Buizza R.,European Center for Medium Range Weather Forecasts
Quarterly Journal of the Royal Meteorological Society | Year: 2010
The impact of horizontal resolution increases from spectral truncation T95 to T799 on the error growth of ECMWF forecasts is analysed. Attention is focused on instantaneous, synoptic-scale features represented by the 500 and 1000 hPa geopotential height and the 850 hPa temperature. Error growth is investigated by applying a three-parameter model, and improvements in forecast skill are assessed by computing the time limits when fractions of the forecast-error asymptotic value are reached. Forecasts are assessed both in a realistic framework against T799 analyses, and in a perfect-model framework against T799 forecasts. A strong sensitivity to model resolution of the skill of instantaneous forecasts has been found in the short forecast range (say up to about forecast day 3). But sensitivity has shown to become weaker in the medium range (say around forecast day 7) and undetectable in the long forecast range. Considering the predictability of ECMWF operational, high-resolution T799 forecasts of the 500 hPa geopotential height verified in the realistic framework over the Northern Hemisphere (NH), the long-range time limit τ(95%) is 15.2 days, a value that is one day shorter than the limit computed in the perfect-model framework. Considering the 850 hPa temperature verified in the realistic framework, the time limit τ(95%) is 16.6 days for forecasts verified in the realistic framework over the NH (cold season), 14.1 days over the SH (warm season) and 20.6 days over the Tropics. Although past resolution increases have been providing continuously better forecasts especially in the short forecast range, this investigation suggests that in the future, although further increases in resolution are expected to improve the forecast skill in the short and medium forecast range, simple resolution increases without model improvements would bring only very limited improvements in the long forecast range. © 2010 Royal Meteorological Society.
Lopez P.,European Center for Medium Range Weather Forecasts
Monthly Weather Review | Year: 2011
Direct four-dimensional variational data assimilation (4D-Var) of NCEP stage IV radar and gauge precipitation observations over the eastern United States has been developed and tested in ECMWF's global Integrated Forecasting System. This is the natural extension of earlier work using a two-step 1D14D-Var approach. Major aspects of the implementation are described and discussed in this paper. In particular, it is found that assimilating 6-h precipitation accumulations instead of the original hourly data substantially improves the behavior of 4D-Var, especially as regards the validity of the tangent-linear assumption. The comparison of background and analysis precipitation departures demonstrates that most of the information contained in the new precipitation observations is properly assimilated. Experiments run over the periods April-May and September-October 2009 also show that local precipitation forecasts become significantly better for ranges up to 12 h, which indicates that a genuine precipitation analysis can now be obtained over the eastern United States. Geopotential, temperature, moisture, and wind forecast scores are generally neutral or slightly positive for all regions of the globe and at all ranges, which is consistent with previous 1D14D-Var results. The most crucial issue that remains unsolved is the treatment of nonprecipitating model background occurrences because of the corresponding absence of sensitivity in the linearized moist physics. For the moment, only points where both model background and observations are rainy are assimilated. Operational implementation using U.S. data is planned in 2011 and one can hope that new networks of radars (and maybe rain gauges) can be added in the 4D-Var assimilation process in the future. © 2011 American Meteorological Society.
Dragani R.,European Center for Medium Range Weather Forecasts
Quarterly Journal of the Royal Meteorological Society | Year: 2011
This article presents an assessment of the quality of the ERA-Interim ozone analyses produced by the European Centre for Medium-Range Weather Forecasts. Ozone retrievals from a number of satellite instruments and an ad hoc generated mean total column ozone (TCO) reference are used to assess the quality of the ERA-Interim ozone analyses during the period from January 1989 to December 2008. The ERA-Interim TCO is typically within ±5 DU (about ±3%) from the TCO reference, while showing up to 2% lower values than the Ozone Monitoring Instrument TCO between 50°S and 50°N. Comparisons with SAGE, HALOE and (UARS and Aura) MLS data show consistent results both in the Tropics and Extratropics, with mean residuals typically within ±5% around 5 hPa and within ±10% in the region of the ozone mixing ration maximum at 10 hPa. However, the comparisons with POAM II and III show mean relative residuals ranging from a few percent in summer to about -40% in winter at high latitudes, partly confirming the known problems of accurately modelling the ozone field during the polar night. Mean residuals of about +10% (but up to +20% at times) and within ±20% are found both in the Tropics and Extratropics for all instruments near 30 hPa and in the lower stratosphere around 65 hPa, respectively. The quality of the ERA-Interim ozone analyses is also compared with that of ERA-40. The study shows that, until December 1995, the ERA-Interim ozone analyses are in better agreement with the independent observations than their ERA-40 equivalent in the upper troposphere and lower stratosphere, but slightly degraded on average in the middle stratosphere. With the start of the assimilation of GOME ozone profiles (January 1996 -December 2002), the agreement between the independent data and the co-located ERA-Interim analyses improves and exceeds that calculated for ERA-40 also in the middle stratosphere. © 2011 Royal Meteorological Society.
