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Sci.Rep.Fukushima Univ.,No.55 (1995) 13

Observation of Atmospheric Circulation with a Boundary_LayerRadar at Fukushima University

Akira Watanabe

Department of Earth Sciences,Faculty of Education,Fukushfma UniversityFukushlma City,960-12,Japan(Received 10 November,1994)

Abstract

A boundary-layer radar is installed at Fukushima University on March 23, 1994.The radar is developed by Kyoto University (Fukao et at.,1994; Hashiguchi et at., 1994) and the Mitsubishi Electrical Company in 1991.This is a small radar operating at a frequency of 1357.5 MHZ (L_band)with peak transmitter power of l kW.It is designed to receive backscattered echoes from refractive index fluctuations,which are mainly generated by the fluctuations of humidity and atmospheric density profiles associated with atmospheric turbulence.The radar has time and height resolutions of about 50 sec and 100m,respectively.This paper describes the first striking meteorological results obtained with this radar system on snow days.

1. Introduction

The boundary layer is recognized as an important height range in which the free atmosphere is connected with the earth's surface and almost all the sources of energy, momentum,and constituents of the earth's atmosphere are distributed.However,mainly because of lack of appropriate observational techniques,the boundary layer has not yet been clarified sufficiently.The height range of the boundary layer is beyond that of

continuous observations at a tower,and the complexity of the boundary layer cannot be resolved by bal1oon observations.Those observational difficulties are now being overcome

by the advance of radio-probing techniques,called low_level wind profilers or boundary_ layer radars.Hashiguchi et ai. (1994)describes in detail several techniques for boundary_layer radar

observation . We can observe directly three-dimensional wind velocity averaged over a sampling volume (5knf in horizontal and 100m in v,ertica1) with a high time_resolution (50 sec).This paper describes the structure of snow_fail ceils observed with this radar on March 25,1994 at Fukushima University.

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14 A.Watanabe Observation of atmospheric circulation with a BLR at Fukushima Univ

The boundary-layer radar The specifications of the radar are

summarized in Table 1. This is a small radar operating at a frequency of 1357. 5 MHZ(L-band)with peak transmit power of l kW.The antenna consist of three parabolic antennas with diameters of 2_ 04 m, which are directed upward, northwestward,and southwestward with zenith angles of 15°.

Figure l shows a block diagram of the boundary-layer radar.The system of the radar consists of an antenna unit, transmitter unit , receiver unit , data acquisition unit, and signal processing unit_ In the observations using the boundary_layer radar,the radio-frequency signal amplified in the transmitter unit is radiated from a parabolic antenna through an SF cable . The weak signal

Table

at Fukushima University1 Specifications of the boundary-layerat Fukushima University.

radar

Transmi tter uf11t Receiver unit Data Acquisition unitFig.1 Block diagram of the boundary-layer radar at Fukushima University

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Sci.Rep.Fukushima Univ.,No.55 (1995) 15

scattered by atmospheric turbulence is gathered by the antenna and is transferred to the receiver unit.The received signal is amplified,detected,and converted te a video signal in the receiver unit.The video signal is transferred to the data acquisition unit,and is

converted te a digital signa1.The digital signal is transferred to the signal processing unit. The I and Q video signals ted from the receiver unit are converted te a digital signal by an analog-to-digital converter (ADC).The digital signal is coherently integrated by a coherent integrator,and stored in the memory unit.The numerical expressions of the coherent integrator and the ADC are 16and 12bits,respectively . Therefore,the number of coherent integrations is limited to t6 times, in order not to lose any bit ot information. When we need coherent integrations of more than t6 times,additional integrations over 16 times should be performed in the signal processing unit.The capacity of the memory unit is 8 Mbytes for one beam,i.e. 24Mbytes for all three beams . For example,when an observation uses FFT points of 512 and the number of incoherent integrations of 32, the necessary memory size is exactly 8 Mbytes for one beam . The stored data are transferred to the signal processing unit.

The signals received by the boundary-layer radar consist of echoes due to atmospheric scattering,noise,and ground clutter . The noise is separated into two elements:the sky noise due to cosmic rays,solar and atmospheric radiation,and the noise generated by the receiver itself. For the frequency of 1357.5 MHZ,the sky-noise temperature is less than 100K,and the receiver - noise temperature is about 670K.The sky noise can be neglected as compared with the receiver noise_The spectrum of the receiver noise is much wider than that of the atmospheric scattering signal and can be regarded as a white noise in Doppler spectra . The ground clutter is an echo scattered by mountains,forests,the sea surface,and others within the side-lobe of an antenna . If the target does not move,the ground clutter appears at the zero Doppler component of the spectrum.

However,the spectrum of the ground clutter sometimes shows a slight spread because of the fluctuation of the atmospheric refraction or the motion of the ground clutter itself like trees.The Doppler spectrum of the atmospheric echo can be parametrized with three spectral parameters:the echo power,the mean Doppler shift (corresponding to the wind velocity),and the spectral width.A atmospheric turbulence can be assumed to move with a background wind,the mean Doppler shift being regarded as the line-of-sight componet

Vr of the wind velocity.This component is given by

Vr= Cfd/2 fo= スfd/2where fd is the mean Doppler shift [Hz],of the operational frequency [Hz] of the radar, スthe wavelength (22.1clf1),and C the speed of light.If the wind velocity is uniform over the radar,the horizontal component of the wind

velocity along the beam direction Vh is given byVh= (Vr-Vz cos 8)/sin e

where Vz; is the vertical velocity,and e is the zenith angle of the beam direction.

