The development of precipitation retrieval techniques, as well as the quality assessment of satellite-based global precipitation estimates, can now benefit from the availability of unique cloud and precipitation observations by the two spaceborne radars: the Dual-frequency Precipitation Radar (DPR) on board the NASA/JAXA Global Precipitation Measurement (GPM) Core Observatory, available since March 2014, and the NASA CloudSat Cloud Profiling Radar (CPR), available since 2006. The multi-year, quasi-global, and complementary DPR and CPR measurements offer a unique and extensive resource to analyze spaceborne microwave radiometer precipitation observational capabilities. This can be particularly useful in remote areas and/or where ground-based observations are sparse or not available (e.g., high latitudes), and in conditions and regimes where PMW precipitation retrieval is more challenging (e.g., light precipitation and snowfall). A recent study (Panegrossi et al., 2017) illustrated the potential of the use of these observational datasets to improve snowfall detection and retrieval techniques for passive MW radiometers at higher latitudes. The analysis of matched GPM Microwave Imager (GMI) and CloudSat CPR snowfall observations (mainly occurring at latitudes between 55° and 65°N) provided insights on microwave multi-frequency signals associated with snowfall. The analysis quantitatively showed the complex interconnection of high-frequency channel response to snowfall with background surface and environmental conditions (e.g., snow cover over land, sea ice concentration over ocean, water vapor content), and with the presence of supercooled droplets (as inferred from CPR/CALIPSO observations). The study also showed how such response depends on cloud vertical structure and snowfall intensity. Based on these findings, SLALOM (Rysman et al., 2018), a new algorithm for GMI to retrieve snow water path (SWP) and surface snowfall rate (SSR), has been developed. SLALOM exploits multi-platform (GPM/CloudSat/CALIPSO) snowfall coincidence dataset, and is based on random forest approach for snowfall and supercooled water detection, and multi-linear regression for SWP and SSR retrieval. Results show very good skills of SLALOM compared to CPR, both in terms of surface snowfall occurrence and quantification. A similar approach is being recently applied to cross-track scanning MW radiometers, such as the Microwave Humidity Sounder (MHS) on board MetOp and NOAA satellites, and the Advanced Technology Microwave Sounder (ATMS), on board Suomi NPP and JPSS-1 platforms, both equipped with high-frequency MW channels, to analyze their potential to detect snowfall at high latitudes.
These findings pave the way towards the definition of future missions for snowfall global monitoring by combining a core satellite equipped with a multi-frequency radar in synergy with current and future MW radiometers (such as the EPS-SG Microwave Imager and Microwave Sounder). This is the topic of the on-going Raincast ESA-ITT. The initial results of the study based on the analysis of the state-of the-art instrumentation, and on exploitation of the recent advancements in inversion methods for the estimation of precipitation variables, focused at identifying and consolidating the science requirements for such a satellite mission, will also be presented.

References
Panegrossi G., J-F. Rysman, D. Casella, A. C. Marra, P. Sanò, and M. S. Kulie, CloudSat-based assessment of GPM Microwave Imager snowfall observation capabilities, Rem. Sensing, 9(12), 1263; doi:10.3390/rs9121263, 2017.

Jean-François Rysman, G. Panegrossi, P. Sanò, A. C. Marra, S. Dietrich, L. Milani, M. S. Kulie, (2018), SLALOM: A snow water path retrieval algorithm for GPM Microwave Imager, Rem. Sensing, 2018, 10, 1278; doi:10.3390/rs10081278

The Global Precipitation Measurement (GPM) mission shows the benefits of synergistic retrievals of passive and active sensors for measuring precipitation. In our study we use synergistic retrievals with focus on clouds. Passive submillimeter and microwave sensors like the Ice Cloud Imager (ICI) and the Microwave Imager (MWI) on the upcoming MetOp-SG-B satellite have a great potential for estimating the bulk mass of clouds. Being the first operational down looking passive submillimeter sensor ICI has an even higher potential for estimating the bulk mass of ice clouds. Passive submillimeter and microwave sensors also have to some extent the potential to provide information about the vertical structure of clouds, but compared to a nadir looking cloud radar their vertical resolution is coarse. Nadir looking cloud radar satellites like CloudSat with its 94 GHz radar provide high resolution vertical profiles of water content (WC) but are limited to along track measurements, whereas ICI and MWI have a wide swath of about 1700 km but provide only limited information about the vertical structure.
Here we aim to bring together the strengths of passive sensors and radar to enhance satellite measurements of clouds. We combine the passive sensors with a hypothetical nadir looking 94 GHz cloud radar to estimate WC using synergistic radar and passive retrievals at the swath center. These swath center estimates are used to set up a database similar to the GPM mission for the passive offcenter retrievals. The passive retrievals are based on a Bayesian Monte Carlo integration approach. Passive and cloud radar observations are consistently simulated using the Atmospheric Radiative Transfer Simulator (ARTS). The atmospheric data for the simulations are taken from high resolution runs of Canada’s high-resolution numerical weather prediction model Global Environmental Multiscale (GEM) and from the ICOsahedral Non-hydrostatic (ICON) model of the German Weather Service (DWD) and the Max Planck Institute for Meteorology (MPI-M).
We assume that the nadir looking cloud radar flies on the same orbit ahead of Metop-SG-B, so that it provides quasi coincidental along track measurements with the oblique looking ICI and MWI. ICI and MWI are conical scanning radiometer with 53° local incidence angle.

