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Michael Tjernström and David P. Rogers

Abstract

The average and turbulence structure of two marine stratocumulus layers, from the Atlantic Stratocumulus Transition Experiment, are analyzed. These layers ware in adjacent air masses with different histories: one cloud layer was in a clean air mass with a marine history and the other was in a continental air mass, which had a higher aerosol content. The air masses were brought together by synoptic-scale flow and are separated by a semiclear transition zone.

The clouds were decoupled from the marine surface mixed layer in both air masses. In the transition zone, the marine mixed layer was deeper than that under either cloud layer. The total depth below the main inversion, including the cloud layer was, however, substantially greater than in the semiclear transition zone. The western clean cloud layer was more well mixed than the eastern aerosol-rich cloud layer, and the turbulence analysis shows that the western cloud layer complies to convective scaling. Buoyancy production of turbulence was also positive in the eastern cloud, but here shear production was larger than the buoyancy production by a factor of 4, and convective scaling fails.

One cause of the stability differences may lie in differences in radiative forcing, both external and internal. The difference in external forcing is due to higher humidity aloft to the east, reducing the net cloud-top longwave cooling. The difference in internal forcing is due to differences in the cloud microphysics. The larger number of smaller drops in the eastern cloud, arising from the abundance of aerosol particles, increases both albedo and absorption, and thus solar heating. Increased solar heating balances the longwave cooling at the cloud top, warms the cloud interior, and decreases the depth of the net-cooled layer in the cloud. All these effects decrease the buoyancy in the eastern cloud layer in comparison to the western one.

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Linda Ström, Michael Tjernström, and David P. Rogers

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Airborne in situ and remote meteorological measurements from around Cape Mendocino, California, sampled during the Coastal Waves 1996 field program are analyzed for three different days: 7, 12, and 26 June 1996. Two days conformed to typical summertime conditions, with a strong northerly downcoast flow, while the wind on the third day was weaker. On the first 2 days, the flow was supercritical in the sense that the Froude number was larger than unity and these 2 days feature expansion fans in the lee of the cape. On the third day, no such phenomenon was observed. All 3 days had a strong thermal wind caused by the marine inversion sloping down toward the coast. On the first 2 days, the flow aloft was westerly or northerly, so that the thermal wind added to the background flow results in a strong jet. On the third day the flow aloft was southerly, and consequently even with the added thermal wind, the northerly flow in the marine layer was too weak to be supercritical. The main difference between the first 2 days was the fact that 7 June is cloud free, while 12 June had a stratocumulus cover.

The 3 days are analyzed as composites, and several scales are identified and described: 1) the large-scale synoptic forcing, determined from the wind aloft and synoptic conditions; 2) blocking by the coastal topography, the thermal wind balance in the near-coast zone, and nondimensional properties; 3) the hydraulic properties in the sub- and supercritical flows as they pass the change in coastline orientation at Cape Mendocino; 4) the impact of the local blocking by the terrain at the cape itself, generating a lee-wave phenomena for the high-wind days, which was the actual cause of the collapse of the marine layer in the lee of the cape; 5) the boundary layer interaction, apparently generating SST anomalies, but with little or no feedback to the wind field.

A momentum budget analysis for the two high-wind days show a significant difference between one day (7 June) when mesoscale perturbation dominated the flow and the other day (12 June), when large-scale forcing dominated and the mesoscale perturbation was smaller. The importance of the cloud layer on 12 June is illustrated, using lidar data.

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Zhiqiang Cui, Michael Tjernström, and Branko Grisogono

Abstract

A fully nonlinear, primitive equation hydrostatic numerical model is utilized to study coastal flow along central California, combining a realistic atmospheric model, with a higher-order turbulence closure, with highly simplified background flow. Local terrain and surface forcing of the model are treated realistically, while the synoptic-scale forcing is constant in time and space. Several different simulations with different background wind directions were performed. The motivation is to isolate the main properties of the local flow dependent on the coastal mesoscale influence only and to facilitate a study of the structure of the coastal atmospheric boundary layer, the mean momentum budget, and the atmospheric forcing on the coastal ocean for simplified quasi-stationary but still typical conditions. The model results feature the expected summertime flow phenomena, even with this simplified forcing. A coastal jet occurs in all simulations, and its diurnal variability is realistically simulated. The coastal topography serves as a barrier, and the low-level coastal flow is essentially coast parallel.

