Sakari M. Uppala

ECMWF Analyses and Forecasts of Stratospheric Winter Polar Vortex Breakup: September 2002 in the Southern Hemisphere and Related Events

Abstract Breakup of the polar stratospheric vortex in the Northern Hemisphere is an event that is known to be predictable for up to a week or so ahead. This is illustrated using data from the 45-yr ECMWF Re-Analysis (ERA-40) for the sudden warmings of January 1958 and February 1979 and operational ECMWF data for February 2003. It is then shown that a similar level of skill was achieved in operational forecasts for the split of the southern stratospheric vortex in late September 2002. The highly unusual flow conditions nevertheless exposed a computational instability of the forecast model. Analyses and forecasts from reruns using improved versions of the forecasting system are presented. Isentropic maps of potential vorticity and specific humidity provide striking pictures of the advective processes at work. Forecasts as well as analyses are shown to be in good agreement with radiosonde measurements of the temperature changes associated with vortex movement, distortion, and breakup during August and Septem...

Identification of Anthropogenic Climate Change Using a Second-Generation Reanalysis

[1] Changes in the height of the tropopause provide a sensitive indicator of human effects on climate. A previous attempt to identify human effects on tropopause height relied on information from ‘first-generation’ reanalyses of past weather observations. Climate data from these initial model-based reanalyses have well-documented deficiencies, raising concerns regarding the robustness of earlier detection work that employed these data. Here we address these concerns using information from the new second-generation ERA-40 reanalysis. Over 1979 to 2001, tropopause height increases by nearly 200 m in ERA-40, partly due to tropospheric warming. The spatial pattern of height increase is consistent with climate model predictions of the expected response to anthropogenic influences alone, significantly strengthening earlier detection results. Atmospheric temperature changes in two different satellite data sets are more highly correlated with changes in ERA-40 than with those in a first-generation reanalysis, illustrating the improved quality of temperature information in ERA-40. Our results provide support for claims that human activities have warmed the troposphere and cooled the lower stratosphere over the last several decades of the 20th century, and that both of these changes in atmospheric temperature have contributed to an overall increase in tropopause height. INDEX TERMS: 0350 Atmospheric Composition and Structure: Pressure, density, and temperature; 0370 Atmospheric Composition and Structure: Volcanic effects (8409); 1620 Global Change: Climate dynamics (3309); 1640 Global Change: Remote sensing; KEYWORDS: climate change, detection, reanalysis

Mean age of air and transport in a CTM : Comparison of different ECMWF analyses

[1] A comparison of the stratospheric circulation achieved by various data assimilation winds has been performed using multiannual simulations of the TOMCAT/SLIMCAT off-line 3-D chemical transport model (CTM). Data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.K. Met Office (UKMO) have been used to drive the CTM. We find that important improvements have been made in the ECMWF stratospheric winds during recent years. Therefore, a more realistic Brewer-Dobson (B-D) circulation, and subtropical mixing are achieved when ECMWF operational or new interim reanalyses are used instead of ERA-40 analyses. Age-of-air and trajectory calculations show that more realistic vertical and horizontal transport is achieved with the new ECMWF assimilated winds. The modelled tape recorder signal further shows the improvement in the tropical transport with the new winds. Overall, these results show that with the recent ECMWF reanalyses, off-line CTMs can produce stratospheric tracer transport over multiannual timescales more realistically than with other previous (re)analyses. Reasons for the improvements in the new reanalysis are discussed. Citation: Monge-Sanz, B. M., M. P. Chipperfield, A. J. Simmons, and S. M. Uppala (2007), Mean age of air and transport in a CTM: Comparison of different ECMWF analyses, Geophys. Res. Lett., 34, L04801, doi:10.1029/ 2006GL028515.

THE NEED FOR A DYNAMICAL CLIMATE REANALYSIS

Reanalyses are used primarily for atmospheric model validation. However, reanalyses suffer from a number of limitations that unfortunately restrict their general use. A reanalysis workshop held recently at the University of Reading, United Kingdom, highlighted a number of central issues in climate research. Attendees discussed how these research needs may benefit from a dedicated reanalysis. Specific topics covered included a more in-depth understanding of the general circulation of the atmosphere and a more reliable assessment of climate trends, the hydrological cycle and the calculation of energy fluxes over the oceans.

FGGE Research Activities at ECMWF

A presentation of the First GARP Global Experiment (FGGE) Research Programme at the European Centre for Medium Range Weather Forecasts (ECMWF) is given. An excellent data coverage in areas previously practically void of observations has made it possible to analyze synoptic features in the tropics and the Southern Hemisphere in great detail. The studies strongly suggest that the winter circulation in the Southern Hemisphere is more intense than previously assumed. The tropical circulation shows several examples of episodes of very active interhemispheric exchange. The large-scale circulation in the tropics is dominated by a giant ascending cell over the western Pacific having a particularly strong component in the equatorial plane. This circulation is especially pronounced during the Northern Hemisphere summer. Prediction experiments show increased skill, particularly in the Southern Hemisphere and the tropics. Comparison with operational forecasts performed at ECMWF after FGGE, as well as with observing s...

