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C. E. Dorman and C. D. Winant

Abstract

The Santa Barbara Channel is a region characterized by coupled interaction between the lower-level atmosphere, the underlying ocean, and the elevated topography of the coastline. The nature of these interactions and the resulting weather patterns vary between summer and winter.

During summer, synoptic winds are largely controlled by the combined effect of the North Pacific anticyclone and the thermal low located over the southwestern United States, resulting in persistent northwesterly winds. A well-defined marine atmospheric boundary layer (MABL) with properties distinct from the free atmosphere above is a conspicuous feature during the summer. The wind has different characteristics in each of three zones. Maximum winds occur in the area extending south and east from Pt. Conception (zone 1), where they initially increase as they turn to follow the coast, then decrease farther east. Winds are usually weak in zone 2, located in the easternmost part of the channel, offshore from the Oxnard plain. Winds are also weak in zone 3, sometimes reversing to easterly at night, in a narrow band located along the mainland coast. Summer air temperature at the surface follows the SST closely and varies significantly with location. Summer sea level pressure gradients are large, with the lowest pressure occurring on the northeast end of the Santa Barbara Channel. Diurnal variations are strongest in summer, although the modulation is weakest in zone 1. The diurnal variation is parallel to the coast in all of zone 3 but the Oxnard plain, where it is perpendicular to the coast. The height of the marine layer varies between 300 m in late afternoon and 350 m in late morning.

In winter, synoptic conditions are driven by traveling cyclones and sometimes accompanied by fronts. These are usually preceded by strong southeast winds and followed by strong northwest winds. Atmospheric parameters are distributed more uniformly than in summer, and diurnal variations are greatly reduced. Sea level air temperature and pressure are more spatially uniform than in the summer.

Spatial variations in the observed fields in the summer are consistent with a hydraulic model of the MABL as a transcritical expansion fan. The summertime situation is governed by a coupled interaction between the atmosphere and the underlying water. The ocean influences the density of the MABL to the extent that it behaves distinctly from the free atmosphere above, resulting in strong winds polarized in the direction parallel to the coast. In turn, these winds provoke an upwelling response in the coastal ocean, which in part determines the surface properties of the water.

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C. E. Dorman, A. G. Enriquez, and C. A. Friehe

Abstract

The structure of the lower atmosphere over the northern California coastal ocean upwelling area was studied during the Shelf Mixed Layer Experiment in the winter of 1989. Surface data were collected at seven automated coastal stations and six buoys. Boundary layer soundings were made using balloons at the coast and a research aircraft over the ocean. The aircraft was also used to map the low-level (30 m) mean and flux fields over the 80 km × 120 km shelf area.

The wintertime coastal weather conditions were more variable than in summer and were observed to fit into three categories: strong northerly (downcoast) winds, strong southerly (upcoast) winds, and weak winds. The variability was caused by the passage of wintertime cyclones interspersed with periods of small pressure gradients. The strong wind cases had small diurnal variations, whereas the diurnal variations were large for the weak wind case.

The vertical structure of the coastal boundary layer was more uniform compared to that in summer, with weak or nonexistent temperature inversions. Winds below 600 m were not correlated with those above 1.5 km except during strong alongshore winds. The presence of a coastal mountain ridge suppresses low-level cross-shore flow. The horizontal structure over the ocean shelf measured by the low-level aircraft tracks showed an area of large positive wind stress curl [over 1 Pa (100 km)−1] west of Point Arena for both directions of the strong wind cases. This implies positive Ekman pumping of the shelf waters in this area regardless of wind direction.

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C. E. Dorman, E. Enriques, and C. F. Friehe

Abstract

No abstract available

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C. E. Dorman, T. Holt, D. P. Rogers, and K. Edwards

Abstract

Data from surface stations, profilers, long-range aircraft surveys, and satellites were used to characterize the large-scale structure of the marine boundary layer off of California and Oregon during June and July 1996. To supplement these observations, June–July 1996 averages of meteorological fields from the U.S. Navy’s operational Coupled Ocean–Atmospheric Mesoscale Prediction System (COAMPS) model were generated for the region. Model calculations show a broad band of fast northerly surface winds exceeding 7 m s−1 extending along the California–Oregon coast. Buoy-measured peaks of 7.1 m s−1 off Bodega Bay, 7.2 m s−1 off Point Piedras Blancas, and 8.8 m s−1 near Point Conception were reported. Mean winds at the buoys located 15–25 km offshore are generally faster than those at coastal stations, and all station winds are faster in the afternoon.