Hersbach H.,European Center for Medium Range Weather Forecasts
Journal of Atmospheric and Oceanic Technology | Year: 2010
This article describes the evaluation of a C-band geophysical model function called C-band model 5.N (CMOD5.N). It is used to provide an empirical relation between backscatter as sensed by the spaceborne European Remote Sensing Satellite-2 (ERS-2) and Advanced Scatterometer (ASCAT) scatterometers and equivalent neutral ocean vector wind at 10-m height (neutral surface wind) as function of scatterometer incidence angle. CMOD5.N embodies a refit of CMOD5, a C-band model function, which was previously derived to obtain nonneutral surface wind, in such a way that its 28 tunable coefficients lead, for a given backscatter observation, to an enhancement of 0.7 m s-1 in wind speed. The value of 0.7 m s-1 is chosen to be independent of wind speed and incidence angle, and it incorporates the average difference between neutral and nonneutral wind (~0.2 m s-1) and for a known bias of CMOD5 (~0.5 m s-1) when compared to buoy wind data. The quality of theCMOD5.N fit is successfully tested for theActiveMicrowave Instrument (AMI) scatterometer onERS-2 and ASCAT instrument on Meteorological Operational-A (MetOp-A) for July 2007 and January 2008. ASCAT and ERS-2 wind speed obtained from CMOD5.N compares well on average with operational neutral wind from the European Centre for Medium-Range Weather Forecasts (ECMWF). In comparison with nonneutral wind, the local, seasonally dependent biases between scatterometer and ECMWF model are reduced. Besides effects introduced by sea state, orography, and ocean currents, a residual stabilitydependent bias between scatterometer and neutral wind remains, which is likely connected to a nonoptimality in the ECMWF boundary layer formalism that is reported in the literature. © 2010 American Meteorological Society.
Healy S.B.,European Center for Medium Range Weather Forecasts
Quarterly Journal of the Royal Meteorological Society | Year: 2013
Numerical weather prediction (NWP) experiments have been performed to investigate the impact of surface pressure information retrieved from Global Positioning System radio occultation (GPSRO) measurements. The GPSRO measurements are able to reduce the impact of removing all conventional surface pressure observations from the NWP system. The GPSRO measurements can constrain global mean surface pressure errors in analyses and short-range forecasts to the -1 to -1.5 hPa level over a 3-month period, whereas they can be as large as -4 hPa when both GPSRO observations and conventional surface pressure measurements are removed from the NWP system. The large mean errors that arise when both GPSRO and conventional surface pressure measurements are removed are caused by an interaction between the assimilation of the conventional upper-air measurements and the bias correction of radiances with the variational bias correction system (VARBC). In the absence of GPSRO, the VARBC amplifies and spreads errors that are introduced by the conventional upper-air data. However, the GPSRO measurements cannot fully compensate for the loss of the conventional data, particularly in the Northern Hemisphere. It is also shown that the surface pressure results obtained when GPSRO are assimilated are sensitive to biases introduced by other observations, such as aircraft temperature measurements, because these biases can be aliased into surface pressure increments. Experiments designed to compare the amount of information provided by other parts of the global observing system highlight the complementary nature of GPSRO and scatterometer wind retrievals, and show that reasonable surface pressure analyses, with an uncertainty of ~ 1 hPa, can now be obtained by assimilating just satellite data, without using any conventional data to 'anchor' the NWP system. However, the full observing system, including both satellite and conventional observations, remains superior to assimilating just satellite observations. © 2013 Royal Meteorological Society.