Among the conclusions are the following. (i) The boundary layer for a northerly jet is more shallow and more variable than that for a southerly jet. One reason is an interaction between waves generated by the coastal mountains and the boundary layer. A realistic inclusion of the Sierra Nevada is important, even for the near-surface coastal atmosphere. (ii) The transition from southerly to northerly flow, when changing the background flow direction, is abrupt for a change in the latter from west to northwest and more gradual for a change east to south. (iii) The low-level flow is in general semigeostrophic. The across-coast momentum balance is geostrophic, while the along-coast momentum balance is dominated by vertical stress divergence and the pressure gradient. Local acceleration and spatial variability close to the coast arise as a consequence of the balance among the remaining terms. For southeasterly background flow, the across-coast momentum balance is dominated by the background synoptic-scale and the mesoscale pressure gradients, sometimes canceling the forcing, thus making this case transitional. (iv) Smaller-scale flow transitions arise for some background flow directions, including an early morning jet reversal north of Monterey, California, and a morning-to-noon low-level eddy formation in the Southern Californian Bight. (v) The model turbulence parameterization provides realistic patterns of the atmospheric forcing on the coastal ocean. (vi) Characteristic signals measured in propagating wind reversals related to boundary layer depth and inversion structure here are seen to correspond to different quasi-stationary conditions.

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Joseph Sedlar, Matthew D. Shupe, and Michael Tjernström

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Cloud and thermodynamic characteristics from three Arctic observation sites are investigated to understand the collocation between low-level clouds and temperature inversions. A regime where cloud top was 100–200 m above the inversion base [cloud inside inversion (CII)] was frequently observed at central Arctic Ocean sites, while observations from Barrow, Alaska, indicate that cloud tops were more frequently constrained to inversion base height [cloud capped by inversion (CCI)]. Cloud base and top heights were lower, and temperature inversions were also stronger and deeper, during CII cases. Both cloud regimes were often decoupled from the surface except for CCI over Barrow. In-cloud lapse rates differ and suggest increased cloud-mixing potential for CII cases.

Specific humidity inversions were collocated with temperature inversions for more than 60% of the CCI and more than 85% of the CII regimes. Horizontal advection of heat and moisture is hypothesized as an important process controlling thermodynamic structure and efficiency of cloud-generated motions. The portion of CII clouds above the inversion contains cloud radar signatures consistent with cloud droplets. The authors test the longwave radiative impact of cloud liquid above the inversion through hypothetical liquid water distributions. Optically thin CII clouds alter the effective cloud emission temperature and can lead to an increase in surface flux on the order of 1.5 W m−2 relative to the same cloud but whose top does not extend above the inversion base. The top of atmosphere impact is even larger, increasing outgoing longwave radiation up to 10 W m−2. These results suggest a potentially significant longwave radiative forcing via simple liquid redistributions for a distinctly dominant cloud regime over sea ice.

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Wayne M. Angevine, Michael Tjernström, and Mark Žagar

