Search Results
You are looking at 1 - 10 of 164 items for
- Author or Editor: T. Peter x
- Refine by Access: All Content x
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.
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
No abstract available
Abstract
No abstract available
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
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 Community Climate System Model, version 2 (CCSM2) is briefly described. A 1000-yr control simulation of the present day climate has been completed without flux adjustments. Minor modifications were made at year 350, which included all five components using the same physical constants. There are very small trends in the upper-ocean, sea ice, atmosphere, and land fields after year 150 of the control simulation. The deep ocean has small but significant trends; however, these are not large enough that the control simulation could not be continued much further. The equilibrium climate sensitivity of CCSM2 is 2.2 K, which is slightly larger than the Climate System Model, version 1 (CSM1) value of 2.0 K.
Several aspects of the control simulation's mean climate and interannual variability are described, and good and bad properties of the control simulation are documented. In particular, several aspects of the simulation, especially in the Arctic region, are much improved over those obtained in CSM1. Other aspects, such as the tropical Pacific region simulation, have not been improved much compared to those in CSM1. Priorities for further model development are discussed in the conclusions section.
Abstract
The Community Climate System Model, version 2 (CCSM2) is briefly described. A 1000-yr control simulation of the present day climate has been completed without flux adjustments. Minor modifications were made at year 350, which included all five components using the same physical constants. There are very small trends in the upper-ocean, sea ice, atmosphere, and land fields after year 150 of the control simulation. The deep ocean has small but significant trends; however, these are not large enough that the control simulation could not be continued much further. The equilibrium climate sensitivity of CCSM2 is 2.2 K, which is slightly larger than the Climate System Model, version 1 (CSM1) value of 2.0 K.
Several aspects of the control simulation's mean climate and interannual variability are described, and good and bad properties of the control simulation are documented. In particular, several aspects of the simulation, especially in the Arctic region, are much improved over those obtained in CSM1. Other aspects, such as the tropical Pacific region simulation, have not been improved much compared to those in CSM1. Priorities for further model development are discussed in the conclusions section.
Abstract
In mid-autumn 2002, an exceptional 5-day cold spell affected much of the interior Pacific Northwest, with minimum temperatures averaging 13°C below long-term means (1953–2002). On 31 October, minimum temperature records occurred at 98 of the 106 recording stations, with records lowered in some locations by 9°C. Calculation of recurrence intervals of minimum temperatures shows that 50% of the stations experienced a >500-yr event. The synoptic conditions responsible were the development of a pronounced high pressure ridge over western Canada and an intense low pressure area centered in the Intermountain West that promoted strong northeasterly winds. The cold spell occurred near the end of the growing season for an ecologically critical and dominant tree species of the interior Pacific Northwest—western juniper—and followed an extended period of severe drought. In spring 2003, it became apparent that the cold had caused high rates of tree mortality and canopy dieback in a species that is remarkable for its longevity and resistance to climatic stress. The cold event altered western juniper dominance in some areas, and this alteration may have long-term impacts on water budgets, fire intensities and frequencies, animal species interrelationships, and interspecific competition among plant species.
Abstract
In mid-autumn 2002, an exceptional 5-day cold spell affected much of the interior Pacific Northwest, with minimum temperatures averaging 13°C below long-term means (1953–2002). On 31 October, minimum temperature records occurred at 98 of the 106 recording stations, with records lowered in some locations by 9°C. Calculation of recurrence intervals of minimum temperatures shows that 50% of the stations experienced a >500-yr event. The synoptic conditions responsible were the development of a pronounced high pressure ridge over western Canada and an intense low pressure area centered in the Intermountain West that promoted strong northeasterly winds. The cold spell occurred near the end of the growing season for an ecologically critical and dominant tree species of the interior Pacific Northwest—western juniper—and followed an extended period of severe drought. In spring 2003, it became apparent that the cold had caused high rates of tree mortality and canopy dieback in a species that is remarkable for its longevity and resistance to climatic stress. The cold event altered western juniper dominance in some areas, and this alteration may have long-term impacts on water budgets, fire intensities and frequencies, animal species interrelationships, and interspecific competition among plant species.