The RadSnowExp is a multi-platform, multi-sensor (Fig. 1) study that will address the pressing need for provision of precipitation measurements at the global scale. Specific objectives of the RadSnowExp project are:
• Investigation and documentation of spatial and temporal variabilities, intensities, and types of precipitation with near-coincident in-situ and remote sensing measurements that can be used for future precipitation missions. The aims of the investigation include:
o Enhancing the quality of estimates of light and shallow precipitation
o Better characterization of small-scale precipitation events (showers)
• Identification of remote sensing signatures of various types of precipitation including snow over different surface types;
• Evaluation of the sensitivities of multi-frequency polarimetric radar and passive microwave signatures to different precipitation types including intensive snow events over different surface types;
• Evaluation of temporal variability of multi-frequency polarimetric radar and passive microwave during different precipitation

The RadSnowExp field project will be conducted over the Canadian Arctic centred on Iqaluit, Nunavut, Canada (~64N, 68W). For the project, the National Research Council (NRC) Convair-580 aircraft is being instrumented by NRC and Environment and Climate Change Canada (ECCC) with state-of-the-art in-situ sensors, triple frequency (Ka, X, and W-band) polarimetric and Doppler radars, lidars, and a G-band radiometer. The aircraft will be deployed to Iqaluit, NU between 19-30 November, 2018 and will conduct 6-8 flights (totalling 30 hours). Iqaluit is one of ECCC’s Canadian Arctic Weather Science (CAWS) supersites and is equipped with an extensive array of in-situ and remote sensing sensors; including a scanning Ka-band polarimetric radar, multiple lidars and radiation flux sensors. The CAWS supersite operates 24/7 for research purposes so no special arrangements, or additional work, are necessary in advance of the flight campaign. Although satellite cal/val is not a core component of RadSnowExp, attempts will be made to under-fly the GPM, CloudSat, and Aeolus satellites.

Preliminary analyses will be presented at the Symposium of the multi-platform dataset with a focus on triple frequency signatures of particle compositions for Arctic clouds.

Convective storms are the heart of the Earth’s weather and climate system: they convey most of the transport of water and air from near the Earth’s surface to the upper troposphere, they affect the large-scale atmospheric circulation, they are linked to the Earth’s water budget by producing large amounts of rainfall and they influence the Earth’s radiation budget via formation of widespread high clouds. Though convective vertical transport plays a pivotal role, predictions of current weather and future climates remain limited because the parametrizations of such transport are crude; this represents a major roadblock towards the refinement of weather forecasting and climate models. Global observations of convective vertical mass flux are urgently needed for significant progress to occur.
The launch of the first ever cloud 8-mm wavelength radar on a Cubesat (RainCube) in July 2018 has paved the way towards a new era for space-borne observations of convective systems. The small size (10 cm×20 cm×20 cm), moderate mass (21 kg) and low power (10 W peak power) requirement of the instrument enable constellation missions, which can augment our ability to observe weather systems and their dynamics and thermodynamics down to temporal resolutions of few minutes, as required for observing developing convection (see Figure 1).
The paper will present the first results of the mission with specific focus at two topics:
1) The validation of the RainCube precipitation product over US by using NEXRAD co-located ground based measurements;
2) The study of radar-radiometer synergies by looking at co-located observations between RainCube and TEMPEST-D radiometer (frequency between 89 and182 GHz.
Raincube offers a cheaper but unprecedented solution for capturing the storm dynamics. When a constellation of micro-satellites is flying in formation 60-90 seconds apart time-sequenced profiles of radar reflectivity (Z) separated seconds apart (ΔZ) can be acquired. Together Z and ΔZ/ Δt can be used to provide: (i) the mass fluxes of condensed water mass and dry air and (ii) the rates at which the upper regions of convective storms are moistened. The profiles of Z additionally provide profiles of condensed water M in the column and the precipitation falling from convective storms. The radar has demonstrated the maturity of the technology; concepts involving a constellation of Raincubes to better understand convective processes are currently under study.

Figure 1: Left: The RainCube's satellite with its umbrella-like parabolic mesh antenna Right: RainCube data from the airborne campaign as a part of the PECAN campaign (6/28–9/15 2015). Image credit: NASA/JPL-Caltech.

