Evaluation of Ceiling and Visibility Prediction: Preliminary Results over California using the Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS)

A ceiling and visibility (C&V) algorithm developed at the National Center for Atmospheric Research (NCAR) is examined during a six-month period using the Coupled Ocean Atmosphere Modeling Prediction System (COAMPS). Consecutive 12-hour simulations were run using a triply nested (81, 27, 9km) lambert conic conformal projection over the California coast beginning in July 1999 and ending 31 December 1999. During this time period, a qualitative comparison was made between visible satellite imagery and the ceiling height forecasts of the highest resolution nest at the 6th (18Z) and 12th (00Z) forecast hours. A quantitative comparison was made with 3 hourly METARs at Monterey (KMRY), San Francisco (KSFO), Los Angeles (KLAX), San Luis Obispo (KSBP), and Bakersfield (KBFL). The temperature, dewpoint, wind speed, wind direction, ceiling height, and visibility of each METAR were compared with the nearest grid point in the 9 km grid.

COAMPS and the C&V algorithms were found to compare favorably with the visible satellite imagery. In many cases, the structure of the coastal stratus was well represented along the coast, and the periodicity of the cloud coverage was well captured by the model. In contrast, statistical comparisons with the individual stations suggest that the model and algorithms performed very poorly. Occurences of low ceiling and low visibility are underpredicted by the algorithm at KMRY, KSBP, and KLAX and overpredicted at KSFO. An examination of the grid points around these stations, however, shows that in over 50% of the cases in which the model "misses" predicting low ceiling heights, it misses by only a couple grid points (< 20 km). Other possible explanations for the inaccurate forecasts include a lack of aerosol information in the C&V algorithms, terrain misrepresentation within the model where the coastal mountain ranges are smoothed beyond the shoreline, and interpretation of the Automated Surface Observing System (ASOS) ceiling measurements.