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Clive E. Dorman

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Clive E. Dorman

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Clive E. Dorman

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Reed (1980) found that low islands and atolls in the tropical Pacific have a significant effect on measured rainfall. But direct examination of island data leads to the opposite conclusion. Reed and Elliott's (1979) rainfall estimates, the basis of Reed (1980), overestimate dry tropical islands and underestimate wetter islands by a factor of 2 or 3. Further, Reed and Elliott increasingly underestimate the tropical peak toward the west. Other observations are cited, such as satellite cloud analysis, that conflict with Reed (1980). It is concluded that the Reed (1980) analysis is flawed, and that the Reed and Elliott (1980) estimates are in error.

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Clive E. Dorman

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Clouds were observed forming in the lee of Guadalupe Island under a special sequence of events. One cloud was a narrow, linear cloud, 200 km long, that could not have been advected. Instead, a transparent linear wake was advected downwind. Later, synoptic-scale convergence caused the wake to condense, forming the linear cloud trail. That weak synoptic-scale variation may cause a linear cloud trail and has important implications for understanding ship cloud trails.

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Clive E. Dorman

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Clive E. Dorman

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Evidence suggests that an internal solitary Kelvin wave exists in the marine layer along California. The marine layer is lifted over the central coast by a weak cyclonic circulation. This “bump,” initially 850 m high, moves to the north along the coast at 6 m s−1. The undisturbed layer depth is 100–200 m thick. The crest height of the wave decreases to 500 m farther north. Winds under the raised marine layer are southerly. The leading edge of the wave is easily followed by satellite as the thickened marine layer is marked by overcast stratus. A greatly curved offshore leading edge indicates that nonlinear effects are important. Offshore scale in the overcast is about 300 km in the south and 50 km in the north. Surface pressure gradient alongshore is closely related to the marine layer depth. The surface wind shifts when the leading and trailing edge of the wave passes.

Northerly wave progression ceases at the sharp bend formed by Cape Mendocino. At this time, a vortex is formed in the marine layer off Point Arena. This cyclonic vortex, on the order of 50 km across, is designated as the Point Arena Eddy.

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Darko Koračin and Clive E. Dorman

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The authors have performed a numerical experiment using Mesoscale Model 5 (MM5) with a horizontal resolution of 9 km to simulate hourly atmospheric dynamics and thermodynamics along the U.S. California coast for all of June 1996. The MM5 results were evaluated using more than 18 000 data points from wind profilers, radiosondes, buoys, and land stations; the results support the use of modeled dynamics for reliable monthly statistics and calculation of diurnal variations. Month-long mesoscale simulations of the marine atmospheric boundary layer (MABL) and satellite observations have been used to investigate the diurnal variation of near-shore and farther offshore clouds along the U.S. California coast. The authors extended the usual model evaluation with respect to time series and power spectrum analysis to investigate a link between the evaluated dynamics and satellite-derived cloudiness. Two distinct types of cloudiness variation were revealed. One is in the near-shore zone, extending approximately 100 km in the offshore direction, where the diurnal variation of cloudiness develops in response to the formation of MABL wind divergence and convergence fields. Each of the five major capes between southern Oregon and southern California has a satellite-derived, low-cloud maximum albedo on the leeward side and a minimum on the windward side that closely corresponds to “expansion fans” and “compression bulges.” The expansion fan is associated with a divergence field of fast horizontal winds, shallow MABL, and high Froude number. The compression bulge is associated mainly with relatively weak winds (convergent or slightly divergent), a deeper MABL, and smaller Froude number. Simulated divergence in the expansion fan areas shows a significant diurnal trend with the maximum during the late morning through early afternoon. In the compression bulge, either the divergence is an order of magnitude less, or the flow becomes convergent. Going westward, the MABL divergence becomes an order of magnitude less at distances of 30–40 km from the coastline. Since the expansion fan is characteristic of the MABL, the effect of the divergence field decays rapidly in the vertical and, due to mass continuity, reverses into a convergent flow above the MABL.

