Kelvin waves and internal bores in the marine boundary layer inversion and their relationship to coastally trapped wind reversals

Detailed observations of a coastally trapped disturbance, or wind reversal, on 10-11 June 1994 along the California coast provide comprehensive documentation of its structure, based on aircraft, wind profiler, radio acoustic sounding system, and buoy measurements. Unlike the expectations from earlier studies based on limited data, which concluded that the deepening of the marine boundary layer (MBL) was a key factor, the 1994 data show that the perturbation was better characterized as an upward thickening of the inversion capping the MBL. As the event propagated over a site, the reversal in the alongshore wind direction occurred first within the inversion and then 3-4 h later at the surface. A node in the vertical structure (defined here as the altitude of zero vertical displacement) is found just above the inversion base, with up to 200-m upward displacements of isentropic surfaces above the node, and 70-m downward displacements below. Although this is a single event, it is shown that the vertical structure observed is representative of most other coastally trapped wind reversals. This is determined by comparing a composite of the 10-11 June 1994 event, based on measurements at seven buoys, with surface pressure perturbations calculated from aircraft data. These results are compared to the composite of many events. In each case a weak pressure trough occurred between 2.4 and 4.0 h ahead of the surface wind reversal, and the pressure rose by 0.32-0.48 mb between the trough and the wind reversal. The pressure rise results from the cooling caused by the inversion's upward expansion. The propagation and structure of the event are shown to be best characterized as a mixed Kelvin wave-bore propagating within the inversion above the MBL, with the MBL acting as a quasi-rigid lower boundary. If the MBL is instead assumed to respond in unison with the inversion, then the theoretically predicted intrinsic phase speeds significantly exceed the observed intrinsic phase speed. The hybrid nature of the event is indicated by two primary characteristics: 1) the disturbance had a much shallower slope than expected for an internal bore, while at the same time the upward perturbation within the inversion was quasi-permanent rather than sinusoidal, which more closely resembles a bore; and 2) the predicted phase speeds for the "solitary" form of nonlinear Kelvin wave and for an internal bore are both close to the observed intrinsic phase speed

Observations and analysis of the 10-11 June 1994 coastally trapped disturbance

A coastally trapped disturbance (CTD), characterized by southerly flow at the surface on 10-11 June 1994, was observed from the California Bight to Bodega Bay during a field experiment along the California coast. (North-south approximates the coast-parallel direction.) Data from a special observational network of wind profilers, radio acoustic sounding systems, special surface data, balloon ascents, and a research aircraft were used with satellite and synoptic data to explore both the CTD structure and the regional-scale changes before the event. The disruption of the climatological northerly flow along the central California coast, which preconditioned the area for the development of a CTD, began with the eastward movement of a surface high into Washington and Oregon and the amplification of a thermal low in northern California. As with most CTDs in the region, this occurred over the 2-3 days preceding the CTD's initiation. These large-scale changes caused westward advection of warm continental air across much of the California coast, which increased temperatures by 10°-12°C in the layer from 0.4 to 2.0 km above mean sea level (MSL) during the 48 h before southerly flow appeared offshore at the surface. The warming reversed the alongshore sea level pressure gradients near the coast by creating a region of pressure falls extending along 600-1000 km of the coast. This also modified the cross-shore pressure gradient and thus the geostrophic alongshore flow. The warming along the coast also increased the strength of the temperature inversion capping the marine boundary layer (MBL) by a factor of 2-4 over 48 h. The synoptic-scale changes also moved the axis of the climatological near-surface, northerly jet much farther offshore from central California and strengthened this jet near the headlands of Capes Mendocino and Blanco, The development and decay of southerly flow at the surface along the coast coincided roughly with the evolution of a mesoscale low 200 km offshore, and of a coastal ridge roughly 100 km wide. However, the CTD initiation also followed a 500-m thickening of the MBL inversion in the California Bight region where a Catalina eddy was initially present. At surface sites, the CTD was marked by the passage of a pressure trough, followed by a gradual shift to southerly flow and the appearance of clouds. The area of low cloud was not coincident with the region of southerly flow. The transition to southerly flow propagated northward along shore at 1 1.9 ± 0.3 m s-1 on 10 June, stalled for 11-12 h during the part of the diurnal cycle normally characterized by enhanced northerly flow, and then continued propagating northward along shore at 11.6 m s-1. Both the geostrophic wind and the isallobaric component of the ageostrophic wind were consistent with southerly flow at the surface. Southerly flow was observed up to 5 km MSL in this event and in others, which indicates that the synopticscale environment of many CTDs in this region may include a deep tropospheric cyclonic circulation or trough offshore. Both cross-shore and alongshore flights performed by a research aircraft documented the CTD structure and showed that the southerly flow extended at least 100 km offshore and appeared first within the MBL inversion as the inversion thickened upward. While the top of the inversion rose, the height of the inversion's base remained almost unchanged. The thickening of the inversion decreased with distance offshore, and there was no significant change in the MBL depth (i.e., the inversion base height), until 12-14 h after the surface wind shift. Thus, it is suggested that two-layer, shallow water idealizations may be unable to represent this phenomenon adequately. Nonetheless, the gradual wind shift, the thickening inversion, and the correlation between southerly flow and a mesoscale coastal pressure ridge are consistent with a coastally trapped Kelvin wave, albeit one with a higher-order vertical structure that can exist in a two-layer model. However, the semipermanent nature of the changes in the MBL and its inversion is more characteristic of a shallowly sloped internal bore. The temperature increase and lack of southerly flow exceeding the northward phase speed are inconsistent with gravity current behavior

Observations and Analysis of the 10-11 June 1994 Coastally Trapped Disturbance

A coastally trapped disturbance (CTD), characterized by southerly flow at the surface on 10â 11 June 1994, was observed from the California Bight to Bodega Bay during a field experiment along the California coast. (Northâ south approximates the coast-parallel direction.) Data from a special observational network of wind profilers, radio acoustic sounding systems, special surface data, balloon ascents, and a research aircraft were used with satellite and synoptic data to explore both the CTD structure and the regional-scale changes before the event.Both the experimental and analysis work on which this paper is based were partially supported by grants from the Office of Naval Research as part of the Coastal Meteorology Accelerated Research Initiative