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

TROPOSPHERIC WINDS MEASURED WITH THE PIURA ST RADAR:NORMAL VS. “EL NIÑO 1997-98” CONDITIONSLuis A. Flores 1 , Jacqueline La Madrid 1 , Jorge L. Chau 21 Laboratorio de Física, Universidad de Piura, Apartado 353, Piura, Peru2 Radio Observatorio de Jicamarca, Instituto Geofisico del Perú, Apartado 3747, Lima, PeruAbstract. The Piura ST (Stratosphere-Troposphere) radar was installed by NOAA AeronomyLaboratory in 1989 as part of a Trans-Pacific network of wind profilers to study the loweratmospheric dynamics associated with El Niño phenomenon (Gage et al., 1990). In this workwe summarize the statistical characteristics of the horizontal winds observed over Piurabetween 1991 and 2002. The behaviour of the horizontal wind components are shown anddiscussed as a function of their altitudinal, diurnal and seasonal characteristics. The analysishas been performed for both, normal conditions (periods without El Niño) as well as, for thestrong 1997-98 El Niño event. In addition, the horizontal wind characteristics are shown forthe two seasons prior to the El Niño 1997-98 event. Our results suggest the existence of twopotential precursors in the upper tropospheric horizontal winds when a strong event isinvolved.1. Piura VHF Radar.The Piura VHF radar is located in northern Peru (05°10'S, 80° 38'W) approximately 60 km towest of the Andes. It is a ST (Stratosphere, Troposphere) radar. This radar was installed in1989 measuring at the beginning just the vertical wind (Ragaini and Rodríguez, 1992). InFebruary of 1991 the radar system was upgraded with oblique rays (14°S and 14°E) allowingthe measurement of the horizontal wind components. This radar uses a COCO antenna arrayof 100m x 100m (Judasz et al., 1987). More details of the system can be found in Gage et al.[1991].2. Methodology and statistical analysis.We have analysed wind information from February, 1991 to December, 2002 to obtain theseasonal and diurnal characteristics of the horizontal velocity. The seasons considered are (a)Austral summer (21 Dec-20 Mar), (b) Austral autumn (21 Mar-20 Jun), (c) Austral winter (21Jun-22 Sep), and (d) Austral spring (23 Sep-20 Dec). The wind convention is as follows:positive to the east and north for the zonal and meridional wind, respectively.Distribution of frequency We have generated and analysed histograms for the speedcorresponding to several years and to several heights. The characteristics and distribution offrequency are studied for annual periods. Thus we have determined the arithmetic mean,mode, variance, standard deviation, coefficient of asymmetry and coefficient of Kurtosis ofthe horizontal components of wind. Similar analysis we can see in Nastrom et al., 1996.The data collection of zonal and meridional velocity follow a normal distribution. In figures1a and 1b we show histograms of zonal and meridional velocities observed in 1993. Similarresults were obtained for other periods and altitudes.326