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ncar + reanalysi)

Distribution by Scientific Domains

Earth and Environmental Science	80%

Selected Abstracts

Hydrologic response of the Greenland ice sheet: the role of oceanographic warming

HYDROLOGICAL PROCESSES, Issue 1 2009
E. Hanna
Abstract The response of the Greenland ice sheet to ongoing climate change remains an area of great uncertainty, with most previous studies having concentrated on the contribution of the atmosphere to the ice mass-balance signature. Here we systematically assess for the first time the influence of oceanographic changes on the ice sheet. The first part of this assessment involves a statistical analysis and interpretation of the relative changes and variations in sea-surface temperatures (SSTs) and air temperatures around Greenland for the period 1870,2007. This analysis is based on HadISST1 and Reynolds OI.v2 SST analyses, in situ SST and deeper ocean temperature series, surface-air-temperature records for key points located around the Greenland coast, and examination of atmospheric pressure and geopotential height from NCEP/NCAR reanalysis. Second, we carried out a novel sensitivity experiment in which SSTs were perturbed as input to a regional climate model, and document the resulting effects on simulated Greenland climate and surface mass balance. We conclude that sea-surface/ocean temperature forcing is not sufficient to strongly influence precipitation/snow accumulation and melt/runoff of the ice sheet. Additional evidence from meteorological reanalysis suggests that high Greenland melt anomalies of summer 2007 are likely to have been primarily forced by anomalous advection of warm air masses over the ice sheet and to have therefore had a more remote atmospheric origin. However, there is a striking correspondence between ocean warming and dramatic accelerations and retreats of key Greenland outlet glaciers in both southeast and southwest Greenland during the late 1990s and early 2000s. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Intercomparison of global cloud cover fields over oceans from the VOS observations and NCEP/NCAR reanalysis

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2007
Ernst Bedacht
Abstract The paper inter-compares the total cloud cover over the World Ocean from marine visual observations assimilated in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and National Centers of Environmental Prediction/National Center of Atmospheric Research (NCEP-NCAR) reanalysis. The Intercomparison covers the period from 1948 to 2002. NCEP-NCAR reanalysis shows about 10% of fractional cloud cover smaller than the visual observations do. The largest differences are observed in the mid and sub-polar latitudes. In the tropics, NCEP-NCAR data show slightly higher cloud cover then ICOADS. These systematic differences are quite persistent through the year with somewhat stronger differences in summer. Comparison of the characteristics of inter-annual variability shows little consistency between visually observed total cloud cover and total cloudiness diagnosed by the reanalysis. Linear trends are primarily positive in the ICOADS cloud data, while in the NCEP-NCAR reanalysis they show downward trends in the tropics and upward tendencies in the mid and high latitudes. Analysis of the effect of sampling in ICOADS shows that sampling inhomogeneity cannot fully explain the disagreements observed. At the same time, the major climate variability patterns such as North Atlantic Oscillation (NAO) and El-Nino,Southern Oscillation (ENSO) are well captured in both ICOADS and NCEP-NCAR cloud cover data sets. Copyright © 2007 Royal Meteorological Society [source]

Simulation of seasonal precipitation and raindays over Greece: a statistical downscaling technique based on artificial neural networks (ANNs)

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2007
K. Tolika
Abstract A statistical downscaling technique based on artificial neural network (ANN) was employed for the estimation of local changes on seasonal (winter, spring) precipitation and raindays for selected stations over Greece. Empirical transfer functions were derived between large-scale predictors from the NCEP/NCAR reanalysis and local rainfall parameters. Two sets of predictors were used: (1) the circulation-based 500 hPa and (2) its combination along with surface specific humidity and raw precipitation data (nonconventional predictor). The simulated time series were evaluated against observational data and the downscaling model was found efficient in generating winter and spring precipitation and raindays. The temporal evolution of the estimated variables was well captured, for both seasons. Generally, the use of the nonconventional predictors are attributed to the improvement of the simulated results. Subsequently, the present day and future changes on precipitation conditions were examined using large-scale data from the atmospheric general circulation model HadAM3P to the statistical model. The downscaled climate change signal for both precipitation and raindays, partly for winter and especially for spring, is similar to the signal from the HadAM3P direct output: a decrease of the parameters is predicted over the study area. However, the amplitude of the changes was different. Copyright © 2006 Royal Meteorological Society [source]

Europe's 2003 heat wave: a satellite view of impacts and land,atmosphere feedbacks

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2006
Benjamin F. Zaitchik
Abstract A combination of satellite imagery, meteorological station data, and the NCEP/NCAR reanalysis has been used to explore the spatial and temporal evolution of the 2003 heat wave in France, with focus on understanding the impacts and feedbacks at the land surface. Vegetation was severely affected across the study area, especially in a swath across central France that corresponds to the Western European Broadleaf (WEB) Forests ecological zone. The remotely sensed surface temperature anomaly was also greatest in this zone, peaking at +15.4 °C in August. On a finer spatial scale, both the vegetation and surface temperature anomalies were greater for crops and pastures than for forested lands. The heat wave was also associated with an anomalous surface forcing of air temperature. Relative to other years in record, satellite-derived estimates of surface-sensible heat flux indicate an enhancement of 48,61% (24.0,30.5 W m,2) in WEB during the August heat wave maximum. Longwave radiative heating of the planetary boundary layer (PBL) was enhanced by 10.5 W m,2 in WEB for the same period. The magnitude and spatial structure of this local heating is consistent with models of the late twenty-first century climate in France, which predict a transitional climate zone that will become increasingly affected by summertime drought. Models of future climate also suggest that a soil-moisture feedback on the surface energy balance might exacerbate summertime drought, and these proposed feedback mechanisms were tested using satellite-derived heat budgets. Copyright © 2006 Royal Meteorological Society. [source]

Individual and combined influence of El Niño,Southern Oscillation and Indian Ocean Dipole on the Tropospheric Biennial Oscillation

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010
Prasanth A. Pillai
Abstract Tropospheric biennial oscillation (TBO) is the tendency of a strong monsoon to be followed by a weaker one and vice versa. It involves both oceanic and atmospheric processes in the tropical Indian and Pacific Ocean regions. The present study analyses the effect of dynamical processes of the Indian and Pacific Oceans like the Indian Ocean Dipole (IOD) and El Niño,Southern Oscillation (ENSO) on the TBO. The 200 hPa velocity potential, 850 hPa zonal wind and sea-surface temperature datasets obtained from NCEP/NCAR reanalysis for the period 1950,2006 are used for the study of the TBO. The IOD and TBO have both in-phase (positive/negative IOD with positive/negative TBO) and out-of-phase (positive/negative IOD with negative/positive TBO) relationships. On the other hand, La Niña is associated with the positive phase of TBO and El Niño with the negative phase. In the presence of El Niño (La Niña), positive (negative) IOD is associated with negative (positive) TBO and in the absence of ENSO, positive (negative) IOD is associated with positive (negative) phase of TBO. When ENSO is associated with TBO, it tends to dominate the biennial transition irrespective of IOD. In-phase Indian to Australian monsoon transition of TBO is controlled by ENSO. IOD,TBO association is strong and significant in the absence of ENSO only. The biennial reversal is confined to the Indian Ocean in the TBO cycle associated with IOD only. Thus IOD can be considered as the local forcing for the biennial monsoon cycle, and ENSO the remote effect. Copyright © 2010 Royal Meteorological Society [source]