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Gunilla Svensson

Abstract

A numerical flow model is presented for the atmospheric boundary layer, including dispersion and chemical transformations of air pollutants. The model is a three-dimensional time-dependent one for the mesoscale based on the conservation equations for mass, heat, motion, water, and chemical species. The present version is hydrostatic, with a turbulence closure of second order. Only gas-phase chemistry is included and the chemical reaction scheme used is a modified condensed version of the carbon bond mechanism.

A parameter study with a one-dimensional version of the model system is performed for the meteorological conditions of a typical summer day. Simulations with constant and diurnally varied deposition velocity are compared. The conclusion is that the difference in the surface concentration is minor. The difference in the maximum concentration between a simulation with a diurnally varied deposition velocity and a simulation with constant value equal to the maximum of the diurnally varied deposition velocity is about 1%. Other parameters affecting the results studied here are the temperature, the influence of pressure and water vapor on the rate constants, and the actinic flux. The resulting concentrations of ozone, peroxyacyl nitrate, and nitric acid are presented. The timing and the magnitude of peak concentrations of these species are sensitive to alterations in the parameters. When comparing the various simulations with a control run, the largest discrepancies are seen in the first simulation day, for the cases with higher albedo, and when deposition is included.

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Gunilla Svensson

Abstract

A three-dimensional coupled mesoscale meteorological and photochemical model has been applied to the Athens basin in Greece. The Athens area experiences episodes of very high air pollution levels a few times every year. A severe episode on 25 May 1990 was chosen for this study. On this day a high pressure system was situated over Greece, and the synoptic forcing was weak. A sea breeze developed in the basin during the day, and the pollutants were transported out of the basin through the gaps between the mountains to the north and northeast. A northward gradient in ozone concentration was present both in the observations and the model results. Unhealthily high concentrations of nitric oxides and hydrocarbons built up during the night, when it was stably stratified and the wind speeds were low.

Variations in parameters, such as terrain influence and deposition processes, are performed to illustrate the sensitivity of the model results. Deposition is shown to be important for the results, although the deposition velocities over an urban area are not well known. The atmospheric flow in the Athens area is, to a great extent, determined by the local terrain. Three main features in the flow that impose consequences on the air pollution in Athens are identified.

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Gunilla Svensson and Jenny Lindvall

Abstract

The diurnal cycles of near-surface variables and turbulent heat fluxes are evaluated in 16 models from phase 5 of CMIP (CMIP5) and compared with observations from 26 flux tower sites. The diurnal cycle of 2-m temperature agrees well in general with what is observed. The amplitude of the diurnal cycle of wind speed shows a large intermodel spread and is often overestimated at midlatitude grassland sites and underestimated at midlatitude forest sites. There is a substantial systematic negative bias in the nighttime net surface radiative flux, which is partly compensated for by the turbulent heat fluxes. Four models (CESM1, BCC_CSM1.1, HadGEM2-A, and IPSL-CM5A) are evaluated in more detail, including the vertical structure of the atmospheric boundary layer, at the ARM Southern Great Plains site in Oklahoma. At that site, all models tend to frequently overestimate the boundary layer depth and the wind turning in the boundary layer reveals large intermodel differences. In summer, these models exhibit a substantial warm bias with particularly high daytime temperatures. These high temperatures are associated with very small latent heat fluxes, indicating that the soil is too dry, which is likely to impact climate change scenarios.

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Gunilla Svensson and Johannes Karlsson

Abstract

Energy fluxes important for determining the Arctic surface temperatures during winter in present-day simulations from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset are investigated. The model results are evaluated over different surfaces using satellite retrievals and ECMWF interim reanalysis (ERA-Interim). The wintertime turbulent heat fluxes vary substantially between models and different surfaces. The monthly median net turbulent heat flux (upward) is in the range 100–200 W m−2 and −15 to 15 W m−2 over open ocean and sea ice, respectively. The simulated net longwave radiative flux at the surface is biased high over both surfaces compared to observations but for different reasons. Over open ocean, most models overestimate the outgoing longwave flux while over sea ice it is rather the downwelling flux that is underestimated. Based on the downwelling longwave flux over sea ice, two categories of models are found. One group of models that shows reasonable downwelling longwave fluxes, compared with observations and ERA-Interim, is also associated with relatively high amounts of precipitable water as well as surface skin temperatures. This group also shows more uniform airmass properties over the Arctic region possibly as a result of more frequent events of warm-air intrusion from lower latitudes. The second group of models underestimates the downwelling longwave radiation and is associated with relatively low surface skin temperatures as well as low amounts of precipitable water. These models also exhibit a larger decrease in the moisture and temperature profiles northward in the Arctic region, which might be indicative of too stagnant conditions in these models.