The aircraft and station observations confirm that an air temperature inversion typically marks the top of the marine boundary layer, which deepens offshore. Along the coast, the marine boundary layer thins between Cape Blanco and Santa Barbara. The inversion base height is at its lowest (195 m) at Bodega Bay in northern California and at its highest at Los Angeles and San Diego (416 m). The inversion strength is strongest between Bodega Bay and Point Piedras Blancas, exceeding 10.8°C. The June–July 1996 marine boundary layer depth from COAMPS shows a gradual deepening with distance offshore.

The model-averaged flow within the marine boundary layer is supercritical (Froude number > 1) in a region between San Francisco and Cape Mendocino that extends offshore to 126.4°W. Smaller isolated supercritical areas occur in the lee of every major cape, with the peak Froude number of 1.3 in the lee of Cape Mendocino. This is consistent with aircraft flights of Coastal Waves ’96, when extensive regions of supercritical flow off central California and downwind of major capes were recorded with highest Froude numbers around 1.5–2.0. A broad, wedge-shaped area of nearly critical flow (Froude number > 0.8) extends from Cape Blanco to Point Piedras Blancas and offshore to about 128.5°W in the model output.

The model wind stress has a broad maximum exceeding 0.3 N m−2 between Cape Mendocino and San Francisco with the highest values found within 100 km of the coast. Stress calculated directly from low aircraft legs is highest in the lee of large capes with peak values exceeding 0.7 N m−2. Overall aircraft magnitudes are similar to the model’s, but a direct comparison with the 2-month average from the model is not possible due to the lesser space and time coverage of the flights. The stress maxima along the California coast shown in the model results are spatially consistent with the region of coldest sea surface temperature observed by satellite.

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Clive E. Dorman, C. A. Paulson, and W. H. Quinn

Abstract

Meteorological and oceanographic data for Ocean Station Vessel N (30N, 14OW) are analyzed over 20 years (1951–70) and 7 years (1964–70), respectively. A rainfall estimate is constructed for the 20-year period. The yearly average rainfall is 23 cm, far less than existing estimates. Daily and seasonal variations are presented. Heat budgets of the surface show that the two decades (1951–60, 1961–70) are distinctly different. Anomalies of the 20 years of all meteorological variables are calculated. The pressure anomaly appears to be loosely correlated with anomalous large-scale events in the equatorial Pacific. Time series cross sections are shown of the mixed layer depth, bottle temperature and salinity. The near-surface density appears to be largely controlled by temperature.

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C. D. Winant, C. E. Dorman, C. A. Friehe, and R. C. Beardsley

Abstract

During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which occurs with generally northerly winds, is characterized by a strong low-level inversion and the spatial structure of the surface wind is correlated with the coastal topography. The inversion acts as a material interface, and the marine layer behaves as a supercritical channel flow, when the Froude number is greater than one: oblique expansion waves and hydraulic jumps, associated with changes in the orientation of the coastline, account for the observed spatial structure of the flow. Observations from mid-latitudes on the eastern side of other ocean basins suggest that similar supercritical conditions in the marine layer may prevail there also.

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C. E. Dorman, D. P. Rogers, W. Nuss, and W. T. Thompson

Abstract

An instrumented C-130 aircraft flew over water around Point Sur, California, on 17 June 1996 under strong northwest wind conditions and a strong marine inversion. Patterns were flown from 30- to 1200-m elevation and up to 120 km offshore. Nearshore, marine air accelerated past Point Sur, reaching a surface maximum of 17 m s−1 in the lee. Winds measured over water in and above the marine layer were alongshore with no significant cross-shore flow. Sea level pressure, 10-m air temperature, and air temperature inversion base generally decreased toward the coast and were an absolute minimum just downcoast of the wind speed maximum. The sea surface temperature also decreased toward the coast, but was an absolute minimum directly off Point Sur. The near-coast, air temperature inversion base height was 400 m north of Point Sur, decreased to a minimum of 50 m in the lee of Point Sur, then increased farther to the south. Wind speeds were at a maximum centered along the air temperature inversion base; the fastest was 27 m s−1 in the lee of Point Sur.

