Intense Arctic mesocyclones

Characteristics of polar mesoscale cyclones (PMCs) over the North European Basin are analyzed using cloud cover distribution data for 1981–1995. Special features of the annual cycle and interannual variations in characteristics of Arctic mesocyclones with a spiral and comma-shaped cloud structure are discussed. Against the background of large interannual variations, no statistically significant trends were found in the characteristics of Arctic mesocyclones over the North European Basin in the late 20th century. It is shown that the cumulative frequency distribution of PMCs is well approximated by an exponential function in a size range for Arctic mesocyclones from 50 to 400 km. The applicability of the Weibull distribution as an approximation of the PMC size distribution of the number of cyclone days is analyzed. It is shown that the correspondence between the real distribution and the Weibull distribution became worse in the 1990s than in the 1980s, especially the first half of the 1980s. Much of this was due to an increased local maximum in the 1990s in the distribution of polar mesocyclones with diameters about 400 km. This local maximum was found for all types of mesoscale vortices and for all analyzed five-year subperiods during 1981–1995. A large discrepancy between the frequency distribution functions for PMCs depending on their diameters was shown to exist for different types of Arctic mesocyclones.     

Tropospheric lapse rate and its relation to surface temperature from reanalysis data

Estimates of the tropospheric lapse rate γ and analysis of its relation to the surface temperature T _s in the annual cycle and interannual variability have been made using the global monthly mean data of the NCEP/NCAR reanalysis (1948–2001). The tropospheric lapse rate γ is about 6.1 K/km in the Northern Hemisphere (NH) as a whole and over the ocean and about 6.2 K/km over the continents. The value of γ decreases from 6.5 K/km at low latitudes to 4.5 K/km at polar latitudes. The values of dγ/dT _s, the parameter of sensitivity of γ to the variation of T _s for the NH in the interannual variability, are found to be about 0.04 km^?1 (0.041 km^?1 for the NH as a whole, 0.042 km^?1 over the ocean, and 0.038 km^?1 over the continents). This corresponds to an increase in γ of approximately 0.7% when the surface temperature of the NH is increased by 1 K. Estimates of dγ/dT _s vary from about 0.05 km^?1 in the subtropics to 0.10 km^?1 at polar latitudes. When dγ/dT _s is positive, the surface and tropospheric warming means a temperature decrease above a certain critical level H _cr. The height of the level H _cr with constant temperature, which is defined by the inverse value (dγ/dT _s)^?1, is about 25 km for the NH as a whole, i.e., above the tropopause. In the subtropics, H _cr is about 20 km. At polar latitudes, H _cr decreases to about 10 km. Positive values of dγ/dT _s characterize a positive climatic feedback through the lapse rate and indicate a general decrease in the static stability of the troposphere during global warming. Along with a general tendency of γ to increase with rising T _s, there are regional regimes with the opposite tendency, mainly over the ocean. The negative correlation of γ with T _s is found over the oceanic tropics and midlatitudes, in particular, over the oceanic belt around Antarctica.     

A comparative analysis of the method of extratropical cyclone identification

Three methods for identification cyclones in extratropical latitudes of the Northern Hemisphere (NH) (20°–80° L) are compared based on reanalysis data (1948–2007) for the fields of the sea level pressure (SLP). Different characteristics of extratropical cyclones, namely, their number, intensity, size, and lifetime, are analyzed. The effect of orographic effects for the identification of cyclones and their trajectories is evaluated. The characteristics of extratropical cyclones are compared based on different reanalysis data (National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR), ERA-40, and ERA-INTERIM) with different spatial resolutions.     

Estimates of the sensitivity of cyclonic activity in the troposphere of extratropical latitudes to changes in the temperature regime

Quantitative estimates of the sensitivity of the number and size of extratropical cyclones in the Northern Hemisphere to changes in the surface temperature are obtained with the use of NCEP/NCAR reanalysis data over a 60-year period and are compared with estimates on the basis of a relatively simple model of the cyclonic and anticyclonic activities in the atmosphere of extratropical latitudes associated with characteristics of atmospheric temperature stratification (MMPKh model). The model estimates are also obtained for a dry and moist atmosphere. With the use of the reanalysis data, extratropical latitudes are, on the whole, characterized by a general decrease in the number of cyclones and the density of their packing in extratropical latitudes as the surface temperature increases. However, in the MMPKh model for moist atmosphere, estimates of the parameter of sensitivity of the number of cyclones at midlatitudes and at extratropical latitudes in the Northern Hemisphere as a whole are close to those based on the reanalysis data. The influences of the meridional gradient of the surface temperature and the vertical temperature gradient in the troposphere on changes in the number and size of extratropical cyclones are estimated from the reanalysis data and model calculations. It is noted that the most significant changes in annual mean variations in the number and size of extratropical cyclones are associated with the vertical temperature gradient in the troposphere. In this case, an increase in the vertical temperature gradient in the troposphere decreases the size of cyclones. The relative influences of the vertical and meridional temperature gradients are different for different latitudinal zones.     

Probability distributions for cyclones and anticyclones from the NCEP/NCAR reanalysis data and the INM RAS climate model

Analysis of statistical characteristics of cyclones and anticyclones in the latitudinal belt between 20° and 80°N has been performed with the NCEP/NCAR reanalysis data and simulations with the general circulation climate model of the Institute of Numerical Mathematics of the Russian Academy of Sciences (INM RAS GCCM). The model results have been analyzed for the second half of the 20th century against the NCEP/NCAR reanalysis data and for the 21st century with the SRES-A2 anthropogenic scenario. Overall for the 20th century, no statistically significant changes in the number of cyclones and anticyclones are obtained from either the NCEP/NCAR reanalysis data [1] or from simulations with the INM RAS GCCM [2]. It is found that the total number of cyclones and anticyclones decreased in the 20th century as compared to the 21st century. It is shown that cumulative distributions of the number of cyclones and anticyclones by their intensities and areas have an exponential form from both the reanalysis data and the model simulations, although the corresponding exponents are different.