Assessment of the operational Doppler radar systems in Japan as a research tool for monitoring clear-air circulations Kenichi Kusunoki Meteorological Research Institute Published in J. Atmos. Oceanic. Technol.,19,

BACKGROUND Operational C-band Doppler radar systems have been widely used in Japan for meteorological applications. Wind shear detection algorithms that operate on the DRAW are tuned to primarily recognize microbursts and gust fronts, however, the detection of the wind shear in a precipitation free environment has been ignored. DRAW Doppler Radar for Airport Weather

Radar data assimilation scheme will be run as part of the operational system, therefore, it will become important to monitor pre-storm wind fields within the clear-air boundary layer. It is planned that the weather surveillance radar network in Japan will have a Doppler capability. Tokyo(2006) Niigata(2007) Nagoya(2007) Sendai(2007)

OBJECTIVE ･ To examine the potential of clear-air echoes for operational Doppler radar systems in Japan. Clear-air echoes have been produces by small insects carried aloft in the air currents (Hardy and Ottersten 1969; Schaefer 1976).

Clear-air echo example Horizontal Convective rolls

Tokyo Bay Clear-air echo example Doughnut-shaped sea breeze front around the Tokyo Bay.

a: Mymaridae (Parastic wasps) a b Sept at 500m AGL Jul at 500m AGL b: Linyphiidae (Aeronaustic spiders)

DATA SOURCES & STUDY AREA Kanto Plain Narita DRAW STUDY PERIOD From July to December 1997 Clear-air echoes: 141 days Narita DRAW (Doppler Radar for Airport Weather)

Kanto Plain Pacific Ocean Narita DRAW (Doppler Radar for Airport Weather) 0 50km

Spatial distribution Clear-air echo areas observed throughout September 1997 are superimposed. The figure indicates the distribution of the echoes ending at the coastline, and a distinct contrast was seen between clear-air echoes observed over land and their absence over the ocean. RESULTS km

The limitations and time schedules concerning routine operational use of clear-air echoes. ▸ 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. The land is the source area of the terrestrial insects and that there are no or few insects over the ocean. LAND OCEAN

Seasonal variation Clear-air echo frequency Temperature (ºC) From July to late November, clear-air echoes were observed on 40%-80% of no-precipitation days. But clear-air echoes never appeared after late November. No clear-air echo

Frequency Surface temperature (ºC) Jul Aug Sept Oct Nov Dec 40%-80% The drop-off in clear-air echo appearances was associated with the trend in daily average surface temperatures dipping below 10ºC. This shows that the warm condition during the NEP did not produce clear-air echoes under the same temperature condition before the NEP. 10ºC No echo period (NEP) Daily average Trend The 10ºC temperature has been critical for insect activity (Wilson et al.1994). While clear-air echoes appeared below 5ºC before the NEP, they never appeared even above 20ºC during the NEP.

Frequency Temperature (ºC) Jul Aug Sept Oct Nov Dec 40%-80% 10ºC No echo period (NEP) This seasonal nature would not implicate surface temperatures directly but the annual life cycles of insects.

The limitations and time schedules concerning routine operational use of clear-air echoes. ▸ 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 10C. During NEP, there is no clear-air echo available for monitoring clear-air circulations.

Diurnal variation The hourly totals of clear-air echo and no echo for each month. Clear-air echo No echo sunrise sunset

The diurnal variation of clear-air echo appearances had two peaks at the periods of the daytime and the twilight. The first peak was in the daytime, starting after sunrise and fading before sunset, while the second one appeared during twilight after sunset for a short time. sunrise sunset Frequency

These two peaks suggest that different types of insect respectively favor daytime and twilight conditions for their activities (Drake and Farrow 1988). Small insects would largely depend upon carrying aloft by thermals in the mixed layer (Drake and Farrow 1988). Nocturnal migrations of strong-flying insects have been observed (e.g., Drake and Farrow 1985; Drake 1985; Vaughn 1985). Nocturnal migrations usually commence at twilight and insects can attain altitudes of m in less than 1h in the stable layer without thermal (Drake and Farrow 1988). The assumption that the clear-air echoes (insects) would be passively carried by wind is not appropriate. Doppler velocity from the echoes may not represent actual wind.

The limitations and time schedules concerning routine operational use of clear-air echoes. ▸ 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.

Relationships of clear-air appearances to surface weather conditions The clear-air echoes scarcely appeared where surface wind speeds exceeded 6 ms -1. The hourly distribution of surface wind speed in Aug 1997.

The limitations and time schedules concerning routine operational use of clear-air echoes. ▸ 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. According to investigations carried by several entomologists, some types of AB have the habit of suspending their takeoff from the surface when wind speeds exceed several m s -1 (insects: Brodsky 1994; spiders: Okuma1974; Yoshikura 1981). < 6ms -1 > 6ms -1 light wind strong wind

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. CONCLUSION

For Routine monitoring clear-air boundary layers # Spatial distribution Clear-air information is not available in the oceanic sector of the radar coverage area. # Annual schedule -- no clear-air echo available after late November. # Daily timetable -- should be limited to almost daytime. -- poor/no clear-air echoes in nighttime boundary layer. -- during early evening, Doppler velocity from the echoes may not represent actual wind. # Clear-air wind shear detection algorithms may not work accurately due to lack of scatterers under strong wind conditions (surface wind speed exceeding 6ms-1).

To obtain reliable information from clear-air echoes, it is necessary to monitor boundary layer conditions such as surface wind speeds, simultaneously with radar observations.

Kanto Plain Pacific Ocean 1) The Kanto plain, which is roughly 100 km × 100 km, is the largest plain in Japan and is not limited by a mountainous area except on the north and west sides. 3) Several researchers have pointed out that clearair echoes appear over land but seldom over large bodies of water (e.g., at Cape Canaveral, Florida; Wilson et al. 1994). 2) The plain includes the Tokyo metropolitan area, which is heavily urbanized and has the busiest commercial airports. Clear-air wind information such as wind shear detection would be required the most in this area. Narita DRAW (Doppler Radar for Airport Weather) 0 50km