Results from our 1994 flights defined the structure of one propagating, coastally trapped "southerly surge" event. We found that it was not a simple two-layer wavelike disturbance, nor was it a two-layer gravity current. The June 10-11, 1994 event was observed to have an overall wavelike character with an internal surface boundary layer imbedded within it, in which a very-near-surface cloud layer (or fog layer) formed. Vertical sections show that the airflow and stratification were clearly more complex than a two-layer system. Results from this work have appeared in a technical/data report [Bane et al., 1995] and in two articles [Ralph et al., 1995; Dorman et al., 1996], and a third article is soon to be submitted [Ralph et al., 1996].
The data set we obtained in 1996 captured five southerly wind episodes, each one different from the other, and all somewhat different from the June 10-11, 1994, case. Three of the 1996 episodes had a surge component, one event was expected to produce a southerly surge but did not, and one event was a transition to synoptic scale southerlies without a surge component.
The four southerly surge cases that we now have measured include two surges between Point Conception and Point Reyes (June 1994 and June 1996; these are referred to as the "southern cases") and two surges between Point Reyes and Cape Blanco (July and September 1996; these are referred to as the "northern cases"). Temperature and velocity sections from these surges suggest that each has a southerly flow leading edge above the MBL supporting the notion that the complex vertical structure observed in June 1994 is the typical situation as opposed to an isolated occurrence. A correct dynamical understanding must include an explanation of this structure.
Vertical sections of potential temperature and alongshore velocity through the 1996 surges show that the leading edge of the southerly wind flow is about 400 m above the surface, overlying northerly flow. There are distinct differences between the two northern cases and the two southern cases that warrant further investigation, including the greater vertical extent of the southerly airflow in the northern cases vs. the southern cases, and the stronger gradient between the southerlies and the northerlies in the northern cases.
Similarities exist between the structure of the June 1994 surge and that of the June 1996 surge, both of which propagated through the Pt. Conception-to-Pt. Reyes region. The intrusive nature of the southerly flow is apparent in each case, as is the general wavelike character of the inversion topography (i.e. elevated inversion top to the south). Differences include the fact that the southerly jet is more centered in the inversion in the 1994 case, while in the 1996 case the jet is at the transition between the mixed layer and the inversion.
It is apparent from these initial comparisons that the two southern cases look somewhat similar to each other and the two northern cases look somewhat similar to each other, while the northern cases differ from the southern cases in a number of ways. This leads to the question: Are these differences due to locational differences between the northern and southern domains (e.g. differing coastline geometry or terrain), is it something else (e.g. the strength of the synoptic forcing during the northern cases vs. the southern cases), or is it coincidence?