Reanalysis and reforecast of three major European storms of the twentieth century using the ECMWF forecasting system. Part I: Analyses and deterministic forecasts

In Part I of this study recent versions of the ECWMF Integrated Forecasting System (IFS) are used together with historical observational data to carry out reanalyses and deterministic reforecasts of three major north-west European wind storms of the twentieth century. The storms considered are the Dutch storm of 1 February 1953, the Hamburg storm of 17 February 1962, and the British October storm of 1987 (Great October Storm). Common to all these storms is their severity, which caused large loss of life and widespread damage. Reanalysis of the storms is based on a 3D-Var and 4D-Var assimilation scheme at a horizontal resolution of TL159(≈ 125 km) and TL511(≈ 50 km), respectively. Similarly, two different horizontal resolutions (TL159 and TL511) are used to investigate the deterministic predictability of these storms. The lower-resolution system is exactly that used in the ERA-40 reanalysis project. The high-resolution system is a more recent version of the ECMWF IFS. It is shown that the basic characteristics of the Dutch and Hamburg storms that gave rise to the storm surge are well predicted by the single deterministic forecasts up to about 48 and 84 hours, respectively, in advance. Our capability to predict the Great October Storm is more difficult to assess. On the one hand, even recent versions of the ECMWF IFS underestimate the severity of the storm in the very short-range (12–24-hour forecasts started at 12 UTC 15 October 1987). On the other hand, the high-resolution version of the ECMWF IFS provides excellent deterministic forecasts of the track and intensity of the storm up to 96 hours in advance. However, there are errors in the timing of the storm (12 hours for the 96 hour forecast). From the results presented in this study it is concluded – bearing in mind the limited number of cases considered – that with the current ECMWF forecasting system reliable deterministic predictions of some European wind storms are possible several days in advance. Copyright

Reanalysis and reforecast of three major European storms of the twentieth century using the ECMWF forecasting system. Part II: Ensemble forecasts

In Part II of this study the ECMWF Ensemble Prediction System (EPS) is used to study the probabilistic predictability of three major European storms of the twentieth century. The storms considered are the Dutch storm of 1 February 1953, the Hamburg storm of 17 February 1962, and the British/French storm of October 1987 (Great October storm). Common to all these storms is their severity that caused large loss of life and widespread damage. In Part I of this study it has been found that deterministic predictability of the Dutch and Hamburg storms amount to 48 and 84 hours, respectively. Here, it is shown that the ensemble forecasts supplement the deterministic forecasts. The large number of members in the 48 and 84 hour ensemble forecasts of the Dutch and Hamburg storms, respectively, suggest that at this forecast range and for these storms the sensitivity of the forecasts to analysis and model uncertainties is rather small. From these results, therefore, it is argued that reliable warnings (i.e. low probability for the occurrence of a forecast failure) for the Dutch and Hamburg storms could have been issued 48 and 84 hours, respectively, in advance, had the current ECMWF EPS been available. For the Great October storm it has been found in Part I of this study that short-range and medium-range forecasts of the intensity and track of the storm were very skilful with a high-resolution model of the ECWMF model. The actual timing of the storm, however, was difficult to predict. Here, it is shown that the EPS is capable of predicting large forecast uncertainties associated with the timing of the Great October storm up to 4 days in advance. It is argued that reliable warnings could have been issued at least 96 hours in advance had the ECMWF EPS been available. From the results presented in this study it is concluded that an Ensemble Prediction System is an important component of every early warning system for it allows an a priori quantification of the probability of the occurrence of severe wind storms. Copyright

Atmospheric Reanalyses and Climate Variations

The Earth’s climate has traditionally been studied by statistical analysis of observations of particular weather elements such as temperature, wind and rainfall. Climatological information, usually expressed as long-term averages and variability, is then presented over a geographical area or at a single location and time series of these quantities or of the observations themselves are examined for evidence of warming, more-frequent severe storms, and so on.

High Clouds over Oceans in the ECMWF 15- and 45-Yr Reanalyses

Abstract The reanalysis programs of numerical weather prediction (NWP) centers provide global, comprehensive descriptions of the atmosphere and of the earth’s surface over long periods of time. The high realism of their representation of key NWP parameters, like temperature and winds, implies some realism for less emblematic parameters, such as cloud cover, but the degree of this realism needs to be documented. This study aims to evaluate the high clouds over open oceans in the ECMWF 15- and 45-yr reanalyses. The assessment is based on a new 23-yr climatology of monthly frequencies of high-cloud occurrence retrieved from the infrared radiances measured by operational polar satellites. It is complemented by data from the International Satellite Cloud Climatology Project. It is shown that the 45-yr ECMWF reanalysis dramatically improves on the previous 15-yr reanalysis for the realism of seasonal and interannual variations in high clouds, despite remaining systematic errors. More than 60% of the observed an...

Extending the satellite sounding archive back in time: the Vertical Temperature Profile Radiometer data

The Vertical Temperature Profile Radiometer (VTPR) was an operational 8-channel infrared sounding system mounted on the NOAA-2 through NOAA-5 spacecraft. The instrument was a predecessor of the High-Resolution Infrared Radiation Sounder (HIRS) on the continuing NOAA polar orbiting satellite series. The VTPR measurements covered more than six years of data from late 1972 to early 1979. Major work has been done to clear erroneous data records. Theoretical biases between similar channels of VTPR and HIRS are derived using a radiative transfer model to show the potential bias features between the observations of the two instruments. The model simulation shows that for about half of the channels the biases can be in the order of 1 K for certain temperature ranges. Because each spacecraft carried two sets of VTPR instrument, but only one set was turned on at a given time, differences between two sets of channel measurement are expected to exist. It is shown that the differences between two sets of VTPR instrument can range from nearly zero for some channels to about 2 K for other channels. To make the VTPR dataset accessible to the general scientific research community, we have processed the whole VTPR data to common formats and placed the data online along with data quality statistics.