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  • Author or Editor: Darko Korac̆in x
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Michael Tjernström
and
Darko Koračin

Abstract

An ensemble-average closure model intended for mesoscale studies is applied to a marine stratocumulus-capped PBL. The intention is to test this model, in particular, for cases where cloud and subcloud layers are decoupled. The test is based on one case from the First ISCCP Regional Experiment, where solid cloud-capped and clear sky areas were found in close proximity.

The model results compare favorably both with the measurements and with results from more complex model formulations. They show the response of the entire boundary layer dynamic structure to stratocumulus formation as well as longwave and shortwave radiative heat transfers. The net result is that the entire turbulent layer in the cloud-capped case is more vigorously mixed, more neutrally stratified, and deeper compared to a cloud-free PBL developing under similar conditions. Surface fluxes of sensible and latent heat, from the measurements as well as simulations thus vary relatively little between the areas in spite of the observed substantial sea surface temperature difference.

All simulations presented here reveal cloud decoupling during daytime. The multilayer structure is, however, seen almost only in profiles of second-order moments. The mean profiles indicate one single, deep well-mixed layer, while the turbulence profiles clearly show two separate well-mixed layers. The turbulent flux of water vapor from the surface thus generally never penetrates to the cloud layer during daytime but may eventually cause formation of a shallow layer of cumuli below the main cloud layer.

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Clive E. Dorman
and
Darko Koračin

Abstract

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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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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Darko Koračin
,
James Frye
, and
Vlad Isakov

Abstract

The authors have developed a method that uses tracer measurements as the basis for comparing and evaluating wind fields. An important advantage of the method is that the wind fields are evaluated from the tracer measurements without introducing dispersion calculations. The method can be applied to wind fields predicted by different atmospheric models or to wind fields obtained from interpolation and extrapolation of measured data. The method uses a cost function to quantify the success of wind fields in representing tracer transport. A cost function, “tracer potential,” is defined to account for the magnitude of the tracer concentration at the tracer receptors and the separation between each segment of a trajectory representing wind field transport and each of the tracer receptors. The tracer potential resembles a general expression for a physical potential because the success of a wind field trajectory is directly proportional to the magnitude of the tracer concentration and inversely proportional to its distance from this concentration. A reference tracer potential is required to evaluate the relative success of the wind fields and is defined by the initial location of any trajectory at the source. Then the method is used to calculate continuously the tracer potential along each trajectory as determined by the wind fields in time and space. Increased potential relative to the reference potential along the trajectory indicates good performance of the wind fields and vice versa. If there is sufficient spatial coverage of near and far receptors around the source, then the net tracer potential area can be used to infer the overall success of the wind fields. If there are mainly near-source receptors, then the positive tracer potential area should be used. If the vertical velocity of the wind fields is not available, then the success of the wind fields can be estimated from the vertically integrated area under the tracer potential curve. A trajectory with a maximum tracer potential is constructed for each daily tracer measurement, and this tracer potential is used to normalize the relative success of the wind fields in reproducing the transport of tracers. The method is not sensitive to the exact form of the cost function because a test with an inverse square root dependence in the cost function rather than an inverse linear distance dependence ranked the wind fields in the same order. The method requires sufficient spatial coverage of tracer receptors in the vicinity of a source and primarily gives credit to the wind fields that are able to approach areas with high tracer concentrations. The method can quantitatively determine which wind fields are best able to reproduce the main transport of tracers and can be used to determine the most successful wind fields to serve as a solid base for necessary improvement of dispersion models. It can also be used as a screening method prior to using dispersion models. Since the measured tracer concentrations are affected by both transport and dispersion, however, the method does not evaluate the capabilities of successful wind fields, as input to dispersion algorithms, to create tracer concentrations at receptors that are similar to measured ones. The tracer potential method has been applied to data from a comprehensive field program that included tracer measurements and was conducted in the Colorado River Valley area in the southwestern United States in 1992. Wind fields obtained from four atmospheric models as well as those derived from the wind profiler measurements were tested, and the results of their comparison are presented. Since data from the tracer experiment are publicly available, this developed method can be used to test other atmospheric models.

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Darko Koračin
and
David P. Rogers

Abstract

The effect of a stable internal boundary layer (IBL) on the cloud-capped marine boundary layer is investigated using a one-dimensional second-order closure model. A stable IBL forms if there is a reversal in the surface buoyancy flux when warm air flows over colder water. These conditions exist in the vicinity of Ocean fronts where sea surface temperature discontinuities of about 2°C in 5 km have been observed. There is a balance between the buoyant consumption and inertial production of kinetic energy so that the layer remains weakly turbulent and can deepen due to shear-driven mixing. The stability of the layer limits momentum exchange with the air above so that there is a significant reduction in the surface stress in the IBL and an acceleration of the flow aloft. There are important implications for cloud development in regions of large ocean temperature gradients because a stable IBL can limit the vertical transfer of moisture from the surface to the upper part of the boundary layer.

In addition, solar radiation is found to heat the cloud layer sufficiently to cause decoupling between the cloud and subcloud layers during the daytime. This effect is important in determining the rate at which the cloud layer evaporates.

