Johannes Karlsson

Subtropical Cloud-Regime Transitions: Boundary Layer Depth and Cloud-Top Height Evolution in Models and Observations

Abstract In this study, the mean and variability of boundary layer height (BLH) are analyzed along a transect in the eastern Pacific Ocean for the summer of 2003 using BLH estimates based on the height of the main relative humidity (RH) inversion and the height of low cloud tops (CTH). The observations and the regional and global model data have been prepared in the context of the Global Energy and Water Cycle Experiment (GEWEX) Cloud System Study (GCSS) Pacific Cross-Section Intercomparison (GPCI). The GPCI transect covers the transition from a stratocumulus-topped marine boundary layer (MBL) off the coast of California to a trade cumulus–topped, less-well-defined, MBL, and finally to the deep-convection regions in the intertropical convergence zone (ITCZ). The Atmospheric Infrared Sounder (AIRS) and the Multiangle Imaging Spectroradiometer (MISR) have been used to derive observational records of the two BLH estimates. Analyses from the ECMWF are also used in the study. Both BLH estimates in the models, ...

On the Arctic Wintertime Climate in Global Climate Models

Energy fluxes important for determining the Arctic surface temperatures during winter in present-day simulations from the Coupled Model lntercomparison Project phase 3 (CMIP3) multimodel dataset ar ...

The Importance of Representing Mixed-Phase Clouds for Simulating Distinctive Atmospheric States in the Arctic*

AbstractObservations 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 c...

Improved Representation of Marine Stratocumulus Cloud Shortwave Radiative Properties in the CMIP5 Climate Models

The radiative properties of subtropical marine stratocumulus clouds are investigated in an ensemble of current-generation global climate models from phase 5 of the Climate Model Intercomparison Pro ...

Remote sensing of aerosols in the Arctic for an evaluation of global climate model simulations.

In this study Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua retrievals of aerosol optical thickness (AOT) at 555 nm are compared to Sun photometer measurements from Svalbard for a period of 9 years. For the 642 daily coincident measurements that were obtained, MODIS AOT generally varies within the predicted uncertainty of the retrieval over ocean (ΔAOT = ±0.03 ± 0.05 · AOT). The results from the remote sensing have been used to examine the accuracy in estimates of aerosol optical properties in the Arctic, generated by global climate models and from in situ measurements at the Zeppelin station, Svalbard. AOT simulated with the Norwegian Earth System Model/Community Atmosphere Model version 4 Oslo global climate model does not reproduce the observed seasonal variability of the Arctic aerosol. The model overestimates clear-sky AOT by nearly a factor of 2 for the background summer season, while tending to underestimate the values in the spring season. Furthermore, large differences in all-sky AOT of up to 1 order of magnitude are found for the Coupled Model Intercomparison Project phase 5 model ensemble for the spring and summer seasons. Large differences between satellite/ground-based remote sensing of AOT and AOT estimated from dry and humidified scattering coefficients are found for the subarctic marine boundary layer in summer. KEY POINTS Remote sensing of AOT is very useful in validation of climate models.

A Simple Model of the Northeast Pacific Stratocumulus to Cumulus Transition Based on the Climatological Surface Energy Budget

AbstractAir advected equatorward by the trade winds off the coast of California is associated with decreasing cloud cover and is subjected to increasingly warmer sea surface temperatures. These gradients imply large gradients in the surface energy fluxes. Based on the surface energy balance and on the assumption of a small net surface energy flux, which is supported by reanalysis data, a cloud cover model of the climatological stratocumulus to cumulus transition in the northeastern subtropical Pacific Ocean is developed. Using climatological meteorological surface variables, the model, despite its simplicity, is able to describe the transition from stratocumulus to cumulus reasonably well in terms of cloud cover.