Roger Lhermitte

Doppler Radar and Radio Observations of Thunderstorms

Three-dimensional motion fields in a thunderstorm are compared to measurements of the radar reflectivity structure of the storm and to the locations of VHF radiation from electrical discharges, in an attempt to determine the physical conditions which prevailed during the electrically active phase of the storm. The observations were obtained from a network of three Doppler radars and a long-baseline radiation source location system during the 1978 Thunderstorm Research International Program (TRIP 78) at Kennedy Space Center, FL. The analysis shows that two cells developed sequentially on the upshear side of the storm. In the second cell, the updraft velocity between 6- and 7-km altitude (-10 to -15°C) increased rapidly to greater than 20 ms-1; and coincided with a substantial increase in the VHF radiation burst rate. Growth of the updraft was accompanied by the simultaneous development of an upper level downdraft on its upshear side. Reflectivity values in excess of 50 dBZ were observed within the vicinity of the updraft, and have been attributed to graupel or hail that was suspended in the updraft and which grew by riming of supercooled water droplets. Initial source locations for each radiation burst formed an umbrella-like pattern above the high-reflectivity core. We interpret the results to be consistent with an ice-ice electrical charging interaction first investigated by Reynolds et al.

Radar Observations of Updrafts, Downdrafts, and Turbulence in Fair-Weather Cumuli

Abstract Observations from a 94-GHz radar are used to define the vertical structure of marine fair-weather cumuli. Doppler spectra obtained from the radar provide mean vertical velocities as well as detailed spectral shapes that can be used to infer small-scale vertical velocity shear, illuminate cloud microphysical processes, and provide estimates of turbulence dissipation rates. These new observations facilitate the analysis and understanding of in-cloud circulations and the physical processes involved, since the cloud boundaries and dimensions are mapped along with the internal structure of the clouds. Coincident observations from a 915-MHz radar (wind profiler) were used to further define the turbulence structure in and around the clouds. The observations document the detailed vertical and horizontal dimensions of updraft and downdraft circulations in the clouds observed. The two cumuli studied in detail have similar circulation patterns—an updraft core surrounded by downdrafts. Although the clouds ha...

Vertical air motion and raindrop size distributions in convective systems using a 94 GHz radar

This paper presents a method for the observation of air vertical velocity and raindrop size distribution in convective storms, based on the interpretation of Mie backscattering oscillations seen in the Doppler spectrum observed at vertical incidence with a 94 GHz Doppler radar. The location of the first minimum allows an accurate determination of vertical air motion, which is used to shift the spectrum so that it relates to the raindrops terminal velocity alone. The drop size distribution is then retrieved from the shifted Doppler spectrum. An example of the structure of two-dimensional updrafts and downdrafts observed in the low levels of a convective storm is presented. Drop size distributions can also be estimated and evidence of small drop removal by the updraft is presented.

Turbulent flow microstructures observed by sonar

This note presents observations of sound wave backscattering in a turbulent water flow, conducted with a frequency scanning Doppler sonar working in a 0.5 to 5 MHz frequency band. The observed sound wave backscattering is attributed to temperature microstructures created by water turbulence. The backscattering intensity decreases systematically as a function of sonar frequency above 1 MHz and falls below detectability for sonar frequencies higher than a given value function of water turbulence intensity. This backscattering cutoff relates to the high wavenumber end, kB, of the turbulent temperature spectrum. This kB estimate is found in excellent agreement with its theoretical calculation based on independent measurement of turbulence dissipation rate made using the Doppler observed velocity variance.