Thursday, 27 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
This paper discusses the potential use of clear-air echoes for operational C-band Doppler radar systems in Japan. For the purpose of this study, over the six-month period (from July to December 1997), a total of 141 clear-air echo days were analyzed. The results of this study suggest that clear-air boundary layer is rich in clear-air echoes and operational scan strategies and algorithms for them would be worth developing in Japan. This study also exhibits the unique characteristics of space distribution, diurnal and seasonal variations, and sensitive dependence on boundary layer conditions. These results will be reflected in the limitations and time schedules concerning routine operational use of clear-air echoes. 1) Concerning the spatial distribution, a disadvantage is that clear-air information is not available in the oceanic sector of the radar coverage area because clear-air echoes only appear over land. 2) On the annual schedule for routine monitoring clear-air boundary layers, it is noticeable that the phase is shifted rapidly to no echo period (NEP) when daily average surface temperatures dipping below 10°C. During NEP, there is no clear-air echo available for monitoring clear-air circulations. 3) The daily timetable for the use of clear-air echoes should be limited to almost daytime. On the other hand, nighttime boundary layer can be scarcely monitored because of poor/no clear-air echoes. Furthermore, during early morning and evening, Doppler velocity from the echoes may not represent actual wind. 4) Clear-air wind shear detection algorithms may not work accurately due to lack of scatterers under strong wind conditions, because the clear-air echo frequency decreased considerably with surface wind speed exceeding 6ms-1. To obtain reliable information from clear-air echoes, it is necessary to monitor boundary layer conditions such as temperature lapse rates and surface wind speeds, simultaneously with radar observations.
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