Abstract

Concentrations of ozone exceeding regulatory standards are regularly observed along the coasts of New Hampshire and Maine in summer. These events are primarily caused by the transport of pollutants from urban areas in Massachusetts and farther south and west. Pollutant transport is most efficient over the ocean. The coastline makes transport processes complex because it makes the structure of the atmospheric boundary layer complex. During pollution episodes, the air over land in daytime is warmer than the sea surface, so air transported from land over water becomes statically stable and the formerly well-mixed boundary layer separates into possibly several layers, each transported in a different direction. This study examines several of the atmospheric boundary layer processes involved in pollutant transport. A three-dimensional model [the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)] run on grids of 2.5 and 7.5 km is used to examine the winds, thermodynamic structure, and structure of tracer plumes emitted from Boston, Massachusetts, and New York City, New York, in two different real cases—one dominated by large-scale transport (22–23 July 2002) and one with important mesoscale effects (11–14 August 2002). The model simulations are compared with measurements taken during the 2002 New England Air Quality Study. The model simulates the basic structure of the two different episodes well. The boundary layer stability over the cold water is weaker in the model than in reality. The tracer allows for easy visualization of the pollutant transport.

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Michael Tjernström, Joseph Sedlar, and Matthew D. Shupe

Abstract

Downwelling radiation in six regional models from the Arctic Regional Climate Model Intercomparison (ARCMIP) project is systematically biased negative in comparison with observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, although the correlations with observations are relatively good. In this paper, links between model errors and the representation of clouds in these models are investigated. Although some modeled cloud properties, such as the cloud water paths, are reasonable in a climatological sense, the temporal correlation of model cloud properties with observations is poor. The vertical distribution of cloud water is distinctly different among the different models; some common features also appear. Most models underestimate the presence of high clouds, and, although the observed preference for low clouds in the Arctic is present in most of the models, the modeled low clouds are too thin and are displaced downward. Practically all models show a preference to locate the lowest cloud base at the lowest model grid point. In some models this happens also to be where the observations show the highest occurrence of the lowest cloud base; it is not possible to determine if this result is just a coincidence. Different factors contribute to model surface radiation errors. For longwave radiation in summer, a negative bias is present both for cloudy and clear conditions, and intermodel differences are smaller when clouds are present. There is a clear relationship between errors in cloud-base temperature and radiation errors. In winter, in contrast, clear-sky cases are modeled reasonably well, but cloudy cases show a very large intermodel scatter with a significant bias in all models. This bias likely results from a complete failure in all of the models to retain liquid water in cold winter clouds. All models overestimate the cloud attenuation of summer solar radiation for thin and intermediate clouds, and some models maintain this behavior also for thick clouds.

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Mark Žagar, Gunilla Svensson, and Michael Tjernström

Abstract

Small-scale variability of the Reynolds wind stress on the surface of the coastal sea, in conditions for which the land is warmer than the sea, is evaluated by idealized numerical simulations. A method for diagnosing the small-scale influences of a coast on the turbulent flux of momentum at the sea surface is proposed. The parameters are defined on the basis of a high-resolution numerical model. This diagnostic method can be used to resolve the surface turbulent momentum flux variations in large-scale models or in the areas with sparse observational coverage. The input data needed are background wind speed and direction, surface temperature contrast at the coastline, and background static stability. The temperature contrast between the land and sea surface introduces horizontal variations in the flux field. The surface turbulent stress at some distance from the coast exhibits an inverse square root dependence on the temperature contrast, not only for offshore but also for onshore wind situations. The turbulent exchange of momentum can be substantially reduced far ahead of the coast, up to a few hundred kilometers in a more stable atmosphere. In general, it is found that the surface stress to the sea near the coast, in a stable marine boundary layer, is almost always smaller than at open sea. The background static stability in general reduces the magnitude of vertical turbulent mixing. Its effect can be introduced in the diagnosis with good confidence. Also the velocity of the cross-coast wind component in the developed sea breeze can be successfully scaled by the atmospheric background stability.

