Godelieve Deblonde

Radiance and Jacobian Intercomparison of Radiative Transfer Models Applied to HIRS and AMSU Channels

The goals of this study are the evaluation of current fast radiative transfer models (RTMs) and line-by-line (LBL) models. The intercomparison focuses on the modeling of 11 representative sounding channels routinely used at numerical weather prediction centers: 7 HIRS (High-resolution Infrared Sounder) and 4 AMSU (advanced microwave sounding unit) channels. Interest in this topic was evident by the participation of 24 scientists from 16 institutions. An ensemble of 42 diverse atmospheres was used and results compiled for 19 infrared models and 10 microwave models, including several LBL RTMs. For the first time, not only radiances but also Jacobians (of temperature, water vapor, and ozone) were compared to various LBL models for many channels. In the infrared, LBL models typically agree to within 0.05-0.15 K (standard deviation) in terms of top-of-the-atmosphere brightness temperature (BT). Individual differences up to 0.5 K still exist, systematic in some channels, and linked to the type of atmosphere in others. The best fast models emulate LBL BTs to within 0.25 K, but no model achieves this desirable level of success for all channels. The ozone modeling is particularly challenging. In the microwave, fast models generally do quite well against the LBL model to which they were tuned. However, significant differences were noted among LBL models. Extending the intercomparison to the Jacobians proved very useful in detecting subtle or more obvious modeling errors. In addition, total and single gas optical depths were calculated, which provided additional insight on the nature of differences.

Evaluation of GPS Precipitable Water over Canada and the IGS Network

Abstract Precipitable water (PW) derived from the GPS zenith tropospheric delay (ZTD) is evaluated (as a first step toward variational data assimilation) through comparison with that of collocated radiosondes (RS_PW), operational analyses, and 6-h forecasts (from the Canadian Global Environmental Multiscale model) of the Canadian Meteorological Centre. Two sources of ZTD data are considered: 1) final ZTD (over Canada), computed by the Geodetic Survey Division (GSD) of Natural Resources Canada, and 2) final ZTD (distributed globally), obtained from the International GPS Service (IGS). The mean GSD GPS–derived PW (GPS_PW) is 14.9 mm (reflecting the relatively cold Canadian climate), whereas that of the IGS dataset is 20.8 mm. Intercomparison statistics [correlation, standard deviation (SD), and bias] between GPS_PW and RS_PW are, respectively, 0.97, 2.04 mm, and 1.35 mm for the GSD data and 0.98, 2.6 mm, and 0.67 mm for the IGS data. Comparisons of GPS_PW with 6-h forecast PW (TRIAL_PW) show slightly lower ...

Impact of the Assimilation of CHAMP Refractivity Profiles on Environment Canada Global Forecasts

Abstract The data assimilation system of Environment Canada (EC) is adapted to accept GPS radio occultation (GPSRO) data. Observations of this type are available with extensive coverage from several satellites. In this study, experiments are performed to compare the skill of EC’s three-dimensional variational data assimilation (3DVAR) system (including all data normally assimilated operationally), with and without the addition of radio occultation refractivity data from the Challenging Minisatellite Payload for Geophysical Research (CHAMP). These data were not available at the time studied as near-real-time (NRT) observations. However, data from this and other radio occultation missions are now available as NRT data, and the conditions (latency, reliability) are improving. It is expected that NRT GPSRO data from a number of satellite missions will continue to be available through the following years. The results of the assimilation tests are evaluated against the following three data types: radiosondes (t...

One-Dimensional Variational Retrievals from SSMIS-Simulated Observations

Abstract Retrievals using synthetic background fields and observations for the Special Sensor Microwave Imager Sounder (SSMIS) instrument are performed using a one-dimensional variational data assimilation (1DVAR) scheme for clear and cloudy nonprecipitating skies over open oceans. Two retrieval techniques are implemented in the 1DVAR and are extensively tested. Profiles of temperature, marine surface wind speed, and skin temperature are retrieved with both techniques. In addition, with technique A, profiles of the natural logarithm of specific humidity and liquid water path are also retrieved. With technique B, the natural logarithm of total water content (sum of specific humidity and liquid cloud water content) is retrieved instead of the natural logarithm of humidity and liquid water path. A function specifies how total water content is split among its two components. In essence, excess water vapor oversaturation leads to cloud formation. Retrievals in clear and cloudy conditions for a variety of exper...

Evaluation of global numerical weather prediction analyses and forecasts using DMSP special sensor microwave imager retrievals: 1. Satellite retrieval algorithm intercomparison study

Special sensor microwave/imager (SMM/I) satellite retrieval schemes covering a wide range of retrieval techniques were compared for environmental parameters that play a key role in the atmospheric hydrological cycle. These were column integrated water vapor (IWV), near-surface oceanic wind speed (SWS), liquid water path (LWP), and surface rain rate (SRR). The objective was to evaluate the accuracy and limitations of the retrieval schemes on a monthly time-scale (October 1993). The retrieval algorithms that were found to be the most accurate were used for the evaluation of an analysis/forecast system whose results are presented in part 2. The agreement between IWV retrieval algorithms was found to be good, in concordance with prior studies. However, depending on the range of IWV values considered, considerable biases were shown to exist. The choice of precipitation screen for which IWV retrievals were rejected had a non-negligible impact on the monthly statistics. Saturation, which is often present in IWV retrieval algorithms, also influenced these statistics. The agreement between different SWS retrieval algorithms was not as good as for IWV. The largest differences between the Wentz [1994] and the Goodberlet et al. [1989] retrieval algorithms were found in the intertropical convergence zone (ITCZ) (largest difference) and in the midlatitudes. The impact of using different precipitation screens was also analyzed. SWS retrieved with the Goodberlet et al. algorithm was sensitive (in particular in the ITCZ) to the amount of data rejected. Among the LWP retrieval algorithms considered, two algorithms gave acceptable and similar results except for a constant factor (1.75). It is argued that the discrepancy of a constant factor was in part due to the lack of in situ measurements for calibration and the fact that the beam-filling problem was taken into account empirically. For SRR retrievals, considerable disagreement was demonstrated between different retrieval algorithms. However, several algorithms gave retrievals that were in reasonable agreement with long-term climatological data sets. This agreement was best in the tropics and decreased considerably in the midlatitudes where the retrieval capability of SRR for the SSM/I is not optimal.

