Amdar Measurements of Wind Reversals Near San Francisco Bay

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, Oakland International Airport, and San Jose International Airport have increased dramatically to as many as 80 per day in 2015. This study uses AMDAR measurements from 2001-2015 in the bay area and focuses on two main objectives: (1) understanding the AMDAR climatology of the lower atmosphere in the bay area, and (2) examining the effectiveness of AMDAR data to identify and quantify precursors to wind reversals along the central California coast. Wind reversals occur during the warm season when the normal northerly surface wind along the coast is periodically interrupted (~1-2 times a month) by southerly winds that are often accompanied by fog or low clouds which can greatly impact land, sea, and air travel. Prior to initiation of a wind reversal, low and mid-level offshore flow is typically present and previous studies have used reanalysis data to link the strength and duration of the offshore flow to the type of wind reversal (e.g., propagating or non-propagating). A limiting factor in past studies of wind reversals was the lack of long-term monitoring of the lower atmosphere. Low-level features in the coastal margins are difficult to simulate, so AMDAR opens up new possibilities to investigate wind reversals. Using AMDAR, a climatological analysis of all wind reversal events from 2001-2015 is performed to better quantify precursors of the wind reversal in central California, which include pronounced changes to the boundary layer depth and the magnitude and duration of the antecedent offshore flow above the boundary layer. Quantifying these key features will provide past analogs that can be used to help forecast the occurrence and type of wind reversal.