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Thorsten Mauritsen and Gunilla Svensson

Abstract

Stably stratified shear-driven turbulence is analyzed using the gradient Richardson number, Ri, as the stability parameter. The method overcomes the statistical problems associated with the widely used Monin–Obukhov stability parameter. The results of the Ri-based scaling confirm the presence of three regimes: the weakly and the very stable regimes and the transition in between them. In the weakly stable regime, fluxes scale in proportion with variance, while in the very stable regime, stress and scalar fluxes behave differently. At large Ri, the velocity field becomes highly anisotropic and the turbulent potential energy becomes approximately equal to half of the turbulent kinetic energy. It appears that even in the strongly stable regime, beyond what is known as the critical gradient Richardson number, turbulent motions are present.

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James G. Hudson and Gunilla Svensson

Abstract

Cloud microphysical measurements off the southern California coast are presented and compared with in situ airborne measurements of cloud condensation nuclei (CCN) spectra. Large-scale variations in cloud droplet concentrations were due to CCN variations, some medium-scale variations may be a result of the conversion of droplets to drops by coalescence, while small-scale variations were due to different proportions of the CCN spectra being activated because of variations in updraft velocity at cloud base. This latter internal mixing process produces an inverse relationship between droplet concentration and mean size and an increase in droplet spectral width with mean droplet size. Drizzle drop concentrations are strongly associated with lower droplet concentrations, larger droplets, and greater droplet spectral width.

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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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Jenny Lindvall, Gunilla Svensson, and Cecile Hannay

Abstract

This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid- and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

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Anders Engström, Johannes Karlsson, and Gunilla Svensson

Abstract

Observations from the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) suggest that the Arctic Basin is characterized by two distinctly different preferred atmospheric states during wintertime. These states appear as two peaks in the frequency distribution of surface downwelling longwave radiation (LWD), representing radiatively clear and opaque conditions. Here, the authors have investigated the occurrence and representation of these states in the widely used ECMWF Interim Re-Analysis (ERA-Interim) dataset. An interannually recurring bimodal distribution of LWD values is not a clearly observable feature in the reanalysis data. However, large differences in the simulated liquid water content of clouds in ERA-Interim compared to observations are identified and these are linked to the lack of a radiatively opaque peak in the reanalysis. Using a single-column model, dynamically controlled by data from ERA-Interim, the authors show that, by tuning the glaciation speed of supercooled liquid clouds, it is possible to reach a very good agreement between the model and observations from the SHEBA campaign in terms of LWD. The results suggest that the presence of two preferred Arctic states, as observed during SHEBA, is a recurring feature of the Arctic climate system during winter [December–March (DJFM)]. The mean increase in LWD during the Arctic winter compared to ERA-Interim is 15 W m−2. This has a substantial bearing on climate model evaluation in the Arctic as it indicates the importance of representing Arctic states in climate models and reanalysis data and that doing so could have a significant impact on winter ice thickness and surface temperatures in the Arctic.

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Cian Woods, Rodrigo Caballero, and Gunilla Svensson

Abstract

This paper examines the wintertime northward moisture flux at 70°N from 1981–2005 in 31 of the CMIP5 models compared with the ERA-Interim reanalysis product. The models’ total zonally integrated northward moisture flux is found to agree reasonably well with the reanalysis, but with large compensating regional biases. Specifically, the models systematically underpredict the moisture flux in the Atlantic sector and overpredict it in the Pacific sector. The biases are predominantly due to misrepresentation of extreme moisture flux events, which are known to exert a significant control on Arctic climate. Biases in these high-intensity fluxes are almost entirely contributed by biases in the meridional velocity, suggesting a link with biases in storm-track activity at lower latitudes. The extent to which the deficit of moisture intrusions in the Atlantic sector and excess in the Pacific sector may account for biases in the climate of the respective sectors is assessed. Biases in the frequency of moisture intrusions explain roughly 17% of surface temperature and 24% of surface downward longwave radiation biases in the Atlantic sector, and about 14% and 16% of the gradient in these respective biases between the two sectors. The predicted bias gradients, while small in amplitude, are very highly correlated with the true bias gradients in the models, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback.

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