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Abstract
A slow-moving weak tropical cyclone passed near Darwin, Australia, in December 1990. Rainbands were observed by a Doppler weather radar and a 50-MHz wind profiler for over 24 h. The principal bands were seen to be organized on two distinct scales. Bands of stratiform precipitation formed at a radius of about 100 km from the center of the storm and moved outward at about 6 m s−1. These decayed after they moved past Darwin over land. A distinct midlevel jet extended along the bands. Within the bands, convective lines formed at regular intervals, propagated against and outward with respect to the mean flow, and acted as a partial barrier to the radial inflow. Deep, active convection was confined to these lines. The vertical motion in the convection showed a distinct acceleration above the freezing level with measured updrafts of up to 10 m s−1. The convection elevated the tropopause height over the rainband. It is hypothesized that an inertia-gravity wave propagating from near the storm eye was responsible for triggering the convection within the lines. This hypothesis, although difficult to test, accounts for the propagation characteristics of the convective lines and offers an explanation of why similar features have not been seen in more intense storms.
Abstract
A slow-moving weak tropical cyclone passed near Darwin, Australia, in December 1990. Rainbands were observed by a Doppler weather radar and a 50-MHz wind profiler for over 24 h. The principal bands were seen to be organized on two distinct scales. Bands of stratiform precipitation formed at a radius of about 100 km from the center of the storm and moved outward at about 6 m s−1. These decayed after they moved past Darwin over land. A distinct midlevel jet extended along the bands. Within the bands, convective lines formed at regular intervals, propagated against and outward with respect to the mean flow, and acted as a partial barrier to the radial inflow. Deep, active convection was confined to these lines. The vertical motion in the convection showed a distinct acceleration above the freezing level with measured updrafts of up to 10 m s−1. The convection elevated the tropopause height over the rainband. It is hypothesized that an inertia-gravity wave propagating from near the storm eye was responsible for triggering the convection within the lines. This hypothesis, although difficult to test, accounts for the propagation characteristics of the convective lines and offers an explanation of why similar features have not been seen in more intense storms.
Abstract
A simple model is used to illustrate the limitations of dual pulse repetition time (PRT) radars for measurements of wind fields with strong horizontal wind gradients. In such conditions, errors may occur even if the radial velocity measurements in the individual rays are not aliased. However, basic circulation and divergent signatures are seen and some improvement is possible with further postprocessing of the data. This is significant, as radars using this procedure are being used operationally for severe weather applications in Australia and Canada.
Abstract
A simple model is used to illustrate the limitations of dual pulse repetition time (PRT) radars for measurements of wind fields with strong horizontal wind gradients. In such conditions, errors may occur even if the radial velocity measurements in the individual rays are not aliased. However, basic circulation and divergent signatures are seen and some improvement is possible with further postprocessing of the data. This is significant, as radars using this procedure are being used operationally for severe weather applications in Australia and Canada.
Abstract
The performance and limitations of the Bureau of Meteorology Research Centre's (BMRC) 50-MHz wind profiler operating at Saipan in the central western Pacific are assessed. Hourly averaged profiler estimates of horizontal wind are compared with 120 radiosonde ascents. This comparison shows the best agreement of any large profiler-sonde comparison published with rms differences of about 1.5 m s−3 at some altitudes and about 2.3 m s−1 for all altitudes below 10 km. The results appear to be almost independent of wind-speed or precipitation conditions.
Abstract
The performance and limitations of the Bureau of Meteorology Research Centre's (BMRC) 50-MHz wind profiler operating at Saipan in the central western Pacific are assessed. Hourly averaged profiler estimates of horizontal wind are compared with 120 radiosonde ascents. This comparison shows the best agreement of any large profiler-sonde comparison published with rms differences of about 1.5 m s−3 at some altitudes and about 2.3 m s−1 for all altitudes below 10 km. The results appear to be almost independent of wind-speed or precipitation conditions.
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Abstract
High-time-resolution wind profiler/RASS observations are used to describe the vertical velocity, temperature, and reflectivity fields of two gust fronts in detail. The first was a freely propagating gust front and the second interacted with a rain cell near the profiler site. The first of these shows a large updraft confined to the warm air ahead of the front. This updraft coincided with the (nonhydrostatic) pressure jump. The vertical motions within the gust front were an order of magnitude smaller. The updraft impinging on the top of the boundary layer excited a clear gravity wave signature in the free troposphere. The interaction of the vertical circulation and the weakly precipitating cloud in the second case coincided with explosive growth of the cell with reflectivities increasing from ∼30 dBZ to >50 dBZ in 6 min. A descending reflectivity core was observed at this time. Precipitation loading played a significant role in a downdraft behind the gust front head leading to adiabatic warming as no evidence of evaporative cooling in the downdraft was seen. A distinct clear air peak was visible in the profiler Doppler spectra even during the heavy rain.
