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     

Performance of a multi-RCM ensemble for South Eastern South America

The ability of four regional climate models to reproduce the present-day South American climate is examined with emphasis on La Plata Basin. Models were integrated for the period 1991–2000 with initial and lateral boundary conditions from ERA-40 Reanalysis. The ensemble sea level pressure, maximum and minimum temperatures and precipitation are evaluated in terms of seasonal means and extreme indices based on a percentile approach. Dispersion among the individual models and uncertainties when comparing the ensemble mean with different climatologies are also discussed. The ensemble mean is warmer than the observations in South Eastern South America (SESA), especially for minimum winter temperatures with errors increasing in magnitude towards the tails of the distributions. The ensemble mean reproduces the broad spatial pattern of precipitation, but overestimates the convective precipitation in the tropics and the orographic precipitation along the Andes and over the Brazilian Highlands, and underestimates the precipitation near the monsoon core region. The models overestimate the number of wet days and underestimate the daily intensity of rainfall for both seasons suggesting a premature triggering of convection. The skill of models to simulate the intensity of convective precipitation in summer in SESA and the variability associated with heavy precipitation events (the upper quartile daily precipitation) is far from satisfactory. Owing to the sparseness of the observing network, ensemble and observations uncertainties in seasonal means are comparable for some regions and seasons.     

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     

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.     

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.     

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.     

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.     

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.     

The Effect of Moisture on the Kinetic Energy Budget of a Mediterranean Cyclone

Summary  A diagnostic energetics analysis is used to study the effects of moisture-related processes on a developing cyclone over the Mediterranean. This is done by using the moist wind component to calculate the energy budget and then the effect due to wind field changes on the kinetic energy budget is illustrated. The horizontal flux convergence serves as a major energy budget source in both cases (actual and moist wind), although the magnitude values of this term are small in the case of the moist wind. Generation of kinetic energy, is generally (in the case of moist wind), a prominent sink during the life cycle of the cyclone, and its values are greater than the corresponding ones for the actual wind field except at the decay period. Subgrid-scale sources of kinetic energy provide a substantial energy gain throughout the life cycle of the cyclone. The values of the dissipation term differ from using the actual or moist components where its values are influenced by the values of the other terms in the budget. The baroclinic generation due to the divergent moist wind component offsets by 80.8% and 12.1% for the barotropic destruction of kinetic energy by the rotational moist wind component. The divergent moist wind component was found to be very important in the synoptic-scale environments of the cyclogenesis. Both demonstrate that the divergent moist wind component is as important as the rotational moist wind component in producing generation and horizontal flux divergence of kinetic energy. Generation of kinetic energy by the divergent moist wind component seems to be a major factor leading to the creation of upper-level wind maxima north of the storm areas. Thus, these diagnostic findings suggest possible modifications to the wind field by investigating the role of the divergent moist wind component and may also be fruitful in exploring the effects of cyclogensis on the large-scale environment. Received April 27, 1998/Revised April 23, 1999