Climatology of the San Francisco Bay and the Initiation of Wind Reversals along the Western United States Coast Determined from AMDAR Data

Abstract

Observations from commercial aircraft through the Aircraft Meteorological Data Relay (AMDAR) automated weather reports provide a higher frequency sampling of the lower atmosphere than the twice daily radiosonde launches performed by the National Weather Service. In the San Francisco Bay area, the number of profiles from flights arriving or departing San Francisco International Airport (SFO), Oakland International Airport (OAK), and San Jose International Airport (SJC) have increased dramatically from 2001 to 2016. Low-level features in the coastal margins are difficult to simulate, so AMDAR opens up new possibilities to investigate coastal phenomena. This study uses AMDAR measurements from 2001-2016 in the bay area and focuses on three main objectives: (1) understanding the AMDAR climatology of the lower atmosphere in the bay area, (2) examining the effectiveness of AMDAR data to identify and quantify precursors to wind reversals along the central California coast, and (3) use the quantified magnitudes of the precursors to forecast wind reversals. A limiting factor in past studies of wind reversals was the lack of long-term monitoring of the lower atmosphere. While soundings from the aircraft at OAK and SFO were similar and more influenced by the marine environment, SJC had more continental features. Significant anomalies of temperature and wind occurred more than 24 hours ahead of the passage of a wind reversal. A forecast metric was developed using the anomalies, but the metric was not skillful.​