Heiner Körnich

Evidence for interhemispheric stratosphere‐mesosphere coupling derived from noctilucent cloud properties

Evidence for interhemispheric stratosphere-mesosphere coupling derived from noctilucent cloud properties

How well do reanalyses represent the southern African precipitation

Monthly-mean precipitation observations over southern Africa are used to evaluate the performance of eight global reanalyses: ERA-40, ERA-interim, JRA-25, MERRA, CFSR, NCEP-R1, NCEP-R2 and 20CRv2. All eight reanalyses reproduce the regionally averaged seasonal cycle fairly well; a few spatial mismatches with the observations are found in the climate mean for the rainy season. Principal component analyses show a dipole in the leading modes of all reanalyses, however with crucial differences in its spatial position. Possible reasons for the differences between the reanalyses are discussed on the basis of the ERA-interim and 20CRv2 results. A comparison between the moisture transports shows that ERA-interim manifests a very strong moisture convergence over the eastern equatorial Atlantic, resulting in the strong precipitation here. This excessive convergence may be due to the water–vapor assimilation and convection parameterization. Over the Indian Ocean, the ITCZ is shifted northward in ERA-interim compared to its position in 20CRv2. This discrepancy is most likely attributable to the meridional SST gradients in the Indian Ocean which are significantly larger in the ERA-interim than those in the 20CRv2, and the resulting atmospheric response prevents a southward shift of the ITCZ. Overall, the consistent description of the dynamical circulation of the atmosphere and the hydrological cycle appears as a crucial benchmark for reanalysis data. Based on our evaluation, the preferential reanalysis for investigating the climate variability over southern Africa is 20CRv2 that furthermore spans the longest time period, hence permitting the most precise investigations of interannual to decadal variability.

Northern Hemisphere Stationary Waves in Future Climate Projections

The response of the atmospheric large-scale circulation to an enhanced greenhouse gas (GHG) forcing varies among coupled global climate model (CGCM) simulations. In this study, sixteen CGCM simulat ...

Corrigendum to "Climate change between the mid and late Holocene in northern high latitudes – Part 1: Survey of temperature and precipitation proxy data" published in Clim. Past, 6, 591–608, 2010

th sentence The available reconstructions indicate that the north- ern high latitudes were warmer in both summer, winter and the annual mean temperature at the mid-Holocene

On the fog variability over south Asia

An increasing trend in fog frequencies over south Asia during winter in the last few decades has resulted in large economical losses and has caused substantial difficulties in the daily lives of people. In order to better understand the fog phenomenon, we investigated the climatology, inter-annual variability and trends in the fog occurrence from 1976 to 2010 using observational data from 82 stations, well distributed over India and Pakistan. Fog blankets large area from Pakistan to Bangladesh across north India from west to east running almost parallel to south of the Himalayas. An EOF analysis revealed that the fog variability over the whole region is coupled and therefore must be governed by some large scale phenomenon on the inter-annual time scale. Significant positive trends were found in the fog frequency but this increase is not gradual, as with the humidity, but comprises of two distinct regimes shifts, in 1990 and 1998, with respect to both mean and variance. The fog is also detected in ERA-Interim 3 hourly, surface and model level forecast data when using the concept of “cross-over temperature” combined with boundary layer stability. This fog index is able to reproduce the regime shift around 1998 and shows that the method can be applied to analyze fog over south Asia. The inter-annual variability seems to be associated with the wave train originating from the North Atlantic in the upper troposphere that when causing higher pressure over the region results in an increased boundary layer stability and surface-near relative humidity. The trend and shifts in the fog occurrence seems to be associated with the gradual increasing trend in relative humidity from 1990 onwards.

The Role of Stationary Waves in the Maintenance of the Northern Annular Mode as Deduced from Model Experiments

Abstract The influence of stationary waves on the maintenance of the tropospheric annular mode (AM) is examined in a simple global circulation model with perpetual January conditions. The presented model experiments vary in the configurations of stationary wave forcing by orography and land–sea heating contrasts. All simulations display an AM-like pattern in the lower troposphere. The zonal momentum budget shows that the feedback between eddies with periods less than 10 days and the zonal-mean zonal wind is generally the dominating process that maintains the AM. The kinetic energy of the high-frequency eddies depends on the stationary wave forcing, where orographic forcing reduces and thermal forcing enhances it. The AMs in the model experiments differ in the superposed anomalous stationary waves and in the strength of the zonally symmetric component. If only orographic stationary wave forcing is taken into account, the mountain torque decelerates the barotropic wind anomaly, and thus acts to weaken the A...

Large-Scale Dynamical Response to Subgrid-Scale Organization Provided by Cellular Automata

Due to the limited resolution of numerical weather prediction (NWP) models, sub-grid scale physical processes are parameterized, and represented by grid-box means. However, some physical processes ...

A Conceptual Model of the Surface Salinity Distribution in the Oceanic Hadley Cell

A conceptual model of the salinity distribution in the oceanic Hadley cell is presented. The model pertains to the region of tropical easterly surface winds, where the surface salinity increases po ...

Climate sensitivity and variability examined in a global climate model

The climate system covers a large span of spatial and temporal time-scales, which range from molecular length scale and fractions of a second to global extension and thousands of years. The former are displayed in the micro physics of a cloud drop, while the latter can be found in the deep circulation of the ocean. The scale interaction gives rise to the complexity of the climate system. In the presented projects, two important aspects of the climate system are explored: its sensitivity to external forcing and its internal variability.

Combining the mid-latitudinal and equatorial mass/wind balance relationships in global data assimilation

Multivariate data assimilation is achieved by introducing mass/wind balance relationships in the background term. Geostrophy is commonly used as such a relationship in mid-latitudes. For tropical latitudes, a newbalance relationship on the basis of equatorial waves has been proposed. In order to combine the equatorial with the mid-latitudinal formulation, a new data assimilation scheme is developed. The balance relationships are formulated in terms of Hough modes. For the minimization of the cost function, a control variable is constructed, where the background error is projected onto an appropriate selection of Hough modes which are weighted with their respective background covariances. The covariance structures of the data assimilation scheme are examined with single observation experiments and the increments are discussed with respect to the balance relationships. Finally, the new proposed assimilation scheme is tested in a simple observing system simulation experiment. The application of an incomplete balance relationship based on geostrophy leads to a misinterpretation of observational data and thus to enhanced analysis errors. Only the combined balance relationship improves the tropical analysis. This improvement is expected to play an important role for the analysis quality, when future wind observations from the Earth Explorer Atmospheric Dynamics Mission are available.