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Michael Tjernström

Abstract

The amount of liquid water in stratus clouds or fog is discussed from the point of view of estimating visibility variations in areas with complex terrain. The average vertical profile of liquid water from numerical simulations with a higher-order closure mesoscale model is examined, and runs with the model for moderately complex terrain are utilized to estimate the of low-level liquid water content variability and thus, indirectly, the variations in horizontal visibility along a slope.

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Michael Tjernström

Abstract

The vertical turbulence structure in the marine atmosphere close to a coastline is investigated using airborne measurements. The measurements are from a field experiment close to the coast in the southeast of Sweden, in the Baltic Sea. The Baltic Sea has two main properties that make it particularly interesting to study: significant annual lag in sea surface temperature compared to inland surface temperatures and the fact that it is surrounded by land in all directions within advection distances of from a few hours up to 10–15 hours in normal meteorological conditions.

The present results are mostly from spring or early summer with mainly cool water that is, with a stable or neutral marine boundary layer but with substantial heating of the land area during daytime, thus considerable thermal contrasts. When the daytime inland convective boundary layer is advected out over the cool sea, there is a frictional decoupling in space analogous to the same nocturnal process in time. This sometimes creates a residual layer, a remnant of the inland convective boundary layer, that can be advected for considerable distances over the sea. At the top of this layer, wind shear gives rise to a local increase in turbulent kinetic energy. These layers are used for an analysis of turbulent scales for free shear flow in stable stratification. The analysis is based on different length scales used in numerical model closures for turbulence processes and reveals the asymptotic behavior of different scales in the neutral limit and their functional form, and also illustrates the nonlinear relationship between scales for different properties. The applicability of some often used formulations is also discussed.

The profiles from the aircraft are taken from 25 slant soundings performed in connection to low-level boundary-layer flights. The results are calculated from turbulence data extracted through filtering techniques on instantaneous time (space) series (individual profiles). The calculated turbulence parameters from all profiles are lumped together and finally averaged compositely over all profiles.

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Joseph Sedlar and Michael Tjernström

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Measurements from spaceborne sensors have the unique capacity to fill spatial and temporal gaps in ground-based atmospheric observing systems, especially over the Arctic, where long-term observing stations are limited to pan-Arctic landmasses and infrequent field campaigns. The AIRS level 3 (L3) daily averaged thermodynamic profile product is widely used for process understanding across the sparsely observed Arctic atmosphere. However, detailed investigations into the accuracy of the AIRS L3 thermodynamic profiles product using in situ observations over the high-latitude Arctic are lacking. To address this void, we compiled a wealth of radiosounding profiles from long-term Arctic land stations and included soundings from intensive icebreaker-based field campaigns. These are used to evaluate daily mean thermodynamic profiles from the AIRS L3 product so that the community can understand to what extent such data records can be applied in scientific studies. Results indicate that, while the mid- to upper-troposphere temperature and specific humidity are captured relatively well by AIRS, the lower troposphere is susceptible to specific seasonal, and even monthly, biases. These differences have a critical influence on the lower-tropospheric stability structure. The relatively coarse vertical resolution of the AIRS L3 product, together with infrared radiation through persistent low Arctic cloud layers, leads to artificial thermodynamic structures that fail to accurately represent the lower Arctic atmosphere. These thermodynamic errors are likely to introduce artificial errors in the boundary layer structure and analysis of associated physical processes.

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Leif Enger and Michael Tjernström

Abstract

The effects on the regional precipitation climate by the construction of an artificial lake, in a semiarid region are studied. The study is performed using a mesoscale model to identify the larger-scale meteorological conditions when precipitation enhancement is to be expected and to estimate the amount of precipitation enhancement in such situations. The model results are combined with a ten-year synoptic observations dataset to estimate the mean annual increase in precipitation for the region. The results show a significant increase in precipitation partly over the artificial lake itself, due to land/sea-breeze type circulations during periods when the large-scale wind is weak and the surface temperature of the sea is higher than that of the surrounding areas, and partly over the mountains north of the area in cases with sufficiently strong southerly winds causing the air to be lifted. Being aware of the uncertainty in the quantitative estimates presented here, this paper shows how a combination of mesoscale model “sensitivity runs” and climatological data can be used to estimate at least the order of magnitude change in precipitation and also identify the areas where it is likely to occur.

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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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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

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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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