Two decades of spaceborne cloud and precipitation radar data provided by the TRMM’s Precipitation Radar (PR), CloudSat’s Cloud Profiling Radar (CPR) and GPM’s Dual-frequency Precipitation Radar (DPR) have enabled unprecedented advancements in the global mapping of occurrence and vertical structure of most types of meteorological events. After the immense success of these radars, the vision of moving from the current storm structure observation strategy to a new one that is also capable to provide information pertaining the processes that drive the evolution of clouds and precipitation has emerged. The last decade has been focused on achieving two essential improvements necessary to enable this vision : improved capability and improved feasibility.
In this presentation we will provide a high-level overview of radar instruments, and measurement technique concepts, that have been developed at JPL in the past decade with the goal of addressing several needs expressed by the cloud and precipitation remote sensing scientific community. They include the Ka-band 6U CubeSat RainCube demonstrated in space in 2018; the Ka-/W-band ACERAD concept based on CloudSat technology as first response to the needs of the Aerosol/Clouds/Ecosystem mission concept; the Ku-/Ka-/W-band electronically scanning 3CPR and MASTR instrument concepts developed to address the needs of unified Cloud and Precipitation observations, the G-band VIPR for water vapor measurements in-cloud; the Displaced Phase Center Antenna application to precipitation radar to enable Doppler measurements with small antennas; the W-band radar/spectrometer GAISR concept originally developed for planetary applications; and the Ka-/W-/G-band CloudCube concept that aims at integrating the RainCube, VIPR and GAISR technologies). Particular emphasis will be placed on the significance of the development cycle and in-orbit demonstration of RainCube, and how that is being merged with other innovative technologies and transferred to the MASTR and CloudCube instrument concepts.
Potential mission concepts are yet to be defined in the near future in response to the Earth Science Decadal Survey report 2018. Advancements such as the ones described in this presentation offer an array of options for the science community to address the needs of the Cloud, Convection and Precipitation Targeted Observable.

1. TRMM/PR and GPM/DPR
The precipitation radar(PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite is the first spaceborne precipitation radar in the world. The aim of the mission is to reveal the role of the tropical precipitation on the global circulation and therefore the requiring the accurate precipitation estimation as well as the three dimensional structure of the precipitation systems. TRMM/PR has been contributed to improve the precipitation estimation from passive microwave radiometer and to characterize the tropical precipitation systems such as diurnal cycles of precipitation and extreme precipitation based on the long term precipitation data. Improvements of the passive microwave radiometer estimation and the real time availability of multiple satellite data, hourly global precipitation map such GSMaP was established. The multiple satellite concept became the basis of the Global Precipitation Measurement (GPM) mission. The dual-frequency precipitation radar (DPR) was developed for the GPM/core satellite. The radar concept of the DPR is similar to the TRMM/PR except for the adding the new channel (Ka-band) in order to realize more accurate precipitation estimation and to observe solid precipitation (snow, graupel, hail etc.) by dual-frequency radar and the observation area expanded up to the 65 degrees in latitude. The GPM/DPR has been producing the estimates of the solid precipitation as well as cloud physical parameters of the precipitation (rain and snow). During the GPMM era, the hourly global precipitation map (GSMaP) has been improved and utilizing for the practical applications such as flood prediction indicating that the GSMaP product became indispensable precipitation product.

2. Direction of future precipitation measuring mission
Regarding precipitation research, a comprehensive understanding of aerosols, clouds and rainfall related to the formation of clouds and precipitation, such as radiation budget related to global warming and assessment of impact of climate change on precipitation systems, is required. Therefore, dynamic observation such as Doppler velocity observation is required in addition to the combined use of sensors (multi-wavelength radar and lidar) is required. It should be noted that the accurate long term precipitation record is also essential for the climate researches. In this context, continuous precipitation observation by radar is also important.
The GSMaP is still required for further improvement of its accuracy and data latency, especially for the practical use, future precipitation measuring mission is required to contribute the better GSMaP precipitation estimation such as improvement of the precipitation estimation by passive microwave radiometer and/or direct utilization of radar data (but it requires better coverage of data in time and space).

3. Precipitation measuring missions planned in Japan
In the future precipitation observation mission, it is necessary to consider the technological feasiblity that satisfies the scientific requirements relating to the global warming and the improvements for GSMaP. The first approach is to upgrade GPM/DPR. Even with existing technology, it is possible to expand the swath width by about 2 times than GPM/DPR and gain the sensitivity by about 100 times (20 dB). On the Doppler velocity measurement, it is difficult to realize with the existing antenna size, so the deployable antenna must be considered. For the improvement of GSMaP rain estimates, there is another approach so called small radar constellation which consists of small size TRMM/PR-like precipitation radar satellites to provide 6 hourly precipitation map over tropical region. One of the ultimate goals of the precipitation radar would be the precipitation observation from the geostationary orbit (precipitation radar on geostationary orbit: GPR). The required technological advancement is the large size (about 30 m in diameter) antenna which may be possible in near future. The advantage of the GPR is easier implementation of the Doppler velocity measurement. Figure 1 shows the roadmap of the spaceborne precipitation radar proposed.