Deciphering the Signatures of Oceanic Convective Rain Cells Using Simultaneous Observations From C‐Band Synthetic Aperture Radar Onboard EOS‐04 Satellite and GPM Measurements

Earth Observation Satellite (EOS)‐04 launched on 14 February 2022, carries a C‐band Synthetic Aperture Radar (SAR) for Agriculture, Forestry, Hydrology, and Flood mapping applications. In this paper, we have used C‐band SAR images and near‐simultaneous observations from the global precipitation measurements (GPM) to study the signatures of multiple convective rain cells. The bright patches are found on C‐band SAR imagery, which depicts the information of hydrometeors such as graupels or hails in the melting layer. For the first time, unambiguously estimated the diameter of the convective core rain cells from the C‐band SAR backscattered signal and compared with near‐simultaneous observations from GPM‐GMI and Ku‐band radar to confirm our findings. Thus, the present study demonstrates the potential of C‐band SAR for identifying the signatures of convective rain cells.

• The present study brought out the signatures of convective rain cells using C-band Synthetic Aperture Radar (SAR) onboard Earth Observation Satellite -04 satellite • For the first time, unambiguously estimated the diameter of the convective core rain cells from the C-band SAR backscattered signal • We compared near-simultaneous observations from GPM-GMI and Ku-band precipitation radar to confirm the present findings

Supporting Information:
Supporting Information may be found in the online version of this article.
In addition to the above applications, SAR measurements are utilized in mapping other ocean and atmospheric phenomena such as oceanic fronts, marine atmospheric boundary layer, and convective rain cells, which affect the SAR signal (e.g., Fu & Holt, 1982). However, the signature of convective rain is exhibited differently with varying SAR operating frequencies. During heavy rainfall conditions, SAR signal gets attenuated by raindrops in the atmosphere. It is well known that the signature of rain cells can be detected on SAR images at all radar frequencies. Negative impacts of rain for L-band SAR is attributed to wave damping and for X-band enhanced SAR signal is observed due to ring waves as well as from the volume scattering (e.g., Alpers et al., 2016;Melsheimer et al., 1998). It becomes more complex, especially at the C-band wavelength, where it lies in the transition region of Bragg scattering (Braun & Gade, 2006). The information on the convective rain cells over the oceanic region is important, where the traditional ground-based weather radars cannot observe due to their range limitation over the oceanic regions. Further, SAR provides high-spatial resolution images, which contain the convective rain signatures providing finer spatial structures of heavy rain events. Combining the SAR observations along with the space borne precipitation, simultaneously (e.g., TRMM-precipitation radar and Global Precipitation Measurements (GPM)), will provide better understanding of the convective rain events, which are intensifying at an alarming rate in the warming climate scenario. Thus, the present study utilizes the C-band SAR data onboard Earth Observation Satellite (EOS)-04 for deciphering the signatures of convective rain events. Also, we employ GPM measurements for confirming the rain signatures on C-band SAR images. The technical details about the C-band SAR, GPM satellite, and methodology are discussed in Section 2. Section 3 describes the results and discussion of the convective rain events observed by SAR and GPM measurements. Section 4 summarizes the results and presents future scope.

Data and Methodology
In the present study, we have used C-band SAR backscattering measurements to observe the signatures of convective rain cells. We have used near-simultaneous observations from GPM microwave Imager (GMI), which is microwave radiometer and Ku-band precipitation radar onboard the GPM satellite, to confirm the occurrence of rain cells. This section describes the C-band SAR and GPM data collection and its processing.

C-Band SAR Data Processing
C-band SAR onboard Earth Observation Satellite (EOS)-04 was launched on 14 February 2022 by the Indian Space Research Organisation (ISRO). This satellite is in a sun-synchronous orbit at an altitude of 529 km and is a follow-on mission to RISAT-1. The operating frequency of SAR is C-band (5-5.35 GHz) and is variable. The scanning technique of SAR is side-looking, both to left-and-right with 20°-49° off-nadir. SAR operates in various modes such as High Resolution Spotlight (HRS), Fine Resolution Stripmap (FRS), Medium Resolution ScanSAR (MRS), and the Coarse Resolution ScanSAR (CRS) mode. The resolution of each SAR pixel is 1-50 m, depending on the operation mode. Table 1 provides the more detailed characteristics of various operation modes of C-band SAR. The data used in the present analysis consist of CRS mode, which has the pixel resolution of 50 m. The SAR collects the backscattered signal from the hypothetical stationary Earth's surface as it moves along track. The integration of SAR depends on the synthetic aperture. The longer the synthetic aperture is, the larger the integration. The final resolution of the SAR image is a function of the viewing angle, pulse width, relative phase among signals received from various pulses, and coherent integration time, which makes the SAR resolution independent of the distance from the Earth's surfaces. The SAR image data were downloaded from the Bhoonidhi portal (https://bhoonidhi.nrsc.gov.in/bhoonidhi/index.html). The processing of C-band SAR radiance data over the convective rain cells has been performed and estimated the σ 0 as shown in below equation: 0 = 10 log 10 ( Rad 2 − ) + 10 log 10 (Sin( )) − where, σ 0 (dB) is the backscattering coefficient, Rad is the radiance available in the product for different polarizations, N is the image noise bias, I a is the per pixel incidence angle, and C (dB) is the calibration constant for different polarizations available in the META data file.