Abstract
High-time-resolution wind profiler/RASS observations are used to describe the vertical velocity, temperature, and reflectivity fields of two gust fronts in detail. The first was a freely propagating gust front and the second interacted with a rain cell near the profiler site. The first of these shows a large updraft confined to the warm air ahead of the front. This updraft coincided with the (nonhydrostatic) pressure jump. The vertical motions within the gust front were an order of magnitude smaller. The updraft impinging on the top of the boundary layer excited a clear gravity wave signature in the free troposphere. The interaction of the vertical circulation and the weakly precipitating cloud in the second case coincided with explosive growth of the cell with reflectivities increasing from ∼30 dBZ to >50 dBZ in 6 min. A descending reflectivity core was observed at this time. Precipitation loading played a significant role in a downdraft behind the gust front head leading to adiabatic warming as no evidence of evaporative cooling in the downdraft was seen. A distinct clear air peak was visible in the profiler Doppler spectra even during the heavy rain.
Abstract
Data from a wind profiler located at Darwin, Australia, have been used to examine the vertical motions and precipitation microphysics in a well-developed squall line. Both a mature and developing convective cell are well sampled. The vertical motions within the mature cell are dominated by the effect of glaciation and a convective downdraft feeding a cold pool. The strong updrafts are accompanied by supercooled water as much as 2 km above the freezing level. The two cells are separated by a narrow region of deep descent. The developing cell has a low-level maximum in upward motion coinciding with high radar reflectivity below 3 km, suggesting warm rain processes. There is a large transition region with deep descent and a stratiform region with a classic up- and downdraft circulation. The precipitation characteristics show the aggregation of ice particles as they descend in the stratiform region. Over half of the rain is seen to evaporate between 4 and 2 km. The cooling implied by this and the heating by the growth of ice particulates above the melting level balance the mesoscale circulation in the stratiform region. The Q 1, heating profile is consistent with previous studies above 4 km but shows a net cooling below this. This may in part be due to the storm being sampled when the system was mature with extensive convective downdrafts.
Abstract
Data from a wind profiler located at Darwin, Australia, have been used to examine the vertical motions and precipitation microphysics in a well-developed squall line. Both a mature and developing convective cell are well sampled. The vertical motions within the mature cell are dominated by the effect of glaciation and a convective downdraft feeding a cold pool. The strong updrafts are accompanied by supercooled water as much as 2 km above the freezing level. The two cells are separated by a narrow region of deep descent. The developing cell has a low-level maximum in upward motion coinciding with high radar reflectivity below 3 km, suggesting warm rain processes. There is a large transition region with deep descent and a stratiform region with a classic up- and downdraft circulation. The precipitation characteristics show the aggregation of ice particles as they descend in the stratiform region. Over half of the rain is seen to evaporate between 4 and 2 km. The cooling implied by this and the heating by the growth of ice particulates above the melting level balance the mesoscale circulation in the stratiform region. The Q 1, heating profile is consistent with previous studies above 4 km but shows a net cooling below this. This may in part be due to the storm being sampled when the system was mature with extensive convective downdrafts.
Abstract
Continuous vertical velocity measurements using a 50-MHz wind profiler located at Darwin in northern Australia during periods of active convection have been analyzed. This dataset is dominated by continental-type convection. Numerous examples of shallow, deep, and decaying convection were seen and it is shown that only the deep systems have substantial tilts to the draft structure. The most intense updrafts occur above the freezing level, but shallow convection also produces large-amplitude vertical motions. The strength of these updrafts in this dataset is very similar to other tropical, oceanic data. That observation is consistent with the idea that the magnitude of the updrafts is much less in the Tropics than for intense midlatitude convection because the convective available potential energy is distributed over a much deeper layer in the Tropics, although more intense updrafts may be present at other tropical locations, such as the Tiwi Islands north of Darwin. The size of the cores, however, is significantly greater here than with oceanic data and is similar to midlatitude results, thus supporting the suggestion that boundary layer depth is important in determining the horizontal scale. There is a net detrainment in the upward cores above the freezing level occurring at all space scales. The mass flux in intense updrafts is almost constant with height below the freezing level but is almost cancelled by downdrafts and the immediate surrounding environment. Two populations of downdrafts are seen, one a dynamical response associated with intense updrafts at all heights and a second driven by precipitation processes below the freezing level. The core size, intensity, and mass flux are all approximately lognormally distributed. It is shown that a wide range of velocity and size scales contribute to the upward mass flux.
Abstract
Continuous vertical velocity measurements using a 50-MHz wind profiler located at Darwin in northern Australia during periods of active convection have been analyzed. This dataset is dominated by continental-type convection. Numerous examples of shallow, deep, and decaying convection were seen and it is shown that only the deep systems have substantial tilts to the draft structure. The most intense updrafts occur above the freezing level, but shallow convection also produces large-amplitude vertical motions. The strength of these updrafts in this dataset is very similar to other tropical, oceanic data. That observation is consistent with the idea that the magnitude of the updrafts is much less in the Tropics than for intense midlatitude convection because the convective available potential energy is distributed over a much deeper layer in the Tropics, although more intense updrafts may be present at other tropical locations, such as the Tiwi Islands north of Darwin. The size of the cores, however, is significantly greater here than with oceanic data and is similar to midlatitude results, thus supporting the suggestion that boundary layer depth is important in determining the horizontal scale. There is a net detrainment in the upward cores above the freezing level occurring at all space scales. The mass flux in intense updrafts is almost constant with height below the freezing level but is almost cancelled by downdrafts and the immediate surrounding environment. Two populations of downdrafts are seen, one a dynamical response associated with intense updrafts at all heights and a second driven by precipitation processes below the freezing level. The core size, intensity, and mass flux are all approximately lognormally distributed. It is shown that a wide range of velocity and size scales contribute to the upward mass flux.
