## 5 Joint Observations with Operational Passive MW Radiometers  
In the above analysis, a \(\Delta\varphi>2\) threshold was used as an indication of heavy precipitation, but the ray-path nature of the RO observation provides little detail on horizontal context. For example, a RO ray path could exceed the \(\Delta\varphi\) threshold but in reality may have traversed through an edge of a convective cloud system, not necessarily indicative of the moisture structure within the convective core region associated with strong vertical air motion. Strong convection lofts heavy precipitation above the freezing level. In ROHP data, large \(\Delta\varphi\) are commonly noted well above 10-km altitude in convective clouds [39]. Heavy precipitation is often associated with horizontal moisture gradients. For example, if the successive RO ray paths were oriented parallel to and spread across a frontal boundary (some RO capture the dry side; other RO the moist side), the surrounding moisture structure will be interpreted differently than if the RO were aligned in a more perpendicular orientation relative to the front (similar moisture captured across all RO). To gather two-dimensional context on the clouds at the time of the constellation overpass, one option is to consult fast-refresh geostationary satellite imagery. The current network of geostationary satellites has common channels in the longwave (near 11 \(\upmu\)m) for tracking the day/night evolution of optically thick cloud tops [40]. With their wide (>2000 km) swath, existing operational passive MW radiometric observations can complement the constellation observations, providing horizontal precipitation context, and an estimate of the condensed water path along the RO ray paths [24]. Figure 9(a) provides one coincidence intersection from a simulated PRO event and a passive MW overpass occurring on 6 August 2018 near 1320 UTC, showing eight occultations, depicting 200-km ray path segments centered at each of the eight TP locations, and the corresponding (descending node) ground track of each simulated PRO satellite. These are overlaid upon the 165.5 GHz brightness temperature (TB) imagery from the Advanced Technology Microwave Sounder (ATMS) during a descending pass of the Joint Polar Satellite System (JPSS) NOAA-20 satellite near 1405 UTC. In this example, the simulated PRO events are just to the left of the satellite orbit track direction (i.e., negative antenna azimuth angle). Using TB < 200 K as a proxy for heavy precipitation, the PRO from satellite 1 traverses the region of heavy precipitation, those from satellites 2 and 3 traverse the edge of the precipitation, and those from satellites 4-8 form the outside-precipitation (surrounding environment) observations (an example of a P/E event). The TB imagery provides horizontal context to the precipitation structure and can be used to depict the water vapor structure in the cloud-free areas away from the precipitation. In this example event, each RO ray path segment was oriented such that was <500 km from the ground track of its respective satellite. However, other events will exhibit a range of offsets, depending upon the relative locations of the satellites during the occultation time. Figure 9(b) shows the distribution of the maximum value of the distance from the receiving satellite subtrack and the endpoints of each ray path segment (Figure 10 in Appendix A). The solid red line represents the cumulative fraction, showing that 90% of the events fall within 2000 km of the satellite subtrack. Since the event could lie to the left or right side relative to the spacecraft velocity vector, this implies that a single passive MW radiometer with a swath width of 4000 km would be needed to cover this same percentage, a value which is  
technically unfeasible from typical low-Earth orbit satellite altitudes \(<\) 800 km. However, a swath width of 1500 km would cover approximately 30% of the events.