Impact of GPS Radio Occultation Data Assimilation on Regional Weather Predictions
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GPS Solut (2010) 14:35–49 DOI 10.1007/s10291-009-0144-1 ORIGINAL ARTICLE Impact of GPS radio occultation data assimilation on regional weather predictions Ching-Yuang Huang • Ying-Hwa Kuo • Shu-Ya Chen • Chuen-Tsyr Terng • Fang-Ching Chien • Pay-Liam Lin • Mien-Tze Kueh • Shu-Hua Chen • Ming-Jen Yang • Chieh-Ju Wang • Anisetty S. K. A. V. Prasad Rao Received: 31 January 2009 / Accepted: 4 October 2009 / Published online: 17 November 2009 Ó Springer-Verlag 2009 Abstract The impact of GPS radio occultation (RO) and Mei-yu systems when COSMIC data were available, data assimilation on severe weather predictions in East ranging from several points in 2006 to a maximum of Asia is introduced and reviewed. Both the local obser- about 60 in 2007 and 2008 in this region. Based on a vation operator that assimilates the retrieved refractivity number of experiments, regional model predictions at as local point measurement, and the nonlocal observation 5 km resolution were not significantly influenced by dif- operator that assimilates the integrated retrieved refrac- ferent observation operators, although the nonlocal tivity along a straight raypath have been utilized in WRF observation operator sometimes results in slightly better 3DVAR to improve the initial analysis of the model. A track forecast. These positive impacts are seen not only in general evaluation of the impact of these approaches on typhoon track prediction but also in prediction of local Asian regional analysis and daily prediction is provided in heavy rainfall associated with severe weather over this paper. In general, the GPS RO data assimilation may Taiwan. The impact of 56 GPS RO soundings on track improve prediction of severe weather such as typhoons prediction of Cyclone Gonu (2007) over the Indian Ocean is also appealing when compared to other tracks assimi- lated with different observations. From a successive evaluation of skill scores for real-time forecasts on Mei-yu frontal systems operationally conducted over a & C.-Y. Huang ( ) Á S.-Y. Chen Á P.-L. Lin Á M.-T. Kueh Á longer period and predictions of six typhoons in 2008, M.-J. Yang Á C.-J. Wang Á A. S. K. A. V. Prasad Rao Department of Atmospheric Sciences, assimilation of GPS RO data appears to have some National Central University, Jhongli, Taiwan positive impact on regional weather predictions, on top of e-mail: [email protected] existent assimilation with all other observations. Y.-H. Kuo University Corporation for Atmospheric Research, Keywords GPS Á Radio occultation Á WRF Á Boulder, CO, USA Data assimilation Á FORMOSAT-3 Á COSMIC Y.-H. Kuo National Center for Atmospheric Research, Boulder, CO, USA Introduction C.-T. Terng With the global coverage of global positioning system Central Weather Bureau, Taipei, Taiwan (GPS) satellites, radio occultation (RO) technique for F.-C. Chien sounding earth’s atmosphere by limb links was demon- Department of Earth Sciences, strated by the proof-of-concept GPS Meteorology (GPS/ National Taiwan Normal University, Taipei, Taiwan MET) experiment in 1995–1997 (Ware et al. 1996). This successful mission of GPS/MET then was followed by S.-H. Chen Department of Land, Air & Water Resources, two similar missions, the Challenging Minisatellite University of California, Davis, CA, USA Payload (CHAMP) and the Satellite de Aplicaciones 123 36 GPS Solut (2010) 14:35–49 Cientificas-C (SAC-C). According to the RO principle The prediction model and preprocessor and retrieval, these global measurements as procured are actually commensurable with radiosonde soundings in The WRF model (version 2) has been used for studies of accuracy (Kursinski et al. 2000). The newly operational impact on weather prediction. The details of the WRF FORMOSAT-3/COSMIC consisting of six satellites to model can be found on the Web site (http://wrf-model.org). provide up to 1800–2200 global soundings daily (Anthes The WRF 3DVAR, the preprocessor of the WRF model, et al. 2008) has drawn attention to the great value of GPS was utilized here to ingest the GPS RO observations with RO measurements in many global climate and weather the observation operator to map model variables onto the applications. Assimilations of the RO retrieved data observables at the observation time and location. (refractivity and bending angle) have exhibited promising impact on regional as well as global weather predictions The observation operators in WRF 3DVAR (Kuo et al. 1997; Zou et al. 1999; Liu and Zou 2003; Huang et al. 2005; Healy et al. 2005; Healy and Thepaut The observables as retrieved or derived from GPS RO are 2006; Cucurull et al. 2006, 2007; Poli et al. 2008; Healy the Abel-retrieved atmospheric refractivity as a path 2008; Chen et al. 2009). For global models, assimilation average refractivity and the excess phase (the integrated of bending angle has the advantage of providing better