Using a Froude number calculation that includes the lower half of the capping layer, the marine layer in the area is determined to have been supercritical. Most of the marine layer had Froude numbers between 1.0 and 2.0 with the extreme range of 0.8–2.8. Temperatures in the air temperature inversion in the lee were substantially greater than elsewhere, modifying the surface pressure gradient. The overall structure was a hydraulic supercritical expansion fan in the lee of Point Sur under the influence of rotation and surface friction.

The Naval Research Laboratory nonhydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) indicated a broad, supercritical marine boundary layer moving to the south along central California and Point Sur during the aircraft flight. The marine boundary layer thinned and accelerated into the lee of Point Sur, which was the site of the fastest sea level wind speed along central California. Isotherms dip and speeds decreased in the lee of Point Sur in the capping inversion well above the marine layer. COAMPS forecasted a compression shock wave initiating off the upwind side of the topography behind Point Sur and other coastal points to the north. Evidence from the model and the aircraft supports the existence of an oblique hydraulic jump on the north side of Point Sur.

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C. E. Dorman, L. Armi, J. M. Bane, and D. P. Rogers

Abstract

A midlevel, coastally trapped atmospheric event occurred along the California coast 10–11 June 1994. This feature reversed the surface wind field along the coast in a northerly phase progression. Along the central California coast, the winds at the coastal stations reverse before the corresponding coastal buoy offshore, then followed hours later by passage of the leading edge of an overcast stratus cloud. The sea surface temperature was much colder in the narrow strip along the coast. The cloud characteristics may be accounted for by a sea surface mixed layer (SSML) model beginning with the wind reversal and growing with the square root of time. Heat is lost from the SSML to the sea surface. A cloud forms when the air temperature at the top of the SSML is equal to the dewpoint. It is suggested that a bore develops on the top of the SSML, increasing the thickness of the SSML and the progression speed of the cloud to 8 m s−1. There is evidence that an undular bore with a leading cloud develops in the thinner inshore SSML.

Advancing beyond Monterey Bay, horizontal density contrast is believed to have caused the bore to change character to a gravity current with a narrower cloud that passed a point inshore before the winds reversed at the buoys. The last trace of a disturbed boundary layer ended at Point Arena where strong northerly winds prevented any further northerly progression and contributed to a cyclonic eddy that was formed in the lee of the point.

Caution is suggested in the interpretation of stratus cloud phase progression without coastal wind measurements.

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F. M. Ralph, L. Armi, J. M. Bane, C. Dorman, W. D. Neff, P. J. Neiman, W. Nuss, and P. O. G. Persson

Abstract

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 11.9 ± 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 synoptic-scale 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.

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Wendell A. Nuss, John ML Bane, William T. Thompson, Teddy Holt, Clive E. Dorman, F. Martin Ralph, Richard Rotunno, Joseph B. Klemp, William C. Skamarock, Roger M. Samelson, Audrey M. Rogerson, Chris Reason, and Peter Jackson

Coastally trapped wind reversals along the U.S. west coast, which are often accompanied by a northward surge of fog or stratus, are an important warm-season forecast problem due to their impact on coastal maritime activities and airport operations. Previous studies identified several possible dynamic mechanisms that could be responsible for producing these events, yet observational and modeling limitations at the time left these competing interpretations open for debate. In an effort to improve our physical understanding, and ultimately the prediction, of these events, the Office of Naval Research sponsored an Accelerated Research Initiative in Coastal Meteorology during the years 1993–98 to study these and other related coastal meteorological phenomena. This effort included two field programs to study coastally trapped disturbances as well as numerous modeling studies to explore key dynamic mechanisms. This paper describes the various efforts that occurred under this program to provide an advancement in our understanding of these disturbances. While not all issues have been solved, the synoptic and mesoscale aspects of these events are considerably better understood.

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