VOCAR

Description

The Variability of Coastal Atmospheric Refractivity (VOCAR) experiment occurred in the Southern California Bight during the period 24 August to 3 September 1993.   A more extensive description of VOCAR appears in Paulus (1995) and Burk and Thompson (1997).  Special high-resolution radiosondes were launched every four hours (except local midnight) at eight sites during the VOCAR experiment.  Additional data was collected from three profiler sites, research aircraft flights, a research ship, numerous surface stations, and two radio transmitter-receiver links making this a rare and valuable data set for mesoscale model validation in a coastal environment.  Figure 1 shows the location of the radiosonde sites and the transmission links from San Clemente Island to Pt. Mugu and from San Clemente Island to San Diego.  The broad goal of VOCAR was to observe the mesoscale variability of the microwave refractivity field in a coastal environment, while collecting transmission data along several lengthy over water paths. 

Results

The VOCAR data is used to validate COAMPS modeling capabilities in the coastal environment and provide a test bed for validating the refractivity data fusion methodology.  An example of the forecast performance of COAMPS is shown by the profiles in Fig. 2 of modified refractivity forecasts and verifying soundings at San Nicolas Island during the middle of the VOCAR IOP.  In these forecasts COAMPS predicts the existence and general structure of the elevated duct rather well, but tends to under forecast the height of the duct base and the duct strength and over forecast the trapping layer thickness in all cases but Fig. 2d.  In Figs. 2b-d COAMPS correctly forecasts the existence of surface-based ducting, while in Fig. 2a COAMPS forecasts a slightly elevated duct while the sounding indicates marginal surface-based duct. 

For the period 24-31 August 1993, statistics are presented in Table 1 for a total of 32 forecast/observational sounding pairs at both the San Nicolas and San Clemente Island sites, and 31 pairs at the NPS ship. Here COAMPS shows considerable skill in MABL forecasts during the VOCAR experiment, although it contains biases for the MABL to be somewhat shallow and for the forecast duct strength, measured by the refractivity jump at MABL top, to be too weak. Nevertheless, for a total of 95 forecasts at these sites, COAMPS correctly forecasts surface-based ducting/no surface-based ducting events 82% of the time, with a false alarm rate of only 0.15. This is shown in contingency Table 2 (e.g., Panofsky and Brier 1958) for observed and forecast cases of surface-based ducting and no-surface-based ducting. Forecasts of varying lengths, from as short as 4 h to as long as 24 h, are incorporated into the statistics at each site. If only the statistics for 24 h forecasts at the 3 stations are examined (20 cases), the results are not significantly different. In this instance 80% of the forecasts are correct (16/20) and the false alarm rate is 0.091 (1/11).

References

See the publication Haack and Burk (2001) for a detailed description of this study as well as the other citations mentioned above.