# A Study on Assimilation of CYGNSS Wind Speed Data for Tropical Convection during 2018 January MJO
## Data and Methods
### The 2018 January MJO and CYGNSS Observation
In this study, we will examine the CYGNSS data for TC Irving and a WWB event during the 2018 January MJO, and evaluate the impact of the CYGNSS data on the short-term numerical prediction of these events. TC Irving initiated from a low-pressure system in the southern Indian Ocean at \(\sim\)1800 UTC on 4 January 2018 and developed into a tropical storm after 18 h. It continued to strengthen and became a tropical cyclone by 0600 UTC on 7 January, with the center minimum sea level pressure (MSLP) falling to 981 hPa and the maximum surface winds reaching 36 m s\({}^{-1}\). TC Irving gained further intensity as it moved southwestward, and reached peak intensity with a MSLP of 962 hPa and maximum surface wind of 49 m s\({}^{-1}\) at 0000 UTC on 8 January. Thereafter, it started to weaken before it dissipated at around 0600 UTC on 11 January 2018. In this study, we focused on the intensification from 0000 UTC on 6-8 January, and the weakening afterwards until 0000 UTC on 9 January 2018. In this study, a few quality control steps were applied to the Level 2 CYGNSS data v2.1. Data with the range corrected gain below 3 and the zenith sun angle lower than zero were removed to filter out the  
Figure 1: (**a**) RMM index phase diagram from 27 December 2017 to 4 February 2018. Different color lines represent the different months (red for December, green for January, and blue for February); dates are annotated to the left of the values. Phases are labelled on the diagram. Points outside the circle indicate a significant MJO event, while phase triangles indicate its primary location. (**b**) CYGNSS daily wind speed averaged within 70\({}^{\circ}\)–80\({}^{\circ}\)E, 10\({}^{\circ}\)S–0\({}^{\circ}\). unreliable observations. Also, the uncertainties in the Block II-F GPS transmitter power and antenna gain patterns can significantly degrade the quality of wind speed products [49]. Therefore, all Level 2 retrievals from Block II-F GPS satellites were excluded from the Level 2 product. Further quality control (e.g., gross check, vertical consistency, dry convective adjustment, etc.) was conducted during WRFDA data process before the CYGNSS wind speed observation was assimilated. Two different wind speed products retrieved by different geophysical model functions (GMFs) are provided in v2.1, i.e., the FDS and the YSLF. The FDS is appropriate for most conditions, while the YSLF data can be used typically for the inner core of TCs. YSLF GMF was developed for conditions when the long-wave portion of the sea state has not responded fully to the local surface winds [40; 41]. Figure 2 displays the sample data of the CYGNSS v2.1 YSLF (Figure 2a) product around 1500 UTC, and FDS product over-plotted on GPM IMERG rainfall (Figure 2b) at around 1500 UTC, and the ASCAT wind speed (Figure 2c) at 1448 UTC on 7 January 2018. In Figure 2, CYGNSS detected TC Irving with a maximum wind of 24 m s\({}^{-1}\) in the FDS product and 44 m s\({}^{-1}\) in the YSLF product over the heavily-precipitating eyewall region. ASCAT views could not penetrate the core region of TC Irving, but a maximum wind speed of 26 m s\({}^{-1}\) was observed in the surrounding area. For the WWB event, IMERG data indicates that convection was observed with a maximum precipitation rate of around 10 mm h\({}^{-1}\). ASCAT observed a portion of the WWB event with maximum wind speed of 14 m s\({}^{-1}\). Strong winds related to this WWB event were detected by CYGNSS with a maximum wind speed of 17 m s\({}^{-1}\) in the FDS product. Figure 2: (**a**) WRF model 9-km and 3-km domains (d01 and d02) and CYGNSS v2.1 YSLF data within 1.5 h from 1500 UTC on 7 January 2018. The black circle shows the high winds in CYGNSS related to the WWB and the red circle shows high winds related to TC Irving. (**b**) CYGNSS v2.1 FDS data in 1400–1500 UTC over-plotted on IMERG 1-h rainfall at 1400 UTC on 7 January 2018. The magenta box shows the approximate location of the WWB event. (**c**) ASCAT wind speed at around 1448 UTC on 7 January 2018.