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Clive E. Dorman and Clinton D. Winant

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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

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

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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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Raymond W. Schmitt, Philip S. Bogden, and Clive E. Dorman

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Estimates of evaporation (E) over the North Atlantic Ocean by Bunker have been combined with estimates of precipitation (P) by Dorman and Bourke to produce new annual and seasonal maps of EP and surface density flux. Although uncertainties about precipitation algorithms and exchange coefficients still presist, it is felt that the high spatial resolution of these data set permits an estimate of EP that is a significant improvement over previous work. The maps of EP show considerably more detail than earlier maps, including a previously uncharted minimum with a northeast to southwest trend across the subtropical gyre. The two regions of maximal EP display a close connection with continental air flows off Africal and North America, suggesting that the relative narrowness of the North Atlantic contributes to its status as a net evaporation basin. The zonally integrated EP values are combined with river runoff estimates to obtain the meridional flux of freshwater, which can be compared with fluxes calculated from oceanographic sections.

Maps of the surface density flux are also presented for the annual and seasonal averages. Areas of net density gain by the ocean correspond to formation regions of subpolar mode water at high latitudes, 18°C water south of the Gulf Stream, and salinity maximum water at low latitudes in midgyre. The contributions of heat and salt to the density flux are separately computed. This reveals that the thermal density flux dominates at high and low latitudes whereas the haline density flux is most important in the subtropics, particularly on the eastern side of the basin. These data should facilitate the development of models of the thermohaline circulation, and aid the identification of regional differences in the dominant air–sea interaction processes.

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Darko Koračin, Clive E. Dorman, and Edward P. Dever

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Month-long simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with a horizontal resolution of 9 km have been used to investigate perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along the California and Baja California coasts during June 1999. The dominant spatial inhomogeneity of the wind stress and wind stress curl is near the coast. Wind and wind stress maxima are found in the lees of major capes near the coastline. Positive wind stress curl occurs in a narrow band near the coast, while the region farther offshore is characterized by a broad band of weak negative curl. Curvature of the coastline, such as along the Southern California Bight, forces the northerly flow toward the east and generates positive wind stress curl even if the magnitude of the stress is constant. The largest wind stress curl is simulated in the lees of Point Conception and the Santa Barbara Channel. The Baja California wind stress is upwelling favorable. Although the winds and wind stress exhibit great spatial variability in response to synoptic forcing, the wind stress curl has relatively small variation. The narrow band of positive wind stress curl along the coast adds about 5% to the coastal upwelling generated by adjustment to the coastal boundary condition. The larger area of positive wind stress curl in the lee of Point Conception may be of first-order importance to circulation in the Santa Barbara Channel and the Southern California Bight.

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Eric D. Skyllingstad, Philip Barbour, and Clive E. Dorman

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A mesoscale model is used to examine the dynamics of northwest flow over the Santa Barbara Channel region. Three cases are considered, each characterized by typical summertime synoptic conditions, but with differences in pressure gradient strength and marine boundary layer depth (MBL). The first case examines a relatively deep MBL and strong pressure gradient. Case 2 is characterized by a more shallow MBL and weaker pressure gradient, and case 3 represents a transition from a deep MBL to shallow conditions. In all cases, simulated surface winds show reasonable agreement with observations over most of the model domain, with the exception of regions near abrupt terrain changes.

Results from the model indicate that the flow with a deep MBL (∼400 m) and strong pressure gradient (case 1) is supercritical, causing regions of acceleration and expansion in the lee of Point Conception. When the MBL is shallow (∼150 m) (case 2), a transcritical flow scenario exists with subcritical flow upstream from Point Conception and a supercritical flow region over the Santa Barbara Channel and downstream from the Channel Islands. Flow over the channel is strongly affected by diurnal heating in shallow MBL cases, reversing direction in step with a land breeze circulation induced by nighttime cooling. The land breeze forces an internal wave disturbance that propagates westward across the channel, eliminating the supercritical flow region in the lee of Point Conception. Conditions with a deep MBL (∼400 m) produce less variability in the surface winds, except for the region sheltered by the Santa Ynez Mountains. An expansion fan is still evident in this case, but it is produced by the interaction of the flow with higher terrain north and east of the channel. The low hills on Point Conception and the Channel Islands do not have a large blocking effect on the surface flow when the MBL is deep.

