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Clive E. Dorman

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Clive E. Dorman

Abstract

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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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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Clive E. Dorman

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Clive E. Dorman

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Clive E. Dorman

Abstract

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
and
Robert H. Bourke

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New estimates of rainfall over the Atlantic Ocean between 30°S to 70°N have been constructed based an a technique that uses the present weather observations taken by ships. Annual and quarterly rainfall maps are presented. Between the equator and 60°N, the average annual rainfall depth is 1034 mm and the annual volume is 3.93 × 104km3. Compared to the Pacific, the Atlantic is significantly drier and has less extreme values. Maps of amplitude and phase show that most of the North Atlantic cast of 60°W experiences a inter peak rainfall. The South Atlantic experiences its peak rainfall in the Southern Hemisphere summer.

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Clive E. Dorman
and
Robert H. Bourke

Abstract

By using present weather observations taken by ships and relating them to a given amount of precipitation, new estimates of oceanic rainfall for the Pacific Ocean between 30°S and 60°N have been derived. Satellite microwave measurements and Taylor's (1973) island analysis support our findings. Annual and quarterly rainfall maps, drawn from our estimates, agree with other modem, land-derived values, but provide greater detail. Between the equator and 60°N, the annual depth and volume rainfall totals are 1282 mm and 1.16×105 km3, respectively. Maps of amplitude and phase show that most of the rainfall north of 28°N occurs in winter, while maximum rainfall occurs in July and August in the tropics. Diurnal rainfall, studied at selected locations, is at a minimum at noon in all but the western pan of the North Pacific. Here there is no distinct minimum.

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Darko Koračin
and
Clive E. Dorman

Abstract

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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