GPM Data
To confirm the convective rain features observed by C-band SAR image, near-simultaneous observations from the Global Precipitation Measurements (GPM) Microwave Imager (GPM-GMI) and Ku-band precipitation radar are used to study the vertical and horizontal structures of convective rain cells. The GPM consist of GMI and dual precipitation radar (DPR) of Ku-(13.6 GHz) and Ka-band (35.5 GHz). The swath width of GMI is 885 km and is used to confirm the spatial pattern of convective rain cells as observed by C-band SAR. The Ka-band is sensitive to light-moderate rainfall, while Ku-band is more sensitive to moderate-heavy rainfall. The swath width of Ku (Ka)-band is 240 (120) km, and the vertical range resolution is 250 m. The minimum resolution for Ka (Ku)-band is 0.2 (0.5) mm/hr (Hou et al., 2014). In the present study, we have used level 2A DPR product (https://storm.pps. eosdis.nasa.gov/storm/), which contains the vertical profiles of Ku-and Ka-band radar data to study the vertical structure of convective rain cells and their horizontal width. This data product also contains information on the melting layer. Figure 1a shows an EOS-04 C-band SAR image acquired on 30 May 2022 over the Australian coast at 1017 UTC, and the zoomed image of Figure 1a is shown in Figure 1c in ascending orbit with HV polarization mode. HV denotes that the signal is emitted at horizontal polarization and received at vertical polarization. The white patches are the signatures of the convective rain cell footprints. Figure 1b shows the observations of GPM GMI over the exact location, and the zoomed one is shown in Figure 1d, where it is clearly evident that the rain band has multiple convective systems and the maximum rain rate is found to be above 30 mm/hr. Further, we have utilized the GPM Ku-band precipitation radar measurements to examine the vertical extent of convective systems. Figure 2a depicts the spatial structure of reflectivity at 2 km height level. It shows the presence of packets of high reflective areas, which correspond to multiple rain core regions. The bright areas on C-band SAR are almost colocated with GPM precipitation radar reflectivity observations higher than 40 dBZ. Therefore, this observation suggests that the C-band SAR backscattering signal enhances at higher rain rates. Melshmeimer et al. (1998) also observed a similar feature with ground-based weather radar (NEXRAD) reflectivity in the convective storm along with a C-band SAR image. The maximum reflectivity is found to be around 50 dBZ in the present study ( Figure 2). The vertical structure of reflectivity corresponding to the solid line AB in Figure 2a is shown in Figure 2b. In the present case, the vertical extent of the convective rain cell was found to be around 16.5 km, and the diameter of the convective rain cell is ∼22 km. Figure S1 in Supporting Information S1 provides more details on the vertical extent of convective precipitating system. In general, the diameter of a typical convective rain core is around 30 km (e.g., Mapes & Houze, 1995). Further, we have also estimated the diameter of the convective rain cell using a C-band SAR image, as shown in Figure 3. Figure 3a shows the spatial distribution of σ 0 (dB) and (b) shows the various cross-sections of rain cells, which correspond to solid lines in Figure 3a. From these line profiles, it is evident that the C-band SAR backscattered signal is increased in the presence of convective rain cells resulting from shadowing by hydrometeors in the melting layer, while at L-band, radar signatures consist of dark patches resulting from scattering of the sea surface caused by the roughness reduction due to impinging onto the sea surface (Alpers et al., 2016). But, in the present case, there is no dark patch visible on the right to the bright patch ( Figure 3a). In general, the dark areas followed by the bright areas on C-band SAR occur due to the shadowing of the precipitating convective system (Fu & Holt, 1982). From the previous studies (e.g., Alpers et al., 2016;Melsheimer et al., 1998;Xu et al., 2015), it is assumed that the physical processes which contribute to the signatures of rain over the ocean on SAR image are the scattering of hydrometeors through modification   of the sea surface roughness, which increases in the presence of downdraft by hydrometeors, scattering from raindrop splash on the water surface, and volume scattering and attenuation by hydrometeors in the precipitating convective systems. Previous studies also showed that the signatures of convective rain core regions are often not escorted by dark areas on the C-band SAR images (e.g., Alpers et al., 2016;Fu & Holt, 1982;Melsheimer et al., 1998). The bright patches on the C-band SAR image are caused by the reflections of hydrometeors in the melting layer (e.g., Alpers et al., 2016).

