WRF Data Assimilation Research Articles

Impact of local data assimilation on tropical cyclone predictions over the Bay of Bengal using the ARW model

Abstract. The tropical cyclone (TC) track and intensity predictions over Bay of Bengal (BOB) using the Advanced Research Weather Research and Forecasting (ARW) model are evaluated for a number of data assimilation experiments using various types of data. Eight cyclones that made landfall along the east coast of India during 2008–2013 were simulated. Numerical experiments included a control run (CTL) using the National Centers for Environmental Prediction (NCEP) 3-hourly 0.5 × 0.5° resolution Global Forecasting System (GFS) analysis as the initial condition, and a series of cycling mode variational assimilation experiments with Weather Research and Forecasting (WRF) data assimilation (WRFDA) system using NCEP global PrepBUFR observations (VARPREP), Atmospheric Motion Vectors (VARAMV), Advanced Microwave Sounding Unit (AMSU) A and B radiances (VARRAD) and a combination of PrepBUFR and RAD (VARPREP+RAD). The impact of different observations is investigated in detail in a case of the strongest TC, Phailin, for intensity, track and structure parameters, and finally also on a larger set of cyclones. The results show that the assimilation of AMSU radiances and Atmospheric Motion Vectors (AMV) improved the intensity and track predictions to a certain extent and the use of operationally available NCEP PrepBUFR data which contains both conventional and satellite observations produced larger impacts leading to improvements in track and intensity forecasts. The forecast improvements are found to be associated with changes in pressure, wind, temperature and humidity distributions in the initial conditions after data assimilation. The assimilation of mass (radiance) and wind (AMV) data showed different impacts. While the motion vectors mainly influenced the track predictions, the radiance data merely influenced forecast intensity. Of various experiments, the VARPREP produced the largest impact with mean errors (India Meteorological Department (IMD) observations less the model values) of 78, 129, 166, 210 km in the vector track position, 10.3, 5.8, 4.8, 9.0 hPa deeper than IMD data in central sea level pressure (CSLP) and 10.8, 3.9, −0.2, 2.3 m s−1 stronger than IMD data in maximum surface winds (MSW) for 24, 48, 72, 96 h forecasts respectively. An improvement of about 3–36 % in track, 6–63 % in CSLP, 26–103 % in MSW and 11–223 % in the radius of maximum winds in 24–96 h lead time forecasts are found with VARPREP over CTL, suggesting the advantages of assimilation of operationally available PrepBUFR data for cyclone predictions. The better predictions with PrepBUFR could be due to quality-controlled observations in addition to containing different types of data (conventional, satellite) covering an effectively larger area. The performance degradation of VARPREP+RAD with the assimilation of all available observations over the domain after 72 h could be due to poor area coverage and bias in the radiance data.

Offshore Wind Resources Assessment from Multiple Satellite Data and WRF Modeling over South China Sea

Using accurate inputs of wind speed is crucial in wind resource assessment, as predicted power is proportional to the wind speed cubed. This study outlines a methodology for combining multiple ocean satellite winds and winds from WRF simulations in order to acquire the accurate reconstructed offshore winds which can be used for offshore wind resource assessment. First, wind speeds retrieved from Synthetic Aperture Radar (SAR) and Scatterometer ASCAT images were validated against in situ measurements from seven coastal meteorological stations in South China Sea (SCS). The wind roses from the Navy Operational Global Atmospheric Prediction System (NOGAPS) and ASCAT agree well with these observations from the corresponding in situ measurements. The statistical results comparing in situ wind speed and SAR-based (ASCAT-based) wind speed for the whole co-located samples show a standard deviation (SD) of 2.09 m/s (1.83 m/s) and correlation coefficient of R 0.75 (0.80). When the offshore winds (i.e., winds directed from land to sea) are excluded, the comparison results for wind speeds show an improvement of SD and R, indicating that the satellite data are more credible over the open ocean. Meanwhile, the validation of satellite winds against the same co-located mast observations shows a satisfactory level of accuracy which was similar for SAR and ASCAT winds. These satellite winds are then assimilated into the Weather Research and Forecasting (WRF) Model by WRF Data Assimilation (WRFDA) system. Finally, the wind resource statistics at 100 m height based on the reconstructed winds have been achieved over the study area, which fully combines the offshore wind information from multiple satellite data and numerical model. The findings presented here may be useful in future wind resource assessment based on satellite data.

