THE INFLUENCE OF INSTRUMENTATION, SITING, EXPOSURE HEIGHT, AND TEMPORAL AVERAGING METHODOLOGY ON METEOROLOGICAL MEASUREMENTS FROM SJVAQS AUSPEX/

To be aware of the influence that istrumentation, siting, data process ing, and sensor exposure have upon the measurements. measurements. A u nique opportunity arose to study this influence when in summer 1990, a s part of the collaborative San Joaquin Valley Air Quality Study/Atmos pheric Utility Signatures, Predictions and Experiments study (SJVAQS/A USPEX), a vast meteorological measurement network was established for the central one-third of the state of California. Within the constrain ts of available resources, a variety of instruments, sites, site densi ties, sensor exposures, and data processing techniques were involved i n the study. Based on the dataset from SJVAQS/AUSPEX, this paper was w ritten to identify and quantify some common measurement influences for the benefit of users of the SJVAQS/AUSPEX data as well as the designe rs and users of other networks and datasets. To characterize and quant ify measurement influences, comparisons were made of data associated w ith different instrument types, sites, measurement heights, and tempor al averaging methodologies. Comparative statistics were then developed and summarized. Little variability existed among the characteristics of surface instrumentation, but sounding instrumentation varied greatl y in the degree to which atmospheric vertical structure was resolved. Vertical sampling resolutions ranged from 4.5 to 300 m, and winds were averaged over layer depths ranging from 15 to 600 m. The spatial loca tion of measurements had a great influence on the measured direction o f the wind. Hourly wind directions at neighboring surface sites differ ed by an average of 18 degrees over the network with an rms variabilit y of 55 degrees. Upper-air values of these same statistics were 12 deg rees and 64 degrees, respectively. A functional dependence of wind var iability on site separation distance was apparent from the data. The i nfluence of sensor exposure height on wind speed measurements, when co mparing neighboring sites with different measurement heights, was less than expected. The average ratio in wind speed for such site pairs wa s only 1.18 (1.06 by day and 1.34 at night) for 10-m versus 2-m measur ements. Temporal averaging methodology greatly influenced hourly wind values, especially for wind speeds less than 5 m s(-1). Large differen ces were observed in hourly wind speed averages from scalar versus vec tor averaging, as well as in hourly wind direction averages from resul tant versus unit vector averaging.