THE SUMMERTIME LOW-LEVEL JET OVER THE GULF OF CALIFORNIA

Special pilot balloon and aircraft observations made during the 1990 S outhwest Area Monsoon Project (SWAMP-90) are used to describe the stru cture of a low-level jet (LLJ) observed in the southerly flow over the Gulf of California and surroundings. Mean wind fields based on the 35 days of pilot balloon observations reveal an LLJ strongest over the n orthern Gulf of California, with southerly flow approximately parallel to the axis of the gulf The diurnal variation of the mean wind fields is also shown; there is upslope flow, away from the gulf, in the afte rnoon and downslope flow, toward the gulf, in the morning. Over land, the LLJ is strongest at Yuma, Arizona, where it shows a strong diurnal variation. Here the morning maximum wind speeds in the jet reach 20 m s(-1) and are strongest approximately 300-600 m above the surface. Th e low-level flow undergoes synoptic timescale fluctuations in intensit y, but the jet is present at Yuma on about 75% of the mornings during SWAMP-90. SWAMP aircraft observations show the altitude of the maximum winds to be nearly constant over the Gulf of California (approximatel y 300 m above the surface), and the jet is present on days with widely varying synoptic flow regimes.