Radar radial wind data assimilation using the time-incremental 4D-Var method implemented to the WRFDA system

In this study, we selected a heavy rainfall case over the Korean Peninsula, which was characterised by two localised rainfall maxima. Neither of the two maxima is accurately simulated when no radar data are assimilated or when radar data are assimilated using the three-dimensional variational (3D-Var) method of the Weather Research and Forecasting Data Assimilation (WRFDA) system. Using the four-dimensional variational (4D-Var) method included in the WRFDA system improves the rainfall forecast partially. To obtain further improvements in the rainfall forecast, outer loops and the time-incremental 4D-Var method are used. In the time-incremental 4D-Var method, the length of the assimilation window is increased gradually, and the starting point of the current minimisation task comes from the minimiser of the previous minimisation task. The analysis of the experiment using outer loops or the time-incremental 4D-Var method is closer to the observations than that of the experiment using the 4D-Var method. This is because the first guess is improved progressively by using outer loops or the time-incremental 4D-Var method. The gap between nonlinear and linear growth is reduced via outer loops or the time-incremental 4D-Var method compared to the 4D-Var method, because the nonlinearity is accounted for by consistently updating the nonlinear model trajectory. However, the rainfall forecast is improved only in the experiment using the time-incremental 4D-Var method. Analysis increments of the horizontal wind and convective available potential energy (CAPE) result in proper modifications to the analysis, and finally, an improved subsequent forecast. The quasi-static adjustment in the time-incremental 4D-Var method may contribute to finding the global minimum under the high degree of nonlinearity present in the original minimisation problem.

Impact of assimilating IASI radiance observations on forecasts of two tropical cyclones

The impact of assimilating Infrared Atmospheric Sounding Interferometer (IASI) radiance observations on the analyses and forecasts of Hurricane Maria (2011) and Typhoon Megi (2010) is assessed using Weather Research and Forecasting Data Assimilation (WRFDA). A cloud-detection scheme (McNally and Watts 2003) was implemented in WRFDA for cloud contamination detection for radiances measured by high spectral resolution infrared sounders. For both Hurricane Maria and Typhoon Megi, IASI radiances with channels around 15-μm CO2 band had consistent positive impact on the forecast skills for track, minimum sea level pressure, and maximum wind speed. For Typhoon Megi, the error reduction appeared to be more pronounced for track than for minimum sea level pressure and maximum wind. The sensitivity experiments with 6.7-μm H2O band were also conducted. The 6.7-μm band also had some positive impact on the track and minimum sea level pressure. The improvement for maximum wind speed forecasts from the 6.7-μm band was evident, especially for the first 42 h. The 15-μm band consistently improved specific humidity forecast and we found improved temperature and horizontal wind forecast on most levels. Generally, assimilating the 6.7-μm band degraded forecasts, likely indicating the inefficiency of the current WRF model and/or data assimilation system for assimilating these channels. IASI radiance assimilation apparently improved depiction of dynamic and thermodynamic vortex structures.

Are current sensitivity products sufficiently informative in targeting campaigns? A DTS‐MEDEX‐2009 case study

AbstractThe DTS‐MEDEX‐2009 campaign was a field experiment in which extra observations were adaptively deployed to improve the short‐range forecast of Mediterranean high‐impact weather (HIW) during autumn 2009. For the DTS‐MEDEX‐2009 cases, five different sensitivity analysis techniques were carried out to provide targeting guidance: singular vectors (SV) from the ECMWF; ensemble transform Kalman filter (ETKF) and Kalman filter sensitivity (KFS) from Météo France; and ensemble and adjoint sensitivities from the University of the Balearic Islands. However, the value of the targeting guidance provided by such a variety of sensitivity products has never been assessed for a Mediterranean HIW event. Since radiosonde and AMDAR profiles were the only observational means available during the DTS‐MEDEX targeting campaign, this study tests the ability of each sensitivity product in identifying the region where a plausible sounding leads to a greater impact on the forecast of a potential high‐impact cyclone over southern Italy on 5 December 2009. All targetable radio‐sounding sites are also tested and a severe‐weather meteorologist is used as a confronting reference. The verification testbed comprehends single sounding experiments and multiple sounding strategies by using the WRF Data Assimilation system. Single sounding tests reveal that sensitivity products fail to recognize the best location for a single observation since most of the soundings added over operational radio‐sounding stations have a larger influence on intense cyclone forecast than the points highlighted by the objective sensitivity calculation methods. Additionally, it is shown that human‐based decisions, after evaluating available sensitivity information, are not optimal, either in single or in multiple sounding strategies.

