This is the largest-scale study on clear-air radar echoes in boundary layers in Japan.
Clear-air echoes observed by sensitive Doppler radars are increasingly being used as "tracers" to monitor kinematic details of clear-air winds in boundary layers. The purpose of this paper is to examine a) the origin and b) the climatology of clear-air echoes appearances in Japan, in order to obtain basic information to better use of them.
a) The origin
Direct observations by a balloon-mounted sampler at 300-500m altitudes were performed for five days, together with simultaneous radar observations in Tsukuba. Radar reflectivity values of clear-air echoes were more than 0dBZ and typically 10dBZ-20dBZ, which indicate that the origin is not consistent with Bragg scattering, but with particulate scattering. Various kinds of aerial plankton were sampled in clear-air echoes such as small flies which typically 1mm lengths with maxima as large as 3mm. Only few insects were sampled in a no-echo day.
These results suggest that clear-air echoes are attributable to weakly flying insects that drift with the wind.
b) The climatology of clear-air echo appearances
We used data of the operational Doppler Radar in Narita International Airport, from 16th June to 30th November 1997. During this period, there were 141 days that clear-air echoes were observed. Various statistical characteristics of these echoes have been examined.
(1) The spatial distribution: Clear-air echoes were usually observed over land, together with the absence of over ocean except short distances on strong offshore wind conditions.
(2) The seasonal variation: Between mid-June and mid-November, clear-air echoes were observed constantly, and there was no strong seasonal dependence. These echoes never appeared after mid- November.
(3) The dairy variation: Between mid-June and mid-November, the most prominent dairy characteristic is "the double peaks". The first peak was diurnal, which rose at the sunrise and faded at the sunset. The second one appeared during twilight after the sunset.
(4) The relations to surface wind speeds: There was apparent correlation between clear-air echo appearances and surface wind speeds. In precipitation-free areas, almost of all surface wind speeds were below 6 m/s when clear-air echoes appeared.