# Comparing Winds near Tropical Oceanic Precipitation Systems with and without Lightning
## 2 Materials and Methods  
### Cygnss  
The CYGNSS constellation of 8 small satellites provides observations of wind speeds over the tropical and subtropical oceans between approximately \(\pm 38^{\circ}\) latitude. This study used version 2.1 CYGNSS Level 2 wind data [21]. Version 2.1 data have been validated to meet all CYGNSS level 1 science requirements [20]. However, observations formed by signals from Block IIF GPS satellites are excluded from version 2.1 due to calibration uncertainties. This reduces the available number of wind observations by roughly one third.  
CYGNSS wind retrievals are available for specular reflections of GPS signals, and tracks are formed by a series of reflections corresponding to a single GPS satellite and one of the antennas on a single CYGNSS observatory. As noted by [19; 22], a single track of specular reflections comprises a fundamental and physically consistent grouping of CYGNSS observations. Therefore, this study's analysis focus was on CYGNSS tracks, and other datasets (e.g., ASCAT, IMERG, lightning) were matched to CYGNSS at the track level.  
Version 2.1 Level 2 CYGNSS wind files do not organize CYGNSS observations by track. However, tracks can be identified and grouped via organizing CYGNSS winds by contributing antenna, observatory number, and GPS satellite number (pseudorandom noise or PRN code). However, on a given day there can be multiple tracks with these common values. Therefore, the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) [23] clustering algorithm was applied to these common groups to further decompose to individual tracks. The track-separation algorithm allowed up to \(1^{\circ}\) difference in latitude/longitude and a time gap of 60 s before a new track was identified from a set of common antenna, observatory, and PRN numbers. The software used to create these tracks was the open source Python Interface to CYGNSS Wind Dataset (PyGNSS) [24].  
Each track--including geolocation and timing information, range-corrected gain (RCG), and wind speeds from the Fully Developed Seas (FDS) and Young Seas Limited Fetch (YSLF) retrieval algorithms--was saved to an individual Network Common Data Format (netCDF) file. YSLF wind speeds are generally only valid near tropical cyclones, while FDS wind speeds are accurate in other circumstances [20]. However, it remains useful to study the utility of YSLF winds near more conventional tropical precipitation, so this study analyzed both types of retrievals. Note that the dataset included tropical cyclones, but since these are rare (relative to conventional tropical precipitation systems) they likely did not impact overall results significantly.  
Tracks were identified for May 2017 through December 2019. With the full constellation operational, typically 2000-3000 tracks are available per day, and each track can contain an average of 200-300 wind observations over a time period lasting several minutes. CYGNSS samples along an individual track at 1 Hz [20], so based on orbit speed the average spacing along a track is 6 km. This provides roughly 4:1 oversampling relative to the nominal CYGNSS resolution of 25 km. However, data gaps often occur, but, as mentioned previously, the track identification algorithm described above allows for that.