Farther offshore, the cloudiness variation is at a minimum around midday as well, but that is mainly a consequence of radiative heat transfer effects within the cloud. Marine atmospheric boundary layer divergence does not have a significant diurnal trend in that area. Daytime offshore cloud clearing begins first in the northern domain, where the marine layer and clouds are shallower. The clearing propagates southward until the marine layer and clouds are too deep; generally the clouds persist throughout the entire day.

The study shows the importance of dynamics on the evolution of observed cloudiness and constitutes an approach to indirectly evaluate modeled dynamics using satellite-derived cloudiness.

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Clive E. Dorman and Darko Koračin

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A summer wind speed maximum extending more than 200 km occurs over water around Point Conception, California, the most extreme bend along the U.S. West Coast. The following several causes were investigated for this wind speed maximum: 1) synoptic conditions, 2) marine layer hydraulic flow effects, 3) diurnal variations, 4) mountain leeside downslope flow, 5) sea surface temperature structure, and 6) island influence. Synoptic conditions set the general wind speed around Point Conception, and these winds are classified as strong, moderate, or weak. The strong wind condition extends about Point Conception, reaching well offshore toward the southwest, and the highest speeds are within 20 km to the south. Moderate wind cases do not extend as far offshore, and they have a moderate maximum wind speed that occurs over a smaller area in the western mouth of the Santa Barbara Channel. The weak wind speed case consists of light and variable winds about Point Conception. Each category occurs about one-third of the time. Atmospheric marine layer hydraulic dynamics dominate the situation after the synoptic condition is set. This includes an expansion fan on the south side of the point and a compression bulge on the north side. The expansion fan significantly increases the wind speeds over a large area that extends to the southwest, south, and east of Point Conception, and the maximum wind speed is increased for the strong and moderate synoptic cases as well. The horizontal sea surface temperature pattern contributes to the sea surface wind maximum through the Froude number, which links the potential temperature difference between the sea surface temperature and the capping inversion temperature with marine layer acceleration in an expansion fan. A greater potential temperature difference across the top of the marine layer also causes more energy to be trapped in the marine layer, instead of escaping upward. The thermally driven flow resulting from differential heating over land in the greater Los Angeles, California, coastal and elevated area to the east is not directly related to the wind speed maximum, either in the Santa Barbara Channel or in the open ocean extending farther offshore. The effects of the thermally driven flow extend only to the east of the Santa Barbara Channel. The downslope flow on the south side of the Santa Ynez Mountains that is generated by winds crossing the Santa Ynez Mountain ridge contributes neither to the high-speed wind maximum in the Santa Barbara channel nor to that extending farther offshore. Fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) simulations do support a weak leeside flow in the upper portions of the Santa Ynez Mountains. The larger Channel Islands have a significant effect on the marine layer flow and the overwater wind structure. One major effect of the Santa Barbara Channel Islands is the extension of the zone of high-speed winds farther to the south than would otherwise be the case.

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Clive E. Dorman and J. F. T. Saur

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Over the eight years of expendable bathythermograph observations from merchant ship transits between San Francisco and Honolulu have been analyzed to determine the nature of subsurface temperature anomalies. The irregularly distributed data were interpolated for 0, 90, 170 and 400 m by an objective analysis and then contoured. Statistical properties which had to be computed for the gridding procedure are described and presented.

The statistical properties and anomaly patterns in the upper layers are contrastingly different from those in the main thermocline and below. In the upper layers the significant correlation of anomalies is limited to time separation of less than 40 days, but extends to distance separations beyond 900 km. At the 170 m level in the main thermocline, anomalies are correlated to 100 days, but extend to only 190 km. The standard deviation increases from the surface to 170 m and then decreases to a minimum at 500 m. The peak of the standard deviation at a level shifts west as depth increases. Vertical correlations reveal that temperature anomalies at the surface are uncorrelated with those in the main thermocline. The main thermocline anomalies move along the route toward Honolulu at about 2.9 cm s−1, which is suggestive of baroclinic Rossby waves.

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