Results and Discussions
In the present case, we have also observed the melting layer from the GPM Ku-band radar measurements, as seen in Figures 2b and 2c. Asterisk symbol denotes the melting layer height, which causes a strong backscattered radar signal and is often found around 4-6 km over the tropical latitude. The melting layer consists of irregular hydrometeors such as ice/snowflake particles undergoing a phase transition from solid to liquid. In general, the melting layer, also called as "bright band" phenomenon in radar meteorology, shows up as higher reflectivity at the zero-degree isotherm level (Houze, 1997). The main reason for the higher reflectivity in the radar signal (Figures 2b and 2c) is that the diameter of falling ice/snowflakes is large but coated with liquid, which has the higher refractive index compared to ice, leading to high reflectivity values near the melting layer as seen in Figures 2b and 2c. In the present case, the height of the melting layer was found to be around 4.8 km.
The radar reflectivity in the melting layer showed an enhancement due to the high reflectivity of snow or ice (Houze, 1997), also called as a bright band. The stratiform region in the convective system is unambiguously identified by the bright band, as seen in Figures 2b and 2c. Thus, the bright patches caused by reflection are not followed by dark areas in the look direction of the C-band SAR antenna, as shown in Figure 1b. Most of the previous studies interpret the bright patches on C-band SAR images associated with a melting layer due to the nonavailability of weather radar observations or other sensors to observe the melting layer independently. Thus, the present study unambiguously demonstrated that the bright oceanic regions on the C-band SAR image are due to the presence of a melting layer, which is confirmed by the GPM Ku-band precipitation radar measurements. Alpers et al. (2016) observed a strong reflection of hydrometeors in the melting layer on the SAR images. The example shown in Figure 1 is in HV polarization, which seems to be strongly associated with SAR backscattered signal from hydrometeors in the melting layer than in HH mode (figure not shown). Browne and Robinson (1952) found more contribution from the melting layer by analyzing the radar cross-polarized backscattered signal, and sometimes, they could detect the melting layer signature only at cross-polarization but not at copolarization.
Further, we have unambiguously estimated the diameter of the convective rain cells. Figure 3b, shows line profiles of σ 0 (dB) at HH (red) and HV (blue) along the pixel of the C-band SAR at three different cross-sections corresponding to solid line in Figure 3a. The σ 0 cross-polarization (HV) values were found to be around −15 to −25 dB, in the present case. This shows a very pronounced signatures of convective rain core at HH (red) and HV (blue) polarizations. The maximum peak values of HH and HV are around −10 and −20 dB, respectively, and the ratio of HH to HV is around −10 dB. The values are consistent with the previous results by Alpers et al. (2016). Alpers et al. (2021) also observed a similar range of SAR cross-section values in the presence of a melting layer. From the profile along A2-B2, the σ 0 increased from pixel 200 to 600, and the convective rain cell consists of around 400 C-band SAR pixels. The resolution of each pixel is 50 m. Therefore, the diameter of the convective rain cell estimated from C-band SAR is about 20 km, coinciding with the GPM Ku-band radar value. At A1-B1 and A3-B3, the diameters of the rain cell are estimated to be around 12.5 and 17.5 km, respectively. From Figure 3, it is evident that the C-band SAR image shows multiple convective rain cells with various diameters. Figure 4 shows another case of simultaneous observations of convective rain cell by C-band SAR (CRS mode) and GPM over the Sierra Leone coast, Africa, on 15 June 2022. The corresponding GMI rain rates, as well as the spatial and vertical structure of radar reflectivity are also shown in Figures 4b-4d. From this figure, it is evident that the convective rain cell tops are reaching around 10 km height. Figure 4e shows the variation of σ 0 profile across A1-B1 solid line in Figure 4a and the diameter of convective rain cell found to be around 25 km, which is confirmed by the GPM radar measurements shown in Figure 4d. Thus, C-band SAR measurements can be utilized to identify the diameter of the convective rain cores within the mesoscale convective precipitating systems. The significance of the present study lies in demonstrating the potential of high spatial resolution of C-band SAR measurements in detecting the convective rain footprints and providing insights into the small-scale precipitation cells, which are embedded into the mesoscale convective precipitating systems.

Summary and Conclusions
In the present study, the signatures of convective rain cells are investigated using near-simultaneous observations from C-band SAR onboard the EOS-04 satellite and GPM-GMI and Ku-band precipitation radar for the first time.
The following are the summary of the results: 1. The footprints of convective rain cells appeared as bright oceanic patches on the C-band SAR image. 2. The bright areas are formed due to the reflections of the C-band SAR backscattered signal from the hydrometeors, such as ice/snowflakes in the melting layer. 3. The GPM observations confirmed the presence of heavy rain rate cells. From the Ku-band precipitation radar, it was observed that the vertical extent of the convective cloud was as high as 16.5 km and the width of the melting layer was around 1.2 km. 4. Diameter of the convective rain core was estimated unambiguously using σ 0 of SAR, and it was found to be around 20 km in the present case, whereas the GPM Ku-band precipitation radar observed the same to be 22 km. 5. The present study thus utilized the near-simultaneous observations from C-band SAR onboard EOS-04 and GPM satellites for unambiguously identifying the signatures of oceanic convective rain cells.
In future, we will attempt to decipher information on convective and stratiform rain signatures in C-band SAR imagery and explore the possibility of detecting lightning events using C-band backscattered signals.
Thus, the present study demonstrated the potential of C-band SAR for identifying the signatures of convective rain cells.