Analysis of the momentum budget supports the conclusion that the boundary layer behaves like a transcritical hydraulic flow when the MBL is shallow. Except for the open ocean region, the Coriolis term is minor in comparison with the pressure and advection terms. Diurnal heating effects are evident in the nearshore pressure term, which varies from offshore during the late evening to onshore in the afternoon. These effects are most significant when the MBL is shallow and can augment the hydraulically forced pressure pattern, causing a stronger expansion fan in the late afternoon and a collapse of the expansion fan during the early morning.

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Darko Koračin, John Lewis, William T. Thompson, Clive E. Dorman, and Joost A. Businger

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A case of fog formation along the California coast is examined with the aid of a one-dimensional, higher-order, turbulence-closure model in conjunction with a set of myriad observations. The event is characterized by persistent along-coast winds in the marine layer, and this pattern justifies a Lagrangian approach to the study. A slab of marine layer air is tracked from the waters near the California–Oregon border to the California bight over a 2-day period. Observations indicate that the marine layer is covered by stratus cloud and comes under the influence of large-scale subsidence and progressively increasing sea surface temperature along the southbound trajectory.

It is hypothesized that cloud-top cooling and large-scale subsidence are paramount to the fog formation process. The one-dimensional model, evaluated with various observations along the Lagrangian path, is used to test the hypothesis. The principal findings of the study are 1) fog forms in response to relatively long preconditioning of the marine layer, 2) radiative cooling at the cloud top is the primary mechanism for cooling and mixing the cloud-topped marine layer, and 3) subsidence acts to strengthen the inversion above the cloud top and forces lowering of the cloud. Although the positive fluxes of sensible and latent heat at the air–sea interface are the factors that govern the onset of fog, sensitivity studies with the one-dimensional model indicate that these sensible and latent heat fluxes are of secondary importance as compared to subsidence and cloud-top cooling. Sensitivity tests also suggest that there is an optimal inversion strength favorable to fog formation and that the moisture conditions above the inversion influence fog evolution.

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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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David P. Rogers, Clive E. Dorman, Kathleen A. Edwards, Ian M. Brooks, W. Kendall Melville, Stephen D. Burk, William T. Thompson, Teddy Holt, Linda M. Ström, Michael Tjernström, Branko Grisogono, John M. Bane, Wendell A. Nuss, Bruce M. Morley, and Allen J. Schanot

Some of the highlights of an experiment designed to study coastal atmospheric phenomena along the California coast (Coastal Waves 1996 experiment) are described. This study was designed to address several problems, including the cross-shore variability and turbulent structure of the marine boundary layer, the influence of the coast on the development of the marine layer and clouds, the ageostrophy of the flow, the dynamics of trapped events, the parameterization of surface fluxes, and the supercriticality of the marine layer.

Based in Monterey, California, the National Center for Atmospheric Research (NCAR) C-130 Hercules and the University of North Carolina Piper Seneca obtained a comprehensive set of measurements on the structure of the marine layer. The study focused on the effects of prominent topographic features on the wind. Downstream of capes and points, narrow bands of high winds are frequently encountered. The NCAR-designed Scanning Aerosol Backscatter Lidar (SABL) provided a unique opportunity to connect changes in the depth of the boundary layer with specific features in the dynamics of the flow field.

An integral part of the experiment was the use of numerical models as forecast and diagnostic tools. The Naval Research Laboratory's Coupled Ocean Atmosphere Model System (COAMPS) provided high-resolution forecasts of the wind field in the vicinity of capes and points, which aided the deployment of the aircraft. Subsequently, this model and the MIUU (University of Uppsala) numerical model were used to support the analysis of the field data.

These are some of the most comprehensive measurements of the topographically forced marine layer that have been collected. SABL proved to be an exceptionally useful tool to resolve the small-scale structure of the boundary layer and, combined with in situ turbulence measurements, provides new insight into the structure of the marine atmosphere. Measurements were made sufficiently far offshore to distinguish between the coastal and open ocean effects. COAMPS proved to be an excellent forecast tool and both it and the MIUU model are integral parts of the ongoing analysis. The results highlight the large spatial variability that occurs directly in response to topographic effects. Routine measurements are insufficient to resolve this variability. Numerical weather prediction model boundary conditions cannot properly define the forecast system and often underestimate the wind speed and surface wave conditions in the nearshore region.

This study was a collaborative effort between the National Science Foundation, the Office of Naval Research, the Naval Research Laboratory, and the National Oceanographic and Atmospheric Administration.

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