Weather Systems (weather + system)

Distribution by Scientific Domains


Selected Abstracts


Suspended dust over southeastern Mediterranean and its relation to atmospheric circulations

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2008
Uri Dayan
Abstract The Middle East deserts are often subjected to dust, which reduces horizontal visibility to 5 km, and sometimes even to < 1 km. The present study examines the annual and inter-annual occurrences of dust events based on 37 years of visibility observations from Hazerim (near Beer Sheba) correlated with PM10 dust concentration. The visibility data was converted to PM10 dust concentration, using concurrent data for three years. We then analyse the linkage between dust and synoptic- to global-scale weather systems. The monthly data indicate that the dust season starts in October and ends in May, with a maximum in March. More than 89% of the total annual dust is accumulated between December and May, the ,high dust season'. The annual totals vary as much as an order of magnitude from year to year. The synoptic system that produces the majority of the dust over the northern Negev is the Cyprus Low, contributing 2/3 of both the total yearly dust yield and of the number of dust observations. This suggests that a positive relationship exists between the dust in the Negev and rainfall in north Israel, both of which are generated by Cyprus Lows. Indeed, a significant (at 0.05 level) correlation of + 0.30 was found between the two. Correlation maps evidence that in dust-rich years the cyclonic activity over the Mediterranean is abnormally high and in poor-dust years it is low. A highly significant negative correlation (,0.66) was found between the dust yield and the intensity of the North Atlantic oscillation (NAO), which modulates the cyclonic activity over Europe and the northern Mediterranean. This may also imply that periods in which more dust accumulated as loess in the northern Negev may indicate the existence of negative NAO phase, and concurrently, warmer conditions over the Sahara, colder conditions over Europe and enhanced rainfall over the Mediterranean Basin. Copyright © 2007 Royal Meteorological Society [source]


Remote weather associated with South Pacific subtropical sea-level high properties

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2004
Richard Grotjahn
Abstract The subtropical highs in sea-level pressure (SLP) are little studied and incompletely understood. In recent years, three groups of theories, i.e. tropical divergent circulations, subtropical Rossby wavetrains, and midlatitude frontal cyclone interactions, have been proposed for remote maintenance of these highs. The latter is presented here as a remote forcing of these highs for the first time in the reviewed literature. The focus of the study is upon illuminating associations between these mechanisms and the South Pacific subtropical high in SLP (SP high). Precipitation, outgoing longwave radiation, velocity potential, and divergent winds are used as proxy markers for the remote forcing mechanisms. The tools used include composites, one-point correlations, autocorrelations, cross-correlations, and cross-spectra. Observational evidence, in monthly and daily data, is examined that appears to support each mechanism. Associations seen in monthly data are better understood in daily data at various lags. Convection over Amazonia, coordinated with suppressed convection in the western tropical Pacific, leads to enhanced SLP on the tropical side of the high. Midlatitude weather systems are the strongest influence upon the maximum SLP and the SLP on the higher latitude side of the high. The western side is associated with both middle-and lower-latitude phenomena, such as the South Pacific convergence zone. Various properties of the high have a strong period around 45 days. Associations to the Madden,Julian oscillation and El Niño,southern oscillation are explored and are strong only for the tropical side of the SP high. Copyright © 2004 Royal Meteorological Society [source]


The flow of energy through the earth's climate system

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 603 2004
Kevin E. Trenberth
Abstract The primary driver of the climate system is the uneven distribution of incoming and outgoing radiation on earth. The incoming radiant energy is transformed into various forms (internal heat, potential energy, latent energy, and kinetic energy), moved around in various ways primarily by the atmosphere and oceans, stored and sequestered in the ocean, land, and ice components of the climate system, and ultimately radiated back to space as infrared radiation. The requirement for an equilibrium climate mandates a balance between the incoming and outgoing radiation, and further mandates that the flows of energy are systematic. These drive the weather systems in the atmosphere, currents in the ocean, and fundamentally determine the climate. Values are provided for the seasonal uptake and release of heat by the oceans that substantially moderate the climate in maritime regions. In the atmosphere, the poleward transports are brought about mainly by large-scale overturning, including the Hadley circulation in low latitudes, and baroclinic storms in the extratropics, but the seamless nature of the transports on about monthly time-scales indicates a fundamental link between the two rather different mechanisms. The flows of energy can be perturbed, causing climate change. This article provides an overview of the flows of energy, its transformations, transports, uptake, storage and release, and the processes involved. The focus is on the region 60°N to 60°S, and results are presented for the solstitial seasons and their differences to highlight the annual cycle. Challenges in better determining the surface heat balance and its changes with time are discussed. Copyright © 2004 Royal Meteorological Society [source]


