# 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  
The MJO is the dominant mode of intraseasonal variability (30-90 days), characterized by eastward propagation of enhanced deep convection at an average speed of 5 m s\({}^{-1}\) during boreal winter [43]. The MJO convective signal typically initiates in the Indian Ocean and propagates to (and often across) the Maritime Continent to the western Pacific. The Real-time Multivariate MJO (RMM) index [44] is a commonly used index in MJO studies and operation forecasts as a measure of the phase and intensity of an MJO event. Figure 1 illustrates the RMM index provided by the Australian Bureau of Meteorology (data available at [[http://www.bom.gov.au/climate/mjo/graphics/rmm.74](http://www.bom.gov.au/climate/mjo/graphics/rmm.74) toRealtime.txt]([http://www.bom.gov.au/climate/mjo/graphics/rmm.74](http://www.bom.gov.au/climate/mjo/graphics/rmm.74) toRealtime.txt)) from late December 2017 to early February 2018. RMM1 and RMM2 represent the first and second principal components of the combined empirical orthogonal functions (EOFs) of outgoing longwave radiation (OLR), as well as zonal wind at 200 and 850 hPa averaged between 15\({}^{\circ}\)N and 15\({}^{\circ}\)S. Points outside the center circle indicate a significant MJO event; the farther from the origin, the stronger the MJO amplitude. The phase triangles indicate the primary location of Indian Ocean, Maritime Continent, western Pacific, or western hemisphere and Africa. The 2018 MJO was one of the strongest events in the last few decades [45]. [PERSON] [45] showed that the MJO progressed eastward in January from the Indian Ocean to the Pacific Ocean in February, and then back to the Indian Ocean in March. The apparent counterclockwise motion shown in Figure 1a indicates the eastward propagation of the event. As in Figure 1a, the MJO was in phases 2 and 3 over the Indian Ocean from January 1 to 16, with maximum RMM of 2.1. The MJO intensified as it passed through Maritime Continent (Phases 4 and 5) from January 17 to 26. The MJO was strong in the western Pacific in phases 6 and 7 from January 27 to February 4, with a peak RMM close to 4. [PERSON] et al. [46] found that synthetic CYGNSS observations were able to measure outflow from convective storms in simulated MJO eventsfrom October and December 2011. Figure 1b shows the CYGNSS surface wind speed averaged over the domain 70\({}^{\circ}\)-80\({}^{\circ}\)E and 0\({}^{\circ}\)-10\({}^{\circ}\)S from 1 to 31 January 2018. This is within the core initiation region of the MJO in the Indian Ocean. The average CYGNSS wind speed showed a peak of 8 m s\({}^{-1}\) on 7-8 January, which is in agreement with the maximum RMM value at the same time in phases 2 and 3. A trough in the average CYGNSS wind speed with a magnitude lower than 4 m s\({}^{-1}\) occurred in 12-16 January. This is also in agreement with the low RMM index in 10-15 January shown in Figure 1a. Another period of large CYGNSS wind over 7 m s\({}^{-1}\) occurred during 24-29 January 2018, which also coincides with strong convection and increasing RMM values during that time period as the MJO signal transitioned to the Maritime Continent. This indicates that, in general, CYGNSS data were able to detect strong winds related to this MJO event. However, it is noteworthy that the average CYGNSS wind speed calculated here is sensitive to WWBs during the MJO, and also to any weather system enhancing wind speeds within the domain; thus this metric should not be viewed as a definitive measure of MJO strength. The late January enhancement in CYGNSS wind speed appears to be similar to that of early January, with an enhanced near-Equator westerly flow associated with the presence of TC Cebile. Previous studies found that CYGNSS measurements during TC overpasses can help in finding the correct center location, estimating the intensity, and constructing the wind distribution within the TC [40; 47; 48]. [PERSON] et al. [46] also showed that synthetic CYGNSS data may be used to monitor westerly wind burst (WWB) events during MJO events.