Impact of stratospheric variability on tropospheric climate change

An improved stratospheric representation has been included in simulations with the Hadley Centre HadGEM1 coupled ocean atmosphere model with natural and anthropogenic forcings for the period 1979–2003. An improved stratospheric ozone dataset is employed that includes natural variations in ozone as well as the usual anthropogenic trends. In addition, in a second set of simulations the quasi biennial oscillation (QBO) of stratospheric equatorial zonal wind is also imposed using a relaxation towards ERA-40 zonal wind values. The resulting impact on tropospheric variability and trends is described. We show that the modelled cooling rate at the tropopause is enhanced by the improved ozone dataset and this improvement is even more marked when the QBO is also included. The same applies to warming trends in the upper tropical troposphere which are slightly reduced. Our stratospheric improvements produce a significant increase of internal variability but no change in the positive trend of annual mean global mean near-surface temperature. Warming rates are increased significantly over a large portion of the Arctic Ocean. The improved stratospheric representation, especially the QBO relaxation, causes a substantial reduction in near-surface temperature and precipitation response to the El Chichón eruption, especially in the tropical region. The winter increase in the phase of the northern annular mode observed in the aftermath of the two major recent volcanic eruptions is partly captured, especially after the El Chichón eruption. The positive trend in the southern annular mode (SAM) is increased and becomes statistically significant which demonstrates that the observed increase in the SAM is largely subject to internal variability in the stratosphere. The possible inclusion in simulations for future assessments of full ozone chemistry and a gravity wave scheme to internally generate a QBO is discussed.     

Use of atmospheric radiation measurement program data from Barrow, Alaska, for evaluation and development of snow-albedo parameterizations

Summary Snow albedo is determined from the ratio of out-going to incoming solar radiation using three years of broadband shortwave radiometer data obtained from the Barrow, Alaska, Atmospheric Radiation Measurement (ARM) site. These data are used for the evaluation of various types of snow-albedo parameterizations applied in numerical weather prediction or climate models. These snow-albedo parameterizations are based on environmental conditions (e.g., air or snow temperature), snow related characteristics (e.g., snow depth, snow age), or combinations of both. The ARM data proved to be well suited for snow-albedo evaluation purposes for a low-precipitation tundra environment. The evaluation confirms that snow-age dependent parameterizations of snow albedo work well during snowmelt, while parameterizations considering meteorological conditions often perform better during snow accumulation. Current difficulties in parameterizing snow albedo occur for long episodes of snow-event free conditions and episodes with a high frequency of snow events or strong snowfall. In a further step, the first two years of the ARM albedo dataset is used to develop a snow-albedo parameterization, and the third year’s data serves for its evaluation. This parameterization considers snow depth, wind speed, and air temperature which are found to be significant parameters for snow-albedo modeling under various conditions. Comparison of all evaluated snow-albedo parameterizations with this new parameterization shows improved snow-albedo prediction. Correspondence: Nicole M?lders, Geophysical Institute and College of Natural Science and Mathematics, University of Alaska Fairbanks, 903 Koyukuk Drive, P.O. Box 757320, Fairbanks, AK 99775-7320, USA     

North Sea-Caspian Pattern (NCP) – an upper level atmospheric teleconnection affecting the Eastern Mediterranean: Identification and definition

Summary An upper level atmospheric teleconnection between grid points: 0°, 55° N; 10° E, 55° N (North Sea) and 50° E, 45° N; 60° E, 45° N (northern Caspian) was identified. This teleconnection, referred as the North Sea-Caspian Pattern (NCP) is evident at the 500 hPa level. The NCP is more pronounced during winter and the transitional seasons. An index (NCPI) measures the geopotential heights differences between the two poles of the NCP. Time series of the NCPI are presented and analysed. Except for September, no significant temporal trends were found. Negative and positive phases of the NCP (NCP(−) and NCP(+), respectively) were defined using standardized scores. A classification of all months into NCP(−), NCP(+) or normal conditions during the analysis period (1958–1998) was prepared and analysed. No significant correlation was found between the NCPI and the NAO index. The anomalous circulation during either NCP(−) or NCP(+) conditions is defined and its possible impact on the regional climate is discussed. Preliminary results show below normal temperatures and above normal precipitation in the Balkans and the Middle East during NCP(+), and the opposite for NCP(−). Received March 8, 2001 Revised July 3, 2001     

Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer

The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored.     

The attenuation of acoustic-gravity waves in a slightly dissipative atmosphere

This paper is concerned with computation of the attenuation coefficient of internal acoustic-gravity waves in a slightly dissipative horizontally stratified atmosphere. Two models are generally used in deriving this attenuation coefficient. In one model, the nonlinear hydrodynamic equations are analyzed by using a Stokes expansion and the WKB solutions to the linearized perturbations are sought by using the coupled mode formulation. In the second model, the attenuation is viewed as the dissipation of pseudoenergy. We show in this paper that in a slightly dissipative atmosphere, these two models give identical results as far as computation of the attenuation coefficient is concerned, although in a highly dissipative atmosphere the results are known to be different.     