Impact of NOAA Ground-based GPS Observations on the Canadian Regional Analysis and Forecast System

Abstract Half-hourly GPS zenith tropospheric delay (ZTD) and collocated surface weather observations of pressure, temperature, and relative humidity are available in near–real time from the NOAA Global Systems Division (GSD) research GPS receiver network. These observations, located primarily over the continental United States, are assimilated in a research version of the Environment Canada (EC) regional (North America) analysis and forecast system. The impact of the assimilation on regional analyses and 0–48-h forecasts is evaluated for two periods: summer 2004 and winter 2004/05. Forecasts are verified against radiosonde, rain gauge, and NOAA GPS network observations. The impacts of GPS ZTD and collocated surface weather observations for the summer period are generally positive, and include reductions in forecast errors for precipitable water, surface pressure, and geopotential height. It is shown that the ZTD data are primarily responsible for these forecast error reductions. The impact on precipitatio...

Evaluation of global numerical weather prediction analyses and forecasts using DMSP special sensor microwave imager retrievals: 2. Analyses/forecasts intercomparison with SSM/I retrievals

Satellite retrievals of environmental parameters for October 1993 were used to validate the Canadian Meteorological Center (CMC) analysis/forecast system and an improved version of the forecast model (including a prognostic equation for cloud water). In part 1 [Deblonde and Wagneur, this issure], SSM/I (special sensor microwave imager) retrievals using different algorithms were intercompared for each environmental parameter (column integrated water vapor (IWV), near-surface oceanic wind speed (SWS), liquid water path (LWP) and surface rain rate (SRR). Here a subset of the SSM/I retrieval algorithms that performed best and other observation data sets are used to evaluate the hydrological cycle of the analysis/forecast system on a monthly timescale. Over the global oceans the agreement between analyzed IWV, forecasted IWV, and SSM/I retrievals was quite good (R ∼0.96). However, where Humsat (GOES) retrieved water vapor profiles were assimilated, significant biases were observed in large areas (up to ∼15 kgm−2). Significant differences were found between analyzed SWS and SSM/I-retrieved SWS. In the tropics, zonally averaged analyzed SWS was underestimated by up to 1.4 ms−1 and in the southern hemisphere midlatitude differences of ∼1 ms−1 were found. Forecasted cloud fraction from the improved model was compared with that observed using the Humsat retrieval system. Over the open oceans, modeled cloud fraction was overestimated by 8%, suggesting that cloud formation was too active. By comparing SSM/I LWP retrievals with those forecasted it was possible to identify mispositioning of cloud systems that were associated with noted biases in the humidity analysis. It was also shown that the choice of function to obtain the liquid phase component of the modeled cloud water modified considerably the magnitude of the monthly mean LWP, particularly in the midlatitudes. The differences in patterns between precipitation short-range forecasts (accumulated over a month) obtained with the operational and improved forecast models were considerably smaller than the differences between each of these forecasted fields and SSM/I retrievals or long-term climatologies.

Assimilation of SSM/I and GOES humidity retrievals with a one-dimensional variational analysis scheme

Abstract Total precipitable water (TPW) retrieved from Special Sensor Microwave/lmager (SSM/I) brightness temperatures and specific humidity retrieved from Geostationary Operational Environmental Satellite (GOES) radiances are assimilated using a one-dimensional (ID) variational analysis technique. The study is divided into two parts. First, collocations with radiosondes are performed to arm the quality of the satellite water vapor retrievals. Collocations are also performed with 6-h forecast Acids. Second, SSM/I TPW and GOES specific humidity are assimilated using a ID variational analysis technique that minimizes the error variance of the analyzed field. A global collocation study over the oceans for SSM/I TPW retrievals and 6-h forecasts of TPW shows that the rmse (with respect to radiosondes) are, respectively, 4.7 and 5.0 kg m−2. A separate collocation study over both the oceans and land for GOES retrieved TPW and 6-h forecasts of TPW yields rmse of 4.6 and 4.4 kg m−2, respectively, in the midlatitud...

Simulation of Satellite Passive Microwave Observations in Rainy Atmospheres at Meteorological Service of Canada

In the present study, a research version of the Meteorological Service of Canada (MSC) Global Environmental Multi-Scale (GEM) mesoglobal weather forecast model is evaluated by comparing simulated SSM/I brightness temperatures (Tbs) with observed ones. Several comparisons based on two-15-day periods, one in winter and one in summer, have been done. Results are compared to those obtained by [F. Chevallier, P. Bauer, Mon. Wea. Rev., vol.131, p.1240-55 (2003)] for a study conducted on the European Centre for Medium-Range Weather Forecast (ECMWF) model. The overall performance of the GEM model is similar to that of the ECMWF model. The model appears to simulate with realism the large-scale rainy systems but with frequent mislocations. Moreover, the model has a tendency to produce intense small-scale precipitating areas that are not observed. The frequency of cloud and rain occurrences is overestimated by the model. The model has the best skill at simulating the 22 GHz Tbs indicating that the simulated water vapour is realistic in cloudy areas. Finally these results are encouraging enough to continue investigation on the assimilation of brightness temperatures in cloudy and rainy skies at MSC