Proceedings with Extended Abstracts (single PDF file) - Radio ...

WIND PROFILER AND TOWER OBSERVATIONS OF A GRAVITY CURRENT ANDA RELATED SOLITARY WAVEAhoro Adachi 1,2 , Wallace L. Clark 2 , Kenneth S. Gage 2 and Takahisa Kobayashi 11Meteorological Research Institute, 1-1 Nagamine Tsukuba, Japan2NOAA/Aeronomy Laboratory, 325 Broadway Boulder, Colorado, USA1. IntroductionA UHF wind profiler was originally developed at the NOAA Aeronomy Laboratory (Ecklundet al. 1988) to provide continuous, high-resolution wind measurements in the first few kilometersof the atmosphere. The accuracy of profilers in measuring horizontal components of windhas been estimated by the comparison with radio sondes and/or towers. However, there are fewstudies on estimating the accuracy of profilers in measuring the vertical component, although ithas a significant influence on the estimation of horizontal wind (Strauch et al. 1987). One of thereasons is that the vertical airflow is not horizontally homogeneous due to convection and/orplumes, which make the comparison difficult. Moreover, the magnitude of the vertical airflowis usually much smaller than that of horizontal airflow and is easily masked by ground clutter.The measurement of vertical airflow is, however, essential to understand the dynamics of mesoscalephenomena, such as mountain wave, gravity current, bore, and the MCS.Gossard et al. (1998) proposed the minimizing the variance of the differences (MVD) methodto estimate vertical component of wind from four oblique radial velocities, which are less sensitiveto ground clutter. They showed that the MVD method can improve the accuracy of RASS.The accuracy of vertical component with this method is, however, still unknown. We, therefore,estimate the accuracy of the MVD method by comparing with a collocated tower at the periodwhen a gravity current passed over our site with strong vertical air motion on Dec. 30, 1997.2982. MethodThe meteorological tower at the MRI (Fig. 1)is 213 m in height and equipped with sonic anemometersthat measure wind vectors includingthe vertical components. We used the data recordedat 200 m in height for the comparison.The data were averaged to 2 minutes to matchwith the profiler observation. Our wind profileris located about 300 m north of the tower. Theprofiler was operated in high and low mode. Thebeam sequence was, Vx, SE, SW, NW, NE, Vy,Vxh, SEh and SWh, where h indicates the highmode. We use only the low mode here. The configurationand operation parameters are summarizedin Table 1.The MVD method determines the vertical airmotion that minimizes the variance between thefour horizontal velocity components calculatedfrom four oblique radial velocities. One of theadvantages of this technique is that it does notrely on vertical beam observation, which is noisydue to clutter. This technique is, however, notavailable in rain.Figure 2 shows the time series trace of verticalairflow derived from the tower and the profiler.We plotted the vertical airflow estimated fromthe vertical beam (hereafter referred as VTB) fora reference. The vertical components whose absolutevalues are larger than 0.6 m s -1 are removedfrom the VTB measurement. The difference intime that arises from the distance between thetower and profiler is compensated by using thefront propagation velocity of the gravity current.Tower (213m)WP300mALOMRIRadioSondeFig. 1. Picture of Meteorological Research Instituteand Aerological Observatory with theirobservational insturuments. WP indicates thelocation of the wind profiler.Frequency1.3575 GHzPeak Power500 WBeam Width 6°Beam elevation 90° and 74.5°Pulth width400 nsFirst Range Gate150 mGate Spacing60 mInter Pulse Period20000 nsTable 1. Parameters of the wind profiler.