Graeme Kelly

ASSIMILATION AND MODELING OF THE ATMOSPHERIC HYDROLOGICAL CYCLE IN THE ECMWF FORECASTING SYSTEM

Several new types of satellite instrument will provide improved measurements of Earths hydrological cycle and the humidity of the atmosphere. In an effort to make the best possible use of these data, the modeling and assimilation of humidity, clouds, and precipitation are currently the subjects of a comprehensive research program at the European Centre for Medium-Range Weather Forecasts (ECMWF). Impacts on weather prediction and climate reanalysis can be expected. The preparations for cloud and rain assimilation within ECMWFs four-dimensional variational data assimilation system include the development of linearized moist physics, the development of fast radiative transfer codes for cloudy and precipitating conditions, and a reformulation of the humidity analysis scheme. Results of model validations against in situ moisture data are presented, indicating generally good agreement—often to within the absolute calibration accuracy of the measurements. Evidence is also presented of shortcomings in ECMWFs h...

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

The Spatial Structure of Observation Errors in Atmospheric Motion Vectors from Geostationary Satellite Data

Abstract This study investigates and quantifies in detail the spatial correlations of random errors in atmospheric motion vectors (AMVs) derived by tracking structures in imagery from geostationary satellites. A good specification of the observation error is essential to assimilate any kind of observation for numerical weather prediction in a near-optimal way. For AMVs, height assignment, tracking of similar cloud structures, or quality control procedures may introduce spatially correlated errors. The spatial structure of the error correlations is investigated based on a 1-yr dataset of pairs of collocations between AMVs and radiosonde observations. Assuming spatially uncorrelated sonde errors, the spatial AMV error correlations are obtained over dense sonde networks. Results for operational infrared and water vapor wind datasets from Meteosat-5 and -7, Geostationary Operational Environmental Satellite-8 and -10 (GOES-8 and -10), and Geostationary Meteorological Satellite-5 (GMS-5) are presented. Winds fr...

Global Observing System Experiments on Operational Statistical Retrievals of Satellite Sounding Data

Abstract We report an observing system experiment on satellite sounding data during a 15.5-day period in January–February 1987, using the operational European Centre for Medium Range Weather Forecasts (ECMWF) system as it was in late July 1988. The forecast results show a negative impact of the satellite sounding data (SATEM) in the Northern Hemisphere, and a strong positive impact in the Southern Hemisphere. The model and analysis developments implemented between July 1987 and July 1988 led to forecast improvements whether or not SATEM data were used. Improvements were larger in the NoSATEM context. Consequently, the neutral Northern Hemisphere impact of SATEM data with the 1987 system became a negative impact with the 1988 system. Thus, recent changes in the analysis–forecast system have made the system more sensitive to data, and therefore more vulnerable to bad data. We show that the statistical retrievals have serious errors and biases. The biases are airmass-dependent and so have strong regional var...

Model Clouds over Oceans as Seen from Space: Comparison with HIRS/2 and MSU Radiances

Radiation observations are a key element in the evaluation of the 40-yr reanalysis at the European Centre for Medium-Range Weather Forecasts. This paper uses the High-Resolution Infrared Radiation Sounder/2 (HIRS/ 2) and Microwave Sounding Unit (MSU) observations on board the National Oceanic and Atmospheric Administration satellites, to assess the characteristics of the cloud fields produced by the forecasting system over midlatitude and tropical oceans. Infrared and microwave radiation have different sensitivities to clouds and are therefore complementary. Observed and model-generated radiances, as well as HIRS/2-derived cloud parameters, are compared. The model clouds are shown to be well distributed, with realistic seasonal cycles. However, deficiencies are identified and discussed: the cloud radiative impact may be too low in the midlatitudes, the frequency of occurrence of high clouds is overestimated in the intertropical convergence zone, and the stratocumulus off the west coast of the continents is underestimated. The methods described here provide a framework for assessing the impact of forthcoming improvements to the cloud scheme.

AMSU-A Land Surface Emissivity Estimation for Numerical Weather Prediction Assimilation Schemes

Abstract This study describes the work performed at the European Centre for Medium-Range Weather Forecasts (ECMWF) to estimate the microwave land surface emissivities at Advanced Microwave Sounding Unit (AMSU)-A frequencies within the specific context and constraint of operational assimilation. The emissivities are directly calculated from the satellite observations in clear-sky conditions using the surface skin temperature derived from ECMWF and the Radiative Transfer for the Television and Infrared Observation Satellite Operational Vertical Sounder (RTTOVS) model, along with the forecast model variables to estimate the atmospheric contributions. The results are analyzed, with special emphasis on the evaluation of the frequency and angular dependencies of the emissivities with respect to the surface characteristics. Possible extrapolation of the Special Sensor Microwave Imager (SSM/I) emissivities to those of the AMSU is considered. Direct calculation results are also compared with emissivity model outputs.

Impact of SSM/I Observations Related to Moisture, Clouds, and Precipitation on Global NWP Forecast Skill

Abstract This paper presents the results from the Observing System Experiments (OSEs) with the current ECMWF data assimilation and modeling system for quantifying the impact on both analysis and forecast quality of Special Sensor Microwave Imager (SSM/I) observations sensitive to moisture and clouds as well as precipitation. SSM/I radiances have been assimilated operationally in clear-sky areas for 8 yr and in cloud- and rain-affected areas since June 2005. This paper examines experiments set up such that clear-sky and rain-affected observations were either added to a baseline with a restricted observing system configuration or withdrawn from the full system. The experiment duration was 10 weeks of which the first 14 days were excluded from the evaluation to allow the system to lose the memory of the initial conditions at day −1. It is shown that both clear-sky and rain-affected observations account for the bulk correction of moisture in the ECMWF analysis. SSM/I data adds 1 day of forecast skill over the...

Impact of a New Cloud Motion Wind Product from Meteosat on NWP Analyses and Forecasts

Abstract Enhanced wind datasets of the European satellite Meteosat are now provided every 90 mins together with the quality indicator (QI) derived by the quality control of the Meteorological Product Extraction Facility (MPEF) at the European Organisation for the Exploitation of Meteorological Satellites. All three channel cloud motion winds and clear sky water vapor motion winds have been passively monitored by comparison with the European Centre for Medium-Range Weather Forecasts model background field. The evaluation of the relationship between the MPEF QI and the observation − background differences indicate possible benefits to be gained from the use of the QI within the observation screening of the assimilation system. The MPEF quality indicator is used as a selection criterion within the screening. The applied thresholds are restricted in the Tropics compared to the extratropical regions where the threshold for high-level winds has been relaxed below the automatic quality control at MPEF. The wind ...

Quality Control of Operational Physical Retrievals of Satellite Sounding Data

Abstract Earlier work identified serious errors and biases in the operational temperature and moisture satellite retrievals produced by statistical methods. We show that similar errors and biases are found in the physical retrievals produced operationally since September 1988. We report experiments on quality control algorithms to deal with the errors in the satellite data. The quality control changes resulting from this work were implemented in the European Centre for Medium Range Weather Forecasts (ECMWF) system in January 1989. The performance of the quality control changes in the period after the change has been satisfactory.