Abstract
A season of operational cell and track data from Darwin, Australia, has been analyzed to explore the statistical characteristics of the convective cell heights. The statistics for the monsoon and break regimes are significantly different with the break season cells being higher for a given reflectivity threshold. The monsoon cells produce more rain, but there are fewer intense cells and there is a much larger contribution from stratiform rain. The monsoon cells are also slightly larger, but shorter lived than the breaks. The shorter lifetime may reflect a more rapid transition to a longer-lived stratiform character. The monsoon regime is shown to be associated with large-scale ascent and higher humidity that may lead to more frequent, but weaker cells. Within regimes, the subset of intense cells generally reach near the tropopause or overshoot. However, there is little evidence in the data for a multimodal distribution of cell heights, except perhaps for the intense monsoon cases. Instead, the picture is a continuous distribution of cell heights with the peak of the distribution shifting to higher values as the distributions are conditioned on higher reflectivity.
Abstract
A season of operational cell and track data from Darwin, Australia, has been analyzed to explore the statistical characteristics of the convective cell heights. The statistics for the monsoon and break regimes are significantly different with the break season cells being higher for a given reflectivity threshold. The monsoon cells produce more rain, but there are fewer intense cells and there is a much larger contribution from stratiform rain. The monsoon cells are also slightly larger, but shorter lived than the breaks. The shorter lifetime may reflect a more rapid transition to a longer-lived stratiform character. The monsoon regime is shown to be associated with large-scale ascent and higher humidity that may lead to more frequent, but weaker cells. Within regimes, the subset of intense cells generally reach near the tropopause or overshoot. However, there is little evidence in the data for a multimodal distribution of cell heights, except perhaps for the intense monsoon cases. Instead, the picture is a continuous distribution of cell heights with the peak of the distribution shifting to higher values as the distributions are conditioned on higher reflectivity.
Abstract
Polarimetric weather radars offer the promise of accurate rainfall measurements by including polarimetric measurements in rainfall estimation algorithms. Questions still remain on how accurately polarimetric measurements represent the parameters of the raindrop size distribution (DSD). In particular, this study propagates polarimetric radar measurement uncertainties through a power-law median raindrop diameter D 0 algorithm to quantify the statistical uncertainties of the power-law regression. For this study, the power-law statistical uncertainty of D 0 ranged from 0.11 to 0.17 mm. Also, the polarimetric scanning radar D 0 estimates were compared with the median raindrop diameters retrieved from two vertically pointing profilers observing the same radar volume as the scanning radar. Based on over 900 observations, the standard deviation of the differences between the two radar estimates was approximately 0.16 mm. Thus, propagating polarimetric measurement uncertainties through D 0 power-law regressions is comparable to uncertainties between polarimeteric and profiler D 0 estimates.
Abstract
Polarimetric weather radars offer the promise of accurate rainfall measurements by including polarimetric measurements in rainfall estimation algorithms. Questions still remain on how accurately polarimetric measurements represent the parameters of the raindrop size distribution (DSD). In particular, this study propagates polarimetric radar measurement uncertainties through a power-law median raindrop diameter D 0 algorithm to quantify the statistical uncertainties of the power-law regression. For this study, the power-law statistical uncertainty of D 0 ranged from 0.11 to 0.17 mm. Also, the polarimetric scanning radar D 0 estimates were compared with the median raindrop diameters retrieved from two vertically pointing profilers observing the same radar volume as the scanning radar. Based on over 900 observations, the standard deviation of the differences between the two radar estimates was approximately 0.16 mm. Thus, propagating polarimetric measurement uncertainties through D 0 power-law regressions is comparable to uncertainties between polarimeteric and profiler D 0 estimates.
Abstract
The implied heating and potential vorticity generation in tropical cyclone rainbands is derived from observed vertical motion profiles. High levels of potential vorticity generation are found in the stratiform rain regions, sufficient to generate substantial wind maxima along the bands within a couple of hours. Such generation may represent a significant source of potential vorticity for the system as a whole and may have implications for cyclone intensity.
Abstract
The implied heating and potential vorticity generation in tropical cyclone rainbands is derived from observed vertical motion profiles. High levels of potential vorticity generation are found in the stratiform rain regions, sufficient to generate substantial wind maxima along the bands within a couple of hours. Such generation may represent a significant source of potential vorticity for the system as a whole and may have implications for cyclone intensity.