amount of the refractivity along a raypath). The neutral accuracy, but at the cost of computational expense (Zou atmospheric refractivity N is given by et al. 1999; Kuo et al. 2000). For regional models, P P assimilation of refractivity has usually been undertaken N ¼ 77:6 þ 3:73 Â 105 w ð1Þ T T2 using a local observation operator or a nonlocal obser- vation operator (Huang et al. 2005; Chen et al. 2009). where Pw is water vapor pressure in hPa, T air temperature GPS RO data have been proven to give a positive in K, and P the pressure of the atmosphere in hPa. In order impact on some typhoon track and rainfall predications to account for nonlocal effects, model local refractivity (Huang et al. 2005) and the prediction of monsoon- may be integrated along a raypath that may be induced rainfall over western Indian coast with Gatt approximated by a straight line (Sokolovskiy et al. 2005). mountain (Huang et al. 2007), based on the control This line integral is treated as a new observable (excess experiment with assimilation of the GPS RO data and the phase) defined as Z denial experiment without the GPS RO data available from two earlier satellites. The FORMOSAT-3/COSMIC S ¼ Ndl ð2Þ satellites are providing plentiful reliable soundings for assimilation into regional models like WRF (Weather where l is the raypath, and S can be calculated for model Research and Forecasting). In this paper, we introduce local refractivity as well as retrieved refractivity (as and review the impact of the GPS RO soundings from observed). Thus, in the S representation, the error induced COSMIC on prediction of recent severe weathers by horizontal gradients can be substantially reduced to give (typhoons and Mei-yu systems) in Asia, in addition to a more accurate analysis (Ma et al. 2009). The nonlocal assessing their general impact on continuous forecasts operator for assimilating excess phase has been imple- over selected periods. This paper highlights the current mented into WRF 3DVAR (Chen et al. 2009), which may status of GPS RO data assimilation in regional weather also assimilate point refractivity simply by deactivating the prediction and their potential impact based on a number ray’s integration. Also, horizontal smear of tangent point of numerical experiments. In ‘‘The methodologies and the trajectories has been taken into account in the nonlocal operators’’, the methodologies and the observation oper- operator of the WRF 3DVAR. ators are introduced. The results for selected cases are presented and described in ‘‘The results’’. Finally, our conclusions are given in ‘‘Conclusions’’. The results Severe weathers such as typhoons and Mei-yu systems The methodologies and the operators were simulated when COSMIC data were available. Gen- eral assessments of continuous forecasts were conducted by The methodologies used in this study include the meteo- CWB (Central Weather Bureau) and NCU (National Cental rological prediction model WRF and its preprocessor using University), respectively, for some monthly periods in three-dimensional variational (3DVAR) method to provide 2007 and 2008. Impacts on typhoon/cyclone prediction in model initial conditions with different observational data 2006 and 2007 and on Mei-yu frontal cases in 2007 and assimilated by their operators. 2008 are explored. Impact on typhoon predictions in 2008 123 GPS Solut (2010) 14:35–49 37 are illustrated, in particular, since the COSMIC data in verified. The verifications are made for the model predic- 2008 have reached maturity in quantity and quality. tions (with and without assimilation of the GPS RO refractivity soundings) against sounding observations in the Simulated severe weather cases in 2006, 2007, and outer model domain. Note that the experiments are for 2008 operational tests and thus include all other routinely available observations (conventional radiosonde sounding, Since the launch of the FORMOSAT-3/COSMIC satellites, thickness and oceanic wind and cloud motion wind from there have been a number of interesting and important satellites, etc.). Figure 1 shows the model verification for severe weather events in Asia. We have conducted predicted temperature against soundings in August 2007 by numerical simulations/forecasts for many of them using CWB using WRF model for the denial run without GPS higher model resolutions (5 km) with focus on the regional RO data and the control run with GPS RO data. As can be area. A list of the simulated events in 2006, 2007, and 2008 clearly seen, both the bias and the standard deviation have is given in Table 1 which includes typhoons/tropical been reduced by assimilation of the GPS RO data, cyclones as well as Mei-yu frontal systems. The GPS RO soundings available for one assimilation time window and the outer model domain (covering most of East Asia) range from 2 to 56, depending on the cases. We will briefly present or describe some results of these simulated events with the available GPS soundings assimilated into the model.