A study of departures from the inverse-barometer response of sea level to air-pressure forcing at a period of 5 days

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004
E. L. Mathers
Abstract The response of the sea level to atmospheric forcing is investigated through the cross-spectral analysis of tide-gauge data from around 100 island and deep-ocean stations and atmospheric pressure records. The focus is at a period of 5 days, coinciding with an atmospheric pressure oscillation (Madden,Julian wave) of the same frequency. Outside the Tropics the energy in synoptic weather systems masks the small amplitude of the pressure wave, resulting in a response indistinguishable from that of an inverse barometer (IB). Within the Tropics, variability of air pressure is smaller in comparison with the amplitude of the pressure wave and departures from the IB model are found. Analysis of several sub-surface pressure records in the tropical Atlantic also yields departures at this time-scale, and indicates the barotropic nature of the response. Findings from a near-global barotropic numerical model forced with realistic air pressure and wind-stress fields show reasonable agreement with results at tide-gauge locations, and provide a more complete view of the sea-level response. These results indicate the possibility of providing an improved simulation of sea-level response to atmospheric-pressure forcing at this time-scale than is currently provided by the IB model, for example in the field of satellite altimetry. Copyright © 2004 Royal Meteorological Society [source]


A comparison of the impact of TOVS arid ATOVS satellite sounding data on the accuracy of numerical weather forecasts

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 569 2000
S. J. English
Abstract The Advanced TIROS Operational Vertical Sounder (ATOVS) was launched on the NOAA-15 satellite in May 1998. This provided a very significant improvement in the information available from meteorological polar-orbiting satellites compared with the previous TIROS Operational Vertical Sounder system, particularly for humidity and vertical resolution of temperature in cloudy areas. In preparation for assimilation of the observations into a three-dimensional analysis of atmospheric temperature and humidity, the observations have been compared with calculated top-of-atmosphere brightness temperatures computed from numerical weather prediction model profiles of temperature and humidity. Differences between observed and modelled brightness temperature are small. In some parts of the tropics and northern hemisphere the standard deviation of these differences for the tropospheric Advanced Microwave Sounding Unit sounding channels is only marginally higher than the radiometric noise of the observations. Early in 1999 a series of observation-system experiments were completed in which ATOVS observations were assimilated using a one-dimensional variational analysis. No use of the new humidity information could be made because of interference problems experienced by the microwave humidity sounder on ATOVS. Nonetheless, these experiments showed that the assimilation of the new temperature information provided by the radiance observations reduces forecast errors by as much as 20% in the southern hemisphere and 5% in the northern hemisphere. Further improvements have been found by assimilating more data over land. The major impact arises from the microwave channels. Whilst forward-model errors may be slightly lower for the microwave channels than the infrared channels the primary reason is the provision of sounding information in active weather systems, which are usually cloudy. [source]


Understanding African easterly waves: a moist singular vector approach

ATMOSPHERIC SCIENCE LETTERS, Issue 3 2009
Rosalind J. Cornforth
Abstract Moist singular vectors (MSV) have been applied successfully to predicting mid-latitude storms growing in association with latent heat of condensation. Tropical cyclone sensitivity has also been assessed. Extending this approach to more general tropical weather systems here, MSVs are evaluated for understanding and predicting African easterly waves, given the importance of moist processes in their development. First results, without initial moisture perturbations, suggest MSVs may be used advantageously. Perturbations bear similar structural and energy profiles to previous idealised non-linear studies and observations. Strong sensitivities prevail in the metrics and trajectories chosen, and benefits of initial moisture perturbations should be appraised. Copyright © 2009 Royal Meteorological Society [source]