Optimisation of spaceborne cloud profiling radar vertical and radiometric resolution requirements for resolving highly layered cloud structures

EarthCARE, a candidate Earth Explorer Core mission of ESA (European Space Agency), aims to improve our knowledge of the impact of clouds and aerosols on the Earth’s radiative budget. If the mission is selected, the satellite will carry two nadir sounding active instruments: a Cloud Profiling Radar (CPR) and a backscatter lidar. In addition, a multispectral cloud-imager and a broadband radiometer complement the payload. The objective of the present study was to optimise the parameters of the CPR for retrieving accurate radiative profiles for highly layered cloud structures. Realistic three-dimensional cloud scenarios taken from ground-based experiments have been used for simulating the spaceborne radar response to cloud layers. A radar simulator was developed initially for one-dimensional simulation of the radar echoes and then improved to a threedimensional simulation. The cloud microphysical properties were retrieved using a model as a function of the reflectivity factor and ice crystals size distribution, based on statistic studies from in-situ measurements. An extensive parametric analysis was performed for various vertical resolutions and sensitivities that have direct impacts on the radar design and necessary resources on-board the satellite. The analysis demonstrated that the proposed radar characteristics will meet the top-of-the-atmosphere radiative flux density estimation accuracy of 10 W/m2 as recommended by WCRP. 1 EarthCARE Candidate Earth Explorer Mission The Earth Clouds, Aerosol and Radiation Explorer (EarthCARE) is a mission jointly proposed by European and Japanese scientists in the frame of the Earth Explorer Core missions (SP-1257(1), 2001). EarthCARE is based upon more than two years of scientific exchanges between Japan and Europe, on the work previously carried on ATMOS-B1 (99P0A1-D011, 1999) (JAXA) and the Earth Radiation MisCorrespondence to: C. Tinel ([email protected]) sion (ESA) and on the joint preparation of the Report for Assessment for the User Consultation Meeting held in Granada in 2001. At the end of the Assessment cycle, EarthCARE was one of the three missions selected for Phase A level study. EarthCARE has been specifically defined with the scientific objectives of determining, for the first time, in a radiatively consistent manner, the global distribution of vertical profiles of cloud and aerosol field characteristics to provide basic essential input data for numerical modelling and global studies of the divergence of radiative energy, the aerosolcloud-radiation interaction, the vertical distribution of water and ice and their transport by clouds, and the vertical cloud field overlap and cloud-precipitation interactions. Reflecting the above requirements on EarthCARE, the following payload elements are required to fulfil the mission objectives: – a backscatter lidar to determine vertical profiles of aerosol physical parameters; – a Cloud Profiling Radar (CPR) for the retrieval of the microand macroscopic properties of clouds, precipitation and their convective motions; – a multi-spectral imager to provide information of the horizontal structure of cloud fields in support of the vertical profiles measured by the active instruments; – a broadband radiometer to measure short-wave (SW) and long-wave (LW) fluxes at the top of the atmosphere (TOA) as a constraint on the radiative flux derived from the vertical profiles of atmospheric properties and in particular the cloud-aerosol profiles measured by the active instruments and other passive instruments on board. In synergy, the backscatter lidar, the CPR and the multispectral imager can retrieve vertical profiles of cloud physical parameters, and the broadband radiometer will validate the TOA flux derived through the retrieved physical parameters. 546 C. Tinel et al.: Optimisation of spaceborne cloud profiling radar 2 CPR Characteristics As a part of the cooperative agreement between ESA, JAXA and NICT (National Institute of Information and Communication Technology), the CPR will be developed by NICT/JAXA as a payload contribution to EarthCARE. A unique feature of this instrument is the emission of microwave pulses that penetrate deep into lower cloud layers, which can not be viewed by passive optical sensors or reached by the lidar. It is designed to attain a high sensitivity for detecting a large majority of so-called ‘radiatively significant clouds’ and has a Doppler capability to measure vertical cloud motions and light precipitations. The latest design of the CPR can be found in Kumagai (2002). Table 1 summarises the instrument design and expected performance, which reflect the result of the optimisation analysis presented in this paper. 3 CPR System Simulator Details concerning equivalent reflectivity profile calculations, detectability and radiometric accuracy requirement are explained in a preliminary study (Tinel et al., 2002). Using 3D scenarios implies the introduction of horizontal distance integration and to take into account the beam filling effect. A simplified inverse model has been used in order to reduce time of calculations. The radar simulator is explained in Sect. 3.1, and Sect. 3.2 and 3.3 respectively present the simplified inverse model and radiative transfert model.