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Ann-Sofi Smedman, Michael Tjernström, and Ulf Högström

Abstract

Data from a marine coastal experiment over the Baltic Sea, comprising airborne measurements and mast measurements, have been used to highlight the turbulence dynamics of a case with most unusual flow characteristics. The boundary layer had a depth of about 1200 m. The thermal stratification was near neutral, with small positive heat flux below 300 m and equally small negative heat flux above. The entire situation lasted about 6 hours. Turbulence levels were unexpectedly high in view of the fact that momentum flux was negligible (in fact positive) in the layers near the surface, and buoyancy flux was also small. The turbulence was found to scale with the height of the boundary layer, giving rise to velocity spectra having the shape of those characteristic of convectively mixed boundary layers. Analysis of the turbulence budget for the entire planetary boundary layer (PBL) revealed that energy was produced from shear instability in the uppermost parts of the PBL and was distributed to the lower parts of the PBL by pressure transport. Dissipation was found to be evenly distributed throughout the entire PBL. Without data on surface wave characteristics, no firm conclusions concerning air–sea interaction processes can be drawn, but there are clear indications that the dynamical decoupling observed at the surface is due to the effect of decaying sea state conditions (high wave age conditions). In any case, the process of active turbulence production in the layers close to the surface observed in “ordinary” near-neutral boundary layers has been effectively turned off here, leaving only turbulence of the “inactive” kind, imported by pressure transport from layers above. The results strongly support the findings reported in the recent literature on “laboratory turbulence” that the process of strong turbulence and shearing stress production near the wall of boundary layers of very different kinds is virtually independent of forcing from large-scale structures embedded in the flow.

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Marie-Luise Kapsch, Rune Grand Graversen, Michael Tjernström, and Richard Bintanja

Abstract

The Arctic summer sea ice has diminished fast in recent decades. A strong year-to-year variability on top of this trend indicates that sea ice is sensitive to short-term climate fluctuations. Previous studies show that anomalous atmospheric conditions over the Arctic during spring and summer affect ice melt and the September sea ice extent (SIE). These conditions are characterized by clouds, humidity, and heat anomalies that all affect downwelling shortwave (SWD) and longwave (LWD) radiation to the surface. In general, positive LWD anomalies are associated with cloudy and humid conditions, whereas positive anomalies of SWD appear under clear-sky conditions. Here the effect of realistic anomalies of LWD and SWD on summer sea ice is investigated by performing experiments with the Community Earth System Model. The SWD and LWD anomalies are studied separately and in combination for different seasons. It is found that positive LWD anomalies in spring and early summer have significant impact on the September SIE, whereas winter anomalies show only little effect. Positive anomalies in spring and early summer initiate an earlier melt onset, hereby triggering several feedback mechanisms that amplify melt during the succeeding months. Realistic positive SWD anomalies appear only important if they occur after the melt has started and the albedo is significantly reduced relative to winter conditions. Simulations where both positive LWD and negative SWD anomalies are implemented simultaneously, mimicking cloudy conditions, reveal that clouds during spring have a significant impact on summer sea ice while summer clouds have almost no effect.

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Michael Tjernström, Ben B. Balsley, Gunilla Svensson, and Carmen J. Nappo

Abstract

The authors report results of a study of finescale turbulence structure in the portion of the nocturnal boundary layer known as the residual layer (RL). The study covers two nights during the Cooperative Atmosphere–Surface Exchange Study 1999 (CASES-99) field experiment that exhibit significant differences in turbulence, as indicated by the observed turbulence dissipation rates in the RL. The RL turbulence sometimes reaches intensities comparable to those in the underlying stable boundary layer.

The commonly accepted concept of turbulence generation below critical values of the gradient Richardson number (Rig) is scale dependent: Ri values typically decrease with decreasing vertical scale size, so that critical Rig values (≈0.25) occur at vertical scales of only a few tens of meters. The very small scale for the occurrence of subcritical Ri poses problems for incorporating experimentally determined Rig -based methods in model closures in models with poor resolution.

There appear to be two distinct turbulence “regimes” in the RL: a very weak but ever-present background turbulence level with minimal temporal and spatial structure and a more intense intermittent regime during which turbulent intensity can approach near-surface nighttime turbulent intensities. It is hypothesized that the locally produced RL turbulence can be related to upward-propagating atmospheric gravity waves generated by flow over the low-relief terrain. The presence of critical layers in the RL, caused by wind turning with height, results in the generation of intermittent turbulence.

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