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Darmstadt, Germany

Vogelzang J.,KNMI | Stoffelen A.,KNMI | Verhoef A.,KNMI | Figa-Saldana J.,EUMETSAT
Journal of Geophysical Research: Oceans | Year: 2011

High-resolution wind products based on space-borne scatterometer measurements by ASCAT and SeaWinds are used widely for various purposes. In this paper the quality of such products is assessed in terms of accuracy and resolution, using spectral analysis and triple collocation with buoy measurements and NWP model forecasts. An experimental ASCAT coastal product is shown to have a spectral behavior close to k-5/3 for scales around 100 km, as expected from theory and airborne measurements. The NWP spectra fall off more rapidly than the scatterometer wind spectra starting at scales of about 1000 km. Triple collocation is performed for four collocated data sets, each with a different scatterometer wind product: ASCAT at 12.5 km and 25 km, and SeaWinds at 25 km processed in two different ways. The spectral difference between scatterometer wind and model forecast is integrated to obtain the representation error which originates from the fact that global weather models miss small-scale details observed by the scatterometers and the buoys. The estimated errors in buoy winds and model winds are consistent over the data sets for the meriodional wind component; for the zonal wind component consistency is less, but still acceptable. Generally, enhanced detail in the scatterometer winds, as indicated at high spatial frequencies by a spectral tail close to k-5/3, results in better agreement with buoys and worse agreement with NWP predictions. The accuracy of the scatterometer winds is about 1 ms -1 or better. The calibration coefficients from triple collocation indicate that on average the ASCAT winds are slightly underestimated. Copyright 2011 by the American Geophysical Union.

Sohn B.J.,Seoul National University | Yeh S.-W.,Hanyang University | Schmetz J.,EUMETSAT | Song H.-J.,Seoul National University
Climate Dynamics | Year: 2013

In order to examine the changes in Walker circulation over the recent decades, we analyzed the sea surface temperature (SST), deep convective activities, upper tropospheric moistening, sea level pressure (SLP), and effective wind in the boundary layer over the 30-year period of 1979-2008. The analysis showed that the eastern tropical Pacific has undergone cooling while the western Pacific has undergone warming over the past three decades, causing an increase in the east-west SST gradient. It is indicated that the tropical atmosphere should have responded to these SST changes; increased deep convective activities and associated upper tropospheric moistening over the western Pacific ascending region, increased SLP over the eastern Pacific descending region in contrast to decreased SLP over the western Pacific ascending region, and enhanced easterly wind in the boundary layer in response to the SLP change. These variations, recognized from different data sets, occur in tandem with each other, strongly supporting the intensified Walker circulation over the tropical Pacific Ocean. Since the SST trend was attributed to more frequent occurrences of central Pacific-type El Niño in recent decades, it is suggested that the decadal variation of El Niño caused the intensified Walker circulation over the past 30 years. An analysis of current climate models shows that model results deviate greatly from the observed intensified Walker circulation. The uncertainties in the current climate models may be due to the natural variability dominating the forced signal over the tropical Pacific during the last three decades in the twentieth century climate scenario runs by CMIP3 CGCMs. © 2012 The Author(s).

Zecchetto S.,CNR Institute of atmospheric Sciences and Climate | Accadia C.,EUMETSAT
Quarterly Journal of the Royal Meteorological Society | Year: 2014

This article aims to understand to what extent the winds fields from an advanced numerical weather prediction system and from a satellite scatterometer describe the same spatial and temporal features of the wind in the Mediterranean Sea. We investigated wind fields for the period February 2010 to February 2012 using the ASCAT scatterometer data with 12.5 km wind vector cells and the analysis wind fields from the ECMWF T1279 global model. The ASCAT-ECMWF mean relative bias and centred root mean square deviation of wind speed, normalized by scatterometer wind speed wsc, Δws/wsc and , have been found to be 7 and 23%. An interesting result is the identification of dependence of both Δws/wsc and on the distance from the coast, indicating the coastal areas as the main source of discrepancy between the two datasets. From 50 to 200 km away from coast, decreases from 40 to 25% and Δws/wsc from 8 to 4%. These results gain more importance considering that the Mediterranean Sea is essentially a coastal sea (50% of its surface lies within 50 km from the coast). Both Δws and have been found to depend nonlinearly on the wind speed. The seasonal variation of Δws/wsc and shows that they are in phase opposition, with higher values of during the warm season (April to October). It is hypothesized that local coastal circulations like land/sea breezes could explain the observed mismatch between model and observations. The reported findings emphasize a common feature of the present atmospheric models forecasting winds over regional basins like the Mediterranean. © 2013 Royal Meteorological Society.

Von Engeln A.,EUMETSAT | Teixeira J.,Jet Propulsion Laboratory
Journal of Climate | Year: 2013

A planetary boundary layer (PBL) height climatology from ECMWF reanalysis data is generated and analyzed. Different methods are first compared to derive PBL heights from atmospheric temperature, pressure, and relative humidity (RH), which mostly make use of profile gradients, for example, in RH, refractivity, and virtual or potential temperature. Three methods based on the vertical gradient of RH, virtual temperature, and potential temperature were selected for the climatology generation. The RH-based method appears to capture the inversion that caps the convective boundary layer very well as a result of its temperature and humidity dependence, while the temperature-based methods appear to capture the PBL better at high latitudes. A validation of the reanalysis fields with collocated radiosonde data shows generally good agreement in terms of mean PBL height and standard deviation for the RH-based method. The generated ECMWF-based PBL height climatology shows many of the expected climatological features, such as a fairly low PBL height near the west coast of continents where stratus clouds are found and PBL growth as the air is advected over warmer waters toward the tropics along the trade winds. Large seasonal and diurnal variations are primarily found over land. The PBL height can exceed 3 km, mostly over desert areas during the day, although large values can also be found in areas such as the ITCZ. The robustness of the statistics was analyzed by using information on the percentage of outliers. Here in particular, the sea-based PBL was found to be very stable. © 2013 American Meteorological Society.

A method is presented to assess whether a given reference ground-based point observation, typically a radiosonde measurement, is adequately collocated and sufficiently representative of space-borne hyperspectral infrared instrument measurements. Once this assessment is made, the ground-based data can be used to validate and potentially calibrate, with a high degree of accuracy, the hyperspectral retrievals of temperature and water vapour. © 2016 Author(s).

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