread occurrence in the tropics and they play an important role in tropical dynamics. ITEX is a joint project of the Bureau of Meteorology Research Centre and Monash University's Centre for Dynamical Meteorology. Preliminary studies have been used to plan an intensive period of observations that was carried out from 20 November to 10 December 1988. The resulting data will provide the basis for a series of analytical and numerical studies of tropical island thunderstorms.
Abstract
The major field phase of the Australian Monsoon Experiment (AMEX Phase II) was conducted over northern Australia from 1 0 January to 1 5 February 1987. It was based on the collection of high-density tropical upper air soundings and radar data at 12 special observation sites. These were complemented by satellite and surface data, the existing upper air network, and two simultaneous aircraft based tropical experiments.
This paper describes the data collected in AMEX and the mean and transient structure of the Australian monsoon circulation during the experiment. Mean soundings across the network am compared with each other and with soundings from other commonly used research datasets.
It is shown that an active monsoon trough lay through the AMEX network, and that the associated convection is located within one of the three global tropical heat sources. Active and inactive periods of monsoon behavior are defined. Monsoon onset occurred within the period of the experiment and four tropical cyclones existed within the enhanced network. Two of these developed inside an array of radiosondes surrounding the Gulf of Carpentaria.
Abstract
The major field phase of the Australian Monsoon Experiment (AMEX Phase II) was conducted over northern Australia from 1 0 January to 1 5 February 1987. It was based on the collection of high-density tropical upper air soundings and radar data at 12 special observation sites. These were complemented by satellite and surface data, the existing upper air network, and two simultaneous aircraft based tropical experiments.
This paper describes the data collected in AMEX and the mean and transient structure of the Australian monsoon circulation during the experiment. Mean soundings across the network am compared with each other and with soundings from other commonly used research datasets.
It is shown that an active monsoon trough lay through the AMEX network, and that the associated convection is located within one of the three global tropical heat sources. Active and inactive periods of monsoon behavior are defined. Monsoon onset occurred within the period of the experiment and four tropical cyclones existed within the enhanced network. Two of these developed inside an array of radiosondes surrounding the Gulf of Carpentaria.
