Search Results
Abstract
Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.
Abstract
Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.
Abstract
Average vertical profiles of the vertical wind obtained under clear sky conditions as weal as under conditions of both light-to-moderate and heavy rainfall am presented from data obtained using a radar wind profiler located on the island of Pohnpei (latitude 7°N, longitude 157°E). The average profiles for the precipitation conditions were obtained, insofar as possible, under conditions similar to those present within the stratiform and convective regions of tropical mesoscale convective complexes. Comparison between the vertical wind profiles obtained from the wind profiler and vertical wind profiles obtained earlier by wore conventional methods (i.e., deduced from the convergence-divergence of mesoscale horizontal winds) shows that, while the general features of the profiles obtained by both techniques are similar, the profiler results exhibit somewhat more detail. The profiler is able to resolve long-term average vertical motions down to the, ∼cm s−1 subsidence that occurs under clear air conditions. Additional evidence for an apparent difference between vertical wind profiles in the Atlantic and Pacific regions in heavy convection reported earlier, is presented.
Abstract
Average vertical profiles of the vertical wind obtained under clear sky conditions as weal as under conditions of both light-to-moderate and heavy rainfall am presented from data obtained using a radar wind profiler located on the island of Pohnpei (latitude 7°N, longitude 157°E). The average profiles for the precipitation conditions were obtained, insofar as possible, under conditions similar to those present within the stratiform and convective regions of tropical mesoscale convective complexes. Comparison between the vertical wind profiles obtained from the wind profiler and vertical wind profiles obtained earlier by wore conventional methods (i.e., deduced from the convergence-divergence of mesoscale horizontal winds) shows that, while the general features of the profiles obtained by both techniques are similar, the profiler results exhibit somewhat more detail. The profiler is able to resolve long-term average vertical motions down to the, ∼cm s−1 subsidence that occurs under clear air conditions. Additional evidence for an apparent difference between vertical wind profiles in the Atlantic and Pacific regions in heavy convection reported earlier, is presented.
Abstract
The authors derive a relationship between the vertical Doppler spectrum of the rain just below the radar bright band and that of the snow just above. It neglects vertical air motions and assumes that each snowflake simply melts to form a raindrop of the same mass, disregarding other possible effects such as aggregation to form larger particles or breakup to create smaller ones. The relationship shows that, regardless of the dependence of particle fallspeed on size, the product of the equivalent reflectivity factor and the mean Doppler velocity of the snow is proportional to the same product for the rain, with a constant proportionality factor of 0.23, which equals the ratio of the dielectric factors of ice and water. Observed values of the reflectivity and mean Doppler velocity above and below the melting layer sometimes agree with this theoretical prediction but more often deviate from it in ways that may be interpreted as indicating the predominance of either aggregation or breakup processes. The data suggest that aggregation is occurring much of the time in the melting layer but that breakup effects become dominant in heavy precipitation. The analysis is extended by assuming relations between particle size and fallspeed for rain and snow. This enables the comparison of measured spectra with those derived theoretically. A simple allowance for aggregation or breakup in the spectral transformation from snow to rain is found to give improved spectral agreement in cases where these effects are indicated.
Abstract
The authors derive a relationship between the vertical Doppler spectrum of the rain just below the radar bright band and that of the snow just above. It neglects vertical air motions and assumes that each snowflake simply melts to form a raindrop of the same mass, disregarding other possible effects such as aggregation to form larger particles or breakup to create smaller ones. The relationship shows that, regardless of the dependence of particle fallspeed on size, the product of the equivalent reflectivity factor and the mean Doppler velocity of the snow is proportional to the same product for the rain, with a constant proportionality factor of 0.23, which equals the ratio of the dielectric factors of ice and water. Observed values of the reflectivity and mean Doppler velocity above and below the melting layer sometimes agree with this theoretical prediction but more often deviate from it in ways that may be interpreted as indicating the predominance of either aggregation or breakup processes. The data suggest that aggregation is occurring much of the time in the melting layer but that breakup effects become dominant in heavy precipitation. The analysis is extended by assuming relations between particle size and fallspeed for rain and snow. This enables the comparison of measured spectra with those derived theoretically. A simple allowance for aggregation or breakup in the spectral transformation from snow to rain is found to give improved spectral agreement in cases where these effects are indicated.