Using Remote Sensing to Assess Russian Forest Fire Carbon Emissions

Russian boreal forests are subject to frequent wildfires. The resulting combustion of large amounts of biomass not only transforms forest vegetation, but it also creates significant carbon emissions that total, according to some authors, from 35–94 Mt C per year. These carbon emissions from forest fires should be considered an important part of the forest ecosystem carbon balance and a significant influence on atmospheric trace gases. In this paper we discuss a new method to assess forest fire damage. This method is based on using multi-spectral high-resolution satellite images, large-scale aerial photography, and declassified images obtained from the space-borne national security systems. A normalized difference vegetation index (NDVI) difference image was produced from pre- and post-fire satellite images from SPOT/HRVIR and RESURS-O/MSU-E images. A close relationship was found between values of the NDVI difference image and forest damage level. High-resolution satellite data and large-scale aerial-photos were used to calibrate the NDVI-derived forest damage map. The method was used for mapping of forest fire extent and damage and for estimating carbon emissions from burned forest areas.     

Quantifying Organization of Atmospheric Turbulent Eddy Motion Using Nonlinear Time Series Analysis

Using three methods from nonlinear dynamics, we contrast the level of organization inthe vertical wind velocity (w) time series collected in the atmospheric surface layer(ASL) and the canopy sublayer (CSL) for a wide range of atmospheric stability ()conditions. The nonlinear methods applied include a modified Shannon entropy, waveletthresholding, and mutual information content. Time series measurements collected overa pine forest, a hardwood forest, a grass-covered forest clearing, and a bare soil, desertsurface were used for this purpose. The results from applying all three nonlinear timeseries measures suggest that w in the CSL is more organized than that in the ASL, and that as the flows in both layers evolve from near-neutral to near-convective conditions, the level of organization increases. Furthermore, we found that the degree of organization in w associated with changes in is more significant than the transition from CSL to ASL.     

Relevance of soil moisture for seasonal climate predictions: a preliminary study

In the framework of the Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of the ARPEGE atmospheric general circulation model has been forced with meteorological observations and analyses in order to produce a two-year (1987–1988) soil moisture climatology at a 1^°×1^° horizontal resolution. This climatology is model dependent, but it is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. In the present study, ensembles of seasonal simulations (March to September) have been performed for 1987 and 1988, in which the total soil water content simulated by ARPEGE is relaxed towards the GSWP climatology. The results indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability, thereby confirming the utility of the GSWP soil moisture data for prescribing initial or boundary conditions in comprehensive climate and numerical weather prediction models. They also demonstrate the relevance of soil moisture for achieving realistic simulations of the Northern Hemisphere summer climate. In order to get closer to the framework of seasonal predictions, additional experiments have been performed in which GSWP is only used for initialising soil moisture at the beginning of the summer season (the relaxation towards GSWP is removed on 1st June). The results show a limited improvement of the interannual variability, compared to the simulations initialised from the ARPEGE climatology. However, some regional patterns of the precipitation differences between 1987 and 1988 are better captured, suggesting that seasonal predictions can benefit from a better initialisation of soil moisture.     

A comprehensive model inter-comparison study investigating the water budget during the BALTEX-PIDCAP period

Summary A comparison of 8 regional atmospheric model systems was carried out for a three-month late summer/early autumn period in 1995 over the Baltic Sea and its catchment area. All models were configured on a common grid using similar surface and lateral boundary conditions, and ran in either data assimilation mode (short term forecasts plus data assimilation), forecast mode (short term forecasts initialised daily with analyses) or climate mode (no re-initialisation of model interior during entire simulation period). Model results presented in this paper were generally post processed as daily averaged quantities, separate for land and sea areas when relevant. Post processed output was compared against available analyses or observations of cloud cover, precipitation, vertically integrated atmospheric specific humidity, runoff, surface radiation and near surface synoptic observations. The definition of a common grid and lateral forcing resulted in a high degree of agreement among the participating model results for most cases. Models operated in climate mode generally displayed slightly larger deviations from the observations than the data assimilation or forecast mode integration, but in all cases synoptic events were well captured. Correspondence to near surface synoptic quantities was good. Significant disagreement between model results was shown in particular for cloud cover and the radiative properties, average precipitation and runoff. Problems with choosing appropriate initial soil moisture conditions from a common initial soil moisture field resulted in a wide range of evaporation and sensible heat flux values during the first few weeks of the simulations, but better agreement was shown at later times. Received September 8, 2000 Revised April 3, 2001