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Peter T. May

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.

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Peter T. May

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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Peter T. May

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Peter T. May

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.

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Peter T. May

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.

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Peter T. May
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Deepak K. Rajopadhyaya

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.

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Peter T. May
and
Deepak K. Rajopadhyaya
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Peter T. May
and
James M. Wilczak

Abstract

A wind profiler-radio acoustic sounding system at Denver collected hourly wind and virtual-temperature data through the boundary layer in the latter half of 1989. Analyzed monthly averages of 24-h time-height cross sections of the daily measurements show a number of significant features. The growth of the nocturnal temperature inversion is observed, followed by a rapid transition to a deep daytime mixed layer. The progression from a strong diurnal temperature signal in the summer to weak diurnal variability in the winter is documented. A mean upslope wind component is found in the middle-to-late afternoon in the summer and autumn months, with a reverse, return flow aloft. Boundary-layer winds show a strong inertial oscillation, with the phase closely following the diurnal heating cycle. Perturbation winds in the return-flow region aloft oscillate almost 180° out of phase with the boundary-layer winds.

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Peter T. May
and
Deepak K. Rajopadhyaya

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.

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Peter T. May
and
Thomas D. Keenan

Abstract

Polarimetric radar data have been used to produce microphysical classifications. This kind of analysis is run in a real-time mode from several research radars, including the C-band polarimetric (C-Pol) radar in Darwin, Australia. However, these classifications have had very little systematic evaluation with independent data. Using surface data is often difficult because of sampling issues, particularly for hail. The approach taken here is to use a combination of 50- and 920-MHz wind profiler estimates of rain and hail to provide validation data for the radar pixels over the profiler. The profilers also observe signals associated with lightning, and some comparisons are made between lightning occurrence and the radar measurements of graupel. The retrievals of hail–rain mixtures are remarkably robust; there are some issues regarding other microphysical classes, however, including difficulties in detecting melting snow layers in stratiform rain. These difficulties are largely due to the resampling of the radar volume data onto a grid and to poor separation of the snow classes.

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