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Relative Vorticity (relative + vorticity)
Selected AbstractsEnvironmental links to reduced tropical cyclogenesis over the south-east CaribbeanINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2007Alexandros P. Georgiadis Abstract Tropical cyclone formation over the Caribbean is not evenly distributed across the basin. Previous work identified the south-western sector as the area that dominates the hurricane activity of the whole basin. The south-eastern sector, in contrast, exhibits a distinct suppression in cyclogenesis. This study seeks to identify the restricting factors that differentiate the cyclogenesis climatology in the south-east Caribbean. It is based on statistical analysis of eight environmental variables using principal component analysis. The first three components are examined in detail. The first component, accounting for 31.7% of the variance within the data, differentiates the Caribbean from the rest of the subtropical Atlantic, primarily in terms of an increased thermodynamic potential for cyclogenesis and enhanced atmospheric humidity in the boundary layer. The second component, accounting for 31% of the variance, marks the south-west Caribbean as the only sub-region within the area of analysis where the easterlies curve southwards and the relative vorticity is cyclonic. The third component, accounting for 20.3% of the variance, differentiates the South Caribbean from the rest of the Atlantic, indicating it to be a region of increased spatial variation in the intensity of the easterlies and in absolute vorticity. The variance of cyclogenesis within the Caribbean is greatest along the second and third components. Thus, the genesis pattern in the basin is likely associated with the low-level wind-field and absolute vorticity. The divergence of the flow, when combined with the vorticity of the wind-field over the Caribbean, renders the background conditions of the eastern sector less prone to developing disturbances than is the case in the western Caribbean. Copyright © 2006 Royal Meteorological Society [source] Near-surface-temperature lapse rates on the Prince of Wales Icefield, Ellesmere Island, Canada: implications for regional downscaling of temperatureINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2007Shawn J. Marshall Abstract Screen temperatures were monitored from May 2001 to April 2003 in an array of 25 sites on the Prince of Wales Icefield, Ellesmere Island, Canada. The observational network covered an area of ca 15 650 km2 and spanned an altitude ranging from 130 to 2010 m above sea level. The spatial array provides a record of near-surface-temperature lapse rates and mesoscale temperature variability on the icefield. The mean daily lapse rate in the 2-year record is , 4.1° C km,1, with an average summer lapse rate of , 4.3° C km,1. Surface-temperature lapse rates in the region are therefore systematically less than the free-air lapse rates that are typically adopted for extrapolations of sea-level temperature to higher altitudes. Steep lapse rates, resembling moist adiabatic rates in the free air (,6 to , 7° C km,1), are more common in summer at our site and are associated with enhanced cyclonic activity (low-pressure and high relative vorticity) and southerly flow aloft. In contrast, northerly, anticyclonic flow prevails when summer lapse rates are weak (above , 2° C km,1). The low surface-temperature lapse rates and their systematic synoptic variability have important implications for applications that require downscaling or extrapolation of surface- or boundary-layer temperatures, such as modelling of glacier mass balance. We illustrate this in an analysis of observed versus modelled snowmelt on the icefield. Copyright © 2006 Royal Meteorological Society. [source] Flow dependence of background errors and their vertical correlations for radiance-data assimilationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010Reinhold Hess Abstract This article examines the dependence of background-error statistics on synoptic conditions and flow patterns. Error variances and vertical correlations of background temperatures as used for variational radiance-data assimilation are estimated for two different weather regimes over Europe using the NMC method. The results are validated with real observations, i.e. radiosonde data and microwave satellite radiances and generalised with half a year of global data from the ECMWF forecasting system, where weather conditions are distinguished using model fields of wind speed, mean sea level pressure, and relative vorticity. Strong winds, low pressure, and cyclonic flow generally induce larger background errors of 500 hPa temperature than calm winds, high pressure, and anticyclonic flow, and also broader temperature correlations in the vertical with other tropospheric levels. Copyright © 2010 Royal Meteorological Society [source] Instability of finite-amplitude lower-neutral Eady wavesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 620 2006M. Fantini Abstract The problem of the stability of the two-dimensional neutral Eady wave, studied numerically by Fantini and Davolio (2001) as a possible mechanism leading to strong orographic cyclogenesis, is re-examined. An approximate analytic solution is found, and a simple condition for the appearance of meridionally structured unstable secondary modes is proposed, based on the relative vorticity of the primary wave. The approximate analytic solution compares well with the numerical simulations and extends previous results. Copyright © 2006 Royal Meteorological Society [source] Structure, genesis and scale selection of the tropical quasi-biweekly modeTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 599 2004Piyali Chatterjee Abstract The quasi-biweekly mode (QBM) and the 30,60 day mode are two major intraseasonal oscillations (ISOs) in the tropics. The QBM is known to have a major influence in determining the active and break conditions of the Indian monsoon during the northern summer. A westward-propagating equatorial Rossby wave with quasi-biweekly period influences the Australian monsoon during the northern winter. Universality between the summer and winter QBM is established through analysis of daily circulation and convection data for 10 years. It is shown that the mean spatial structure of the QBM in circulation and convection resembles that of a gravest meridional mode equatorial Rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4.5 m s,1. However, the maximum zonal wind occurs at around 5°N (5°S) during the northern summer (winter). The wave structure appears to be translated northward (southward) by about 5° during the northern summer (winter). The relationship between outgoing long-wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection. Similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons. An acceptable mechanism for genesis and scale selection of the QBM has been lacking. In the present study, a mechanism for genesis and scale selection of the observed QBM is proposed. A simple 2½-layer model that includes a steady Ekman boundary layer (BL) formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves. Without influence of the background mean flow, moist feedback in the presence of frictional BL convergence drives the gravest meridional mode equatorial Rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator. Potential temperature perturbation associated with the Rossby wave is in phase with relative vorticity perturbation at low level. The BL drives moisture convergence in phase with the relative vorticity at the top of the BL. Release of latent heat associated with the BL convergence enhances the potential temperature leading to a positive feedback. The mean flow over the Indian Ocean and western Pacific at low levels is such that the zero ambient absolute vorticity or the ,dynamic equator' shifts to around 5°N (5°S) during summer (winter) and results in a shift of the unstable Rossby waves towards the north (south) by about 5°. The resulting structure of the unstable Rossby mode resembles the observed structure of the biweekly mode. It is shown that neither evaporation,wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode. However these processes marginally modify the growth rate and make the structure of the unstable wave more realistic. Copyright © 2004 Royal Meteorological Society [source] | ||||||||||