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16 A_Watanabe :Observation of atmospheric circulation with a BLR at Fukushima Univ

3.Observations of snowfall cell with the boundary-layer radar

Figure 2 shows the location of boundary_layer radar. The boundary-layer radar is

located in Fukushima,Japan (37.68°N,140.46°E)between the Ohu mountains and the Abukuma mountainous district and the height is 208.6m above sea levei.The sampling interval of the BLR was 0.67ｵs corresponding to the range of 100m.

Figure 3 shows the fluctuating component ofsoutheastward-vertical wind velocity from Mar。25to Mar.26,1994.During the observation,weaksnowfall was observed at the station . The cellsof snowfall correspond to the relative strongreflection region.The region inclines southeast-ward,and the upper region is observed earlierthan the lower region. The time difference isabout 15 minutes between the upper and lowerregions . The inclination agreed with the meanwind direction,but the southeastward componentof the wind appeared in the center of the

disturbance,and the northwestward component ofwind appeared in the upper region and in frontof the disturbance . It was assumed that the

strong reflective region is flowed by the mean. Fig

wind,the horizonatal scale of the disturbance lsabout 3k確,and the vertical scale is the same.The downdraft wind is observed in front of thedisturbance,and the upstream wind ls observed

2 Geographical features and location ofboundary-layer radar at FukushlmaUniversity (St.)Lnad higher than 500m above sea levelis indicated by l l ,and than t,000mabove sea level indicated by [[mi [[]

in the upper region of the disturbance.The outf1ow is strong in front of the lower region

of the disturbance.The speed is about 6m/sec. The upstream of the upper region is about l m/sec,and the downstream of the lower region is about 0.8m/sec. At the backward of the disturbance ls southeast wind with the upstream component.The structure of the disturbance is the same as that of a convection cell which was indicated Browning et ai. (1976),but the vertical scale is small . Watanabe (1991 a,b,1992) has indicated that a disturbance was formed in winter monsoon by shear instability.However,the present disturbance is not formed by shear instability,but is formed by convective instability.

Figure 4 shows the fluctuation component southwestward-vertical wind velocity at the same period as in Figure 2. The southwestward wind component is perpendicular to the direction of the moving dsiturbance.The perpendicular wind component is small in the disturbance,but the southwestward component wind appears in front of the disturbance. The appearance of this wind indicates a divergence region in front of the disturbance. These wind fields agree with the wind field of convective cells_

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Sci.Rep.Fukushima Univ.,No.55 (19951

25-26 Mar 1994

17

Local Time

Fig.3 Fluctuating component of southeastward-vertical wind velocity,and vertical echo power(shadedregion)

25-26 Mar 1994

Local Time

Fig.4 Fluctuating component of southwestward-vertical wind velocity,and vertical echo power(shadedregion)

4. Discussion and conclusions

A boundary-layer radar has been installed at Fukushima Universlty.We observed a snowfall cell in the winter monsoon by the boundary_layer radar, and analyzed the structure the cel1.The snowfall cell in the monsoon was found to be organized in the same wind field of a severe convective celi.Asai (1970)and Priestley (1962) indicated that

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18 A_Watanabe :Observation of atmospheric circulation with a BLR at Fukushima Univ

a cell is fromed by roll convection,and Watanabe 11991 a,b,1992)indicated that a snowfall cell is formed by shear instability . Although we did not observe the vertical temperature profile,we think that the disturbance of this case is formed by cor1vective instability. However,a relatively strong wind appeared in the upper region of relatively strong reflection . It is possible that the convection cell was formed by shear and convective instability.We can often observe statically stable layer over and under the cloud.We think that wind shear influences the formation of a convective ce11.We wish to observe more disturbances in the monsoon with the boundary-layer radar,and clarify the structure and formation process of the disturbance.

AcknowledgmentsThe author wishes to thank Drs.S_Fukao,M.D_Yamanaka T.Tsuda,and H.Hashiguchi

of the Radio Atmospheric Science Center,Kyoto University,and Dr.A.Sumi,of the Center for Climate System Research,University of Tokyo_They supported the introduction of the boundary_layer radar to Fukushima University.

References

Asai,T.,1970:Three_dimensional features of thermal convection in a piano Couette flow_J_Met.

Soc_Japan,Vol.48, 18-29_

Browning,K.A.,Fankhauser,J_C_,Cha]on,J.P.,Eccles,P_J., Strauch,R.G., Merrem,F_H_,

Musi1,D.J.,May,E.L.and Sand,W.R.,1976:Structure of art evolving hailstorm,V.Synthesis

and implication for hail growth and hail suppression.Mon_Weath.Rev_,Vol. 104, 603-610.Fukao,S.,Hashiguchi,H.,Yamamoto,M.,Tsuda,T.,Sate,T.,Yamanaka,M_D.,Nakamura,T.,

Kate,S., Makihira,T.and Hamatsu,K.,1994:An L-band clear-air Doppler radar for the

atmospheric boundary layer_J.Met.Soc.Japan,submitted.Hashiguchi,H.,Yamanaka,M.D.,Tsuda,T.,Yamamoto,M_,Nakamura,T_,Adachi,T.,Fukao,S_,

Sate,T and Tobjng D_L., 1994:Diurnal variations of the planetary boundary layer observed

with an L_band clear_air Doppler radar_Bound . Layer Met.,submitted.

Priestley,C.H.B.,1962;Width_height ratio of large convection cells_ Telius,Vol. 14, 123-124.

watanabe,A.,1991 a:The structure of disturbances in the winter monsoon_''Peport of MU-rada「Symposium,No.2,52-56.

- ,1991 b :The structure of snowfall ceil.' Tohoku no Yuki to Seikatu, No.6,49- 50.

- ,1992:Observation of a disturbance in the winter monsoon by MU - radar.' Tenki,Vol.32,

652-653_

* in Japanese