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David P. Rogers
and
Darko Korac̆in

Abstract

The effects of longwave and shortwave radiative heating on the coupling between stratocumulus clouds and the boundary layer is investigated using a one-dimensional second-moment turbulence-closure model. The decoupling of a stratiform cloud from the subcloud layer is often a precursor to cloud break up and the transition to scattered cumulus clouds or clear sky. Coupling between cloud and subcloud layers is found to be very sensitive to cloud depth and subcloud layer sensible and latent heat fluxes. In particular, a strong moisture flux can maintain weak coupling between the cloud and subcloud layers so that the lower part of the cloud layer may continue to develop despite the formation of a stable temperature gradient between the top of the subcloud layer and cloud base.The effect of shortwave heating on decoupling is threefold. First, shortwave heating directly offsets the net longwave cooling at cloud top by as much as 30% (in February at latitude 29°N), reducing or eliminating the overall cooling of the cloud layer during part of the day. Second, shortwave heating decreases exponentially from a maximum at cloud top, which tends to stabilize and evaporate the cloud layer. In a deep cloud layer radiative heating is restricted to the upper part of the cloud, which warms at a faster rate than the lower part of the cloud; hence, decoupling can occur within the cloud layer. Vertical mixing in the cloud is limited, and multiple cloud layers may form. Third, the maximum shortwave heating is displaced below the maximum longwave cooling, creating a divergent flux that generates convection in the upper part of the cloud layer that, in turn, promotes entrainment. In a deep cloud layer, shortwave radiative heating can affect the decoupling of a cloud and subcloud layer only if longwave cooling is reduced sufficiently to allow longwave radiative heating of cloud base to warm the lower part of the cloud at a faster rate than the subcloud layer is heated by the sea surface. In a shallow cloud layer, shortwave radiation may penetrate to cloud base to provide an additional heat source to decouple the cloud from the subcloud layer.These results highlight the difficulty of predicting the formation, evolution, and dissipation of marine stratocumulus clouds.

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Craig M. Smith
,
Darko Koračin
, and
Kristian Horvath

Abstract

A detailed description of the meteorological conditions of the Washoe Valley (Nevada) and simulations that examine the predictability of the westerly high wind event known as the Washoe Zephyr are presented. Numerical weather model prediction skill is computed for day-ahead (24–48 h) forecasts of wind speed at a meteorological tower on the Virginia Hills range relative to a persistence forecast based on a seasonal climatology constructed of hourly mean observations. The model predictions are shown to be more skillful than a climatology based on seasonal and hourly means during winter and less skillful than the seasonal-hourly climatology (SHC) during summer. Overall skill of the forecasted winds tends to increase with finer horizontal grid spacing. Phase errors compose the largest component of the error decomposition and large phase errors are associated with the onset and decay of the diurnally forced Washoe Zephyr during summer and synoptically forced high wind events and valley rotors during winter. The correlation coefficient between forecasts and observations for all forecast horizontal grid spacings considered is shown to depend roughly linearly on the ratio of the integrated power spectral density in the synoptic band to the integrated power spectral density in the combined diurnal and subdiurnal band.

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

Abstract

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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Ramesh Vellore
,
Darko Koračin
,
Melanie Wetzel
,
Steven Chai
, and
Qing Wang

Abstract

A numerical study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was performed to assess the impact of initial and boundary conditions, the parameterization of turbulence transfer and its coupling with cloud-driven radiation, and cloud microphysical processes on the accuracy of mesoscale predictions and forecasts of the cloud-capped marine boundary layer. Aircraft, buoy, and satellite data and the large eddy simulation (LES) results during the Dynamics and Chemistry of Marine Stratocumulus field experiment (DYCOMS II) in July 2001 were used in the assessment. Three of the tested input fields (Eta, NCEP, and ECMWF) show deficiencies, mainly in the thermodynamic structure of the lowest 1500 m of the marine atmosphere. On a positive note, the simulated marine-layer depth showed good agreement with aircraft observations using the Eta fields, while using the NCEP and ECMWF datasets underestimated the marine-layer depth by about 20%–30%. The predicted turbulence kinetic energy (inversion strength) was about 50% of that obtained from the LES results (aircraft observed). As a consequence of moisture overprediction, the predicted liquid water path was twice the observed by 1–2 g kg−1. The sensitivity tests have shown that the selections of turbulence and cloud microphysical schemes significantly influence the turbulence estimates and cloud parameters. Two of the tested turbulence schemes (Eta PBL and Burk–Thompson) did not exhibit the coupling with radiation. The significant differences in the simulated turbulence estimates appear to be a consequence of the use of water-conserving potential temperature variables. The microphysical parameterization, which uses the number concentration of cloud drops in the autoconversion process, simulates a realistic evolution of precipitable hydrometeors in the cloudy marine layer on the positive side, but on the other hand enhances the decoupling in the turbulence structure. This study can provide guidance to operational forecasters concerning accuracy issues of the commonly used large-scale analyses for model initialization, and optimal selection of model parameterizations in order to simulate and forecast the cloudy atmospheric boundary layer over the ocean.

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

Abstract

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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