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Mikhail Varentsov, Igor Esau, and Tobias Wolf

Abstract

Detailed temperature maps are required in various applications. Any temperature interpolation over complex terrain must account for differences in land cover and elevation. Local circulations and other small-scale factors can also perturb the temperature. This study considers the surface air temperature T mapping with geostatistical kriging. The kriging methods are implemented for both T and temperature anomalies ΔT, defined as difference between T at a given location and T at the same elevation in the free atmosphere. The study explores the parallelized atmospheric large-eddy simulation (LES) model (PALM) as a source for variogram and external drift in the kriging methods. Ten kriging methods for the temperature mapping have been considered: ordinary kriging (OK) of T and ΔT with variogram derived from the observations (methods 1 and 2, correspondingly); OK of T and ΔT with variogram derived from LES data (3 and 4); universal kriging with external drift (KED) that utilizes the LES data (5 and 6); a weighted combination of KED of T and ΔT (method 7); and methods 5, 6, and 7 enhanced with additional “virtual” points in remote areas (methods 8, 9, and 10). These 10 methods are evaluated for eight typical weather situations observed in Bergen, Norway. Our results demonstrate considerable added value of the LES information for the detailed meteorological temperature mapping. The LES data improve both the variogram and the resulting temperature maps, especially in the remote mountain parts of the domain and along the coast.

Open access
Stephen Outten, Igor Esau, and Odd Helge Otterå

Abstract

This study examines the atmospheric and oceanic heat transports in preindustrial control and historical runs of 15 fully coupled global climate models from the CMIP5 project. The presence of Bjerknes compensation is confirmed in all models by the strong anticorrelation and approximately equal magnitude of the anomalies of these heat transports. Previous studies of Bjerknes compensation in the absence of external forcing have all shown the strongest compensation at high latitudes, where the warm ocean meets the cold Arctic atmosphere. In this study, however, it is found that many of the 15 models have a second and often dominant peak of compensation in the northern midlatitudes, where strong air–sea interaction is often associated with the midlatitude storm tracks. It has also been suggested that variations in heat transport in the ocean lead those in the atmosphere, but this work has found no clear and robust support for this, as only half the models show such a relationship. In the historical simulations where external forcings are applied, Bjerknes compensation continues to be present, but many models show pronounced trends in the heat transports. All of the models show multidecadal variability in heat transports in both preindustrial control and historical simulations. Any anthropogenic climate change signal could potentially be masked or amplified by the natural variability governed by Bjerknes compensation. Given its presence in the CMIP5 models, which are the basis of so much policy and adaptation planning, an improved understanding of Bjerknes compensation may have socioeconomic relevance for the future.

Open access
Alexander V. Chernokulsky, Igor Esau, Olga N. Bulygina, Richard Davy, Igor I. Mokhov, Stephen Outten, and Vladimir A. Semenov

Abstract

A long-term climatology of cloudiness over the Norwegian, Barents, and Kara Seas (NBK) based on visual surface observations is presented. Annual mean total cloud cover (TCC) is almost equal over solid-ice (SI) and open-water (OW) regions of the NBK (73% ± 3% and 76% ± 2%, respectively). In general, TCC has higher intra- and interannual variability over SI than over OW. A decrease of TCC in the middle of the twentieth century and an increase in the last few decades was found at individual stations and for the NBK as a whole. In most cases these changes are statistically significant with magnitudes exceeding the data uncertainty that is associated with the surface observations. The most pronounced trends are observed in autumn when the largest changes to the sea ice concentration (SIC) occur. TCC over SI correlates significantly with SIC in the Barents Sea, with a statistically significant correlation coefficient between annual TCC and SIC of −0.38 for the period 1936–2013. Cloudiness over OW shows nonsignificant correlation with SIC. An overall increase in the frequency of broken and scattered cloud conditions and a decrease in the frequency of overcast and cloudless conditions were found over OW. These changes are statistically significant and likely to be connected with the long-term changes of morphological types (an increase of convective and a decrease of stratiform cloud amounts).

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Thorsten Mauritsen, Gunilla Svensson, Sergej S. Zilitinkevich, Igor Esau, Leif Enger, and Branko Grisogono

Abstract

This paper presents a turbulence closure for neutral and stratified atmospheric conditions. The closure is based on the concept of the total turbulent energy. The total turbulent energy is the sum of the turbulent kinetic energy and turbulent potential energy, which is proportional to the potential temperature variance. The closure uses recent observational findings to take into account the mean flow stability. These observations indicate that turbulent transfer of heat and momentum behaves differently under very stable stratification. Whereas the turbulent heat flux tends toward zero beyond a certain stability limit, the turbulent stress stays finite. The suggested scheme avoids the problem of self-correlation. The latter is an improvement over the widely used Monin–Obukhov-based closures. Numerous large-eddy simulations, including a wide range of neutral and stably stratified cases, are used to estimate likely values of two free constants. In a benchmark case the new turbulence closure performs indistinguishably from independent large-eddy simulations.

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Thomas Spengler, Ian A. Renfrew, Annick Terpstra, Michael Tjernström, James Screen, Ian M. Brooks, Andrew Carleton, Dmitry Chechin, Linling Chen, James Doyle, Igor Esau, Paul J. Hezel, Thomas Jung, Tsubasa Kohyama, Christof Lüpkes, Kelly E. McCusker, Tiina Nygård, Denis Sergeev, Matthew D. Shupe, Harald Sodemann, and Timo Vihma
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