Evaluation of the Advanced Hurricane WRF Data Assimilation System for the 2009 Atlantic Hurricane Season

Abstract Real-time analyses and forecasts using an ensemble Kalman filter (EnKF) and the Advanced Hurricane Weather Research and Forecasting Model (AHW) are evaluated from the 2009 North Atlantic hurricane season. This data assimilation system involved cycling observations that included conventional in situ data, tropical cyclone (TC) position, and minimum SLP and synoptic dropsondes each 6 h using a 96-member ensemble on a 36-km domain for three months. Similar to past studies, observation assimilation systematically reduces the TC position and minimum SLP errors, except for strong TCs, which are characterized by large biases due to grid resolution. At 48 different initialization times, an AHW forecast on 12-, 4-, and 1.33-km grids is produced with initial conditions drawn from a single analysis member. Whereas TC track analyses and forecasts exhibit a pronounced northward bias, intensity forecast errors are similar to (lower than) the NWS Hurricane Weather Research Model (HWRF) and GFDL forecasts for forecast lead times ≤60 h (>60 h), with the largest track errors associated with the weakest systems, such as Tropical Storm (TS) Erika. Several shortcomings of the data assimilation system are addressed through postseason sensitivity tests, including using the maximum 800-hPa circulation to identify the TC position during assimilation and turning off the quality control for the TC minimum SLP observation when the initial intensity is far too weak. In addition, the improved forecast of TS Erika relative to HWRF is shown to be related to having initial conditions that are more representative of a sheared TC and not using a cumulus parameterization.

Estimation of TAMDAR Observational Error and Assimilation Experiments

Abstract Tropospheric Airborne Meteorological Data Reporting (TAMDAR) observations are becoming a major data source for numerical weather prediction (NWP) because of the advantages of their high spatiotemporal resolution and humidity measurements. In this study, the estimation of TAMDAR observational errors, and the impacts of TAMDAR observations with new error statistics on short-term forecasts are presented. The observational errors are estimated by a three-way collocated statistical comparison. This method employs collocated meteorological reports from three data sources: TAMDAR, radiosondes, and the 6-h forecast from a Weather Research and Forecasting Model (WRF). The performance of TAMDAR observations with the new error statistics was then evaluated based on this model, and the WRF Data Assimilation (WRFDA) three-dimensional variational data assimilation (3DVAR) system. The analysis was conducted for both January and June of 2010. The experiments assimilate TAMDAR, as well as other conventional data with the exception of non-TAMDAR aircraft observations, every 6 h, and a 24-h forecast is produced. The standard deviation of the observational error of TAMDAR, which has relatively stable values regardless of season, is comparable to radiosondes for temperature, and slightly smaller than that of a radiosonde for relative humidity. The observational errors in wind direction significantly depend on wind speeds. In general, at low wind speeds, the error in TAMDAR is greater than that of radiosondes; however, the opposite is true for higher wind speeds. The impact of TAMDAR observations on both the 6- and 24-h WRF forecasts during the studied period is positive when using the default observational aircraft weather report (AIREP) error statistics. The new TAMDAR error statistics presented here bring additional improvement over the default error.

The Weather Research and Forecasting Model's Community Variational/Ensemble Data Assimilation System: WRFDA

Data assimilation is the process by which observations are combined with short-range NWP model output to produce an analysis of the state of the atmosphere at a specified time. Since its inception in the late 1990s, the multiagency Weather Research and Forecasting (WRF) model effort has had a strong data assimilation component, dedicating two working groups to the subject. This article documents the history of the WRF data assimilation effort, and discusses the challenges associated with balancing academic, research, and operational data assimilation requirements in the context of the WRF effort to date. The WRF Model's Community Variational/Ensemble Data Assimilation System (WRFDA) has evolved over the past 10 years, and has resulted in over 30 refereed publications to date, as well as implementation in a wide range of real-time and operational NWP systems. This paper provides an overview of the scientific capabilities of WRFDA, and together with results from sample operation implementations at the U.S. ...

Diagnostic analysis of the asymmetric structure of the simulated landfalling typhoon “Haitang”

The landfalling processes of Typhoon “Haitang” near Lianjiang of Fujian Province of China from 00 UTC 19 to 12 UTC 20 July 2005 were reproduced by using the nesting non-hydrostatic WRF model and data assimilation technology (Level II Doppler radar data of Changle of Fujian Province are assimilated to the simulation every one hour from 01 to 06 UTC 19 July 2005). The mesoscale structure and evolution of the typhoon before, during, and after its landfall were discussed. The simulation data show that the assimilation experiment with the high temporal-and-spatial-resolution radar radial velocity and reflectivity data can produce much better simulation of the typhoon track, intensity evolution and landfalling location than the control experiment without assimilating radar data. By using the assimilation experimental data, the mesoscale fine-mesh structure and evolution before and after typhoon landfall were analyzed. Because of the influence of sea–land thermodynamic difference, two asymmetric convective regions were located at ocean and land, respectively. To better understand and investigate the asymmetric-structure characteristics of the landfalling typhoon, several dynamical diagnostic tools, the helicity ( H), the moist potential vorticity (MPV), the convective vorticity vector (CVV), the moist vorticity vector (MVV), which are associated with the development of strong convections, are introduced. Further analysis illuminates that the distributions of these physical diagnostic parameters are totally asymmetric, and subsequently, the associated convections also show distinct asymmetry.