P1.7 Earthcare Towards a Quantification of Cloud-Aerosol-Radiation Interactions
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P1.7 EarthCARE Towards a Quantification of Cloud-Aerosol-Radiation Interactions Tobias Wehr*, Paul Ingmann, Dulce Lajas, Arnaud Heliere European Space Agency, ESTEC, The Netherlands 1. INTRODUCTION • The retrieval of atmospheric radiative heating and cooling through the The Earth Clouds, Aerosols and Radiation combination of retrieved aerosol and cloud Explorer, EarthCARE, has been selected for properties; implementation as the sixth Earth Explorer • The measurements of TOA radiances and Mission of the European Space Agency’s retrieval of fluxes in relation of the Living Planet Programme [1]. The mission will observed clouds and aerosols. measure three-dimensional structures of clouds and aerosols simultaneously with the The key parameters to be addressed are cloud respective outgoing broad band flux. These and aerosol extinction and absorption measurements would significantly contribute to properties, large-scale cloud structure the understanding of key parameters in the (including cloud fraction and overlap) and Earth radiation budget and quantify the impact cloud condensate content, particle size, shape of clouds and aerosols on radiation and flux. and small scale cloud structures. Furthermore, With this, EarthCARE would provide crucial EarthCARE will measure simultaneously data for the improvement of clouds and measure TOA broad-band radiances and aerosols parameterisation in climate models fluxes in order to relate them to the observed and would furthermore contribute to cloud and aerosol properties. improvements of numerical weather prediction [4]. With a target launch date in 2012, the These mission objectives will be address mission would also provide data similar to A- through the measurements of Train missions CloudSAT and CALIPSO, thus • Cloud properties: Cloud top and base supplementing and continuing their respective height (including multi-layer clouds), data sets of lidar and radar observations. fractional cloud cover and overlap (height resolved), occurrence of ice and liquid and The mission is being implemented in super-cooled layers, vertical profiles of ice cooperation with the Japanese Aerospace and liquid water content and effective ice, Exploration Agency, JAXA, which will provide respectively droplet, sizes and ice particle one of the mission core instruments, the shapes, small scale (around 1km) Doppler radar. fluctuations of these properties; • Aerosol properties: extinction profile, boundary layer height, presence of 2. Mission Objectives and Observational anthropogenic and natural aerosols; Requirements • Vertical velocities inside clouds to characterise convection and ice The primary mission objective is to improve sedimentation; the understanding of cloud-aerosol-radiation • Drizzle rain rates and estimates of heavier interactions. This shall be achieved through rainfall rates; • Global observations of vertical • Narrow-band and broad-band reflected distributions of ice and liquid water in solar and emitted thermal TOA radiances clouds, cloud overlaps, cloud-precipitation to retrieve fluxes. interactions and the characterisation of vertical motions within clouds; • Global observations of vertical profiles of 3. Instruments Overview natural and anthropogenic aerosols, their radiative properties and interactions with The EarthCARE payload consists of four clouds; instruments, a high spectral resolution lidar, a Doppler radar, an imager and a broad-band radiometer. The Doppler radar will be provided * Corresponding author address: Tobias Wehr, European Space by JAXA and NiCT, the other instruments and Agency, ESA/ESTEC, Keplerlaan 1, P.O.Box 299, 2200 AG, The Netherlands, +31 (0)71 565 5753, E-mail: [email protected] the satellite will be developed under ESA laser or wavelength tunable Fabry-Pérot contracts. étalon. A cross-polar channel will be implemented in order to support the cloud One of the particular advantages of phase discrimination by using the EarthCARE is the deployment of the four depolarisation ratio. instruments on the same platform. Strict collocation requirements of their fields-of-view The instrument performance is presented in have been established in order to ensure the Figure 1. The signal to noise ratio (SNR) is observation of the same atmospheric scene displayed as a function of altitude for both the Mie and Rayleigh signals. Two geophysical and enable synergistic retrievals of cloud and simulations have been simulated: (1) daytime aerosol properties. The four instruments conditions over a dense cloud deck; (2) footprints (centres of the footprints of lidar, nighttime conditions. The cirrus backscatter radar, radiometer and the imager’s nadir pixel) and extinction coefficients are β=8x10-7 m-1sr-1 will be collocated to an RMS accuracy of 350m and α=4x10-5 m-1, respectively. The full vertical (1000m for radiometer). resolution of 100m is considered for the Mie channel, while data are accumulated in the vertical direction over 300 m for the Rayleigh 3.1 Active Instruments: ATLID channel. A horizontal integration length of The Atmospheric Lidar, ATLID, is a single 10 km is assumed for both channels, in order wavelength lidar operating in the UV with a to achieve the required performance. high spectral resolution receiver that will However, a pulse repetition frequency of 70 to separate the back-scattered particle (Mie) 100Hz would lead to an approximate spacing signal and molecular (Rayleigh) signal. A small of single measurements (i.e. single lidar shots) laser beam footprint of around 6 m combined on ground of 100m or less, of which every with a small telescope footprint (around 30m) measurement will be individually available for is favoured to minimise multiple scattering scientific analysis. effects and reduce solar background. 30 The laser transmitter operates at the 3rd harmonic of an Nd-YAG laser around 355 nm. 25 2 1 This wavelength allows relatively high pulse 20 energy to be used without exceeding eye Rayleigh safety constraints. A conventional design 15 Altitude (km) made of a low power oscillator and a power Thin cirrus: β = 8.10-7 m-1.sr-1 10 amplifier is proposed. The laser will benefit Optical depth = 0.04 from the development activities of the ALADIN 2 5 Thick cloud deck 1 Doppler wind lidar of the ADM-Aeolus Earth Aerosols: optical depth = 0.35 0 Explorer, and in particular for the reference 024681012 laser head the same design will be used. Signal to Noise Ratio Figure 1: ATLID SNR as a function of altitude. The Mie-Rayleigh separation is performed by high resolution Fabry-Pérot étalons, a second 3.2 Active Instruments: CPR unit of which is also used for the suppression of background radiation. This ‘High-Spectral The Cloud Profiling Radar, CPR, is a highly Resolution’ technique uses the reduction in the sensitive 94GHz cloud radar with Doppler molecular return to directly determine the capability. extinction of aerosols and thin clouds, which is then used to correct the backscatter coefficient The CPR measurements will provide profiles in the Mie channel for attenuation, and find the extinction to backscatter ratio of the lidar of ice and liquid water content throughout the signal. Low Light Level CCD’s (L3CCD) and cloud, in particular in regions where the lidar or Photo-Multiplier Tubes (PMT) are considered passive optical sensors could not penetrate. as candidate detectors. The CPR will be designed to achieve a radar reflectivity sensitivity of -35dBZ (Z defined at Wavelength tunability is needed to ensure in- 94GHz and 10°C) at TOA, for a 10 km along- flight relative calibration of the aerosol channel track horizontal integration and -30dBZ over and of the molecular channel and possibly, for 1km horizontal integration, with a vertical Earth Doppler compensation. This can be resolution of 400m (sampling 100m). This achieved either with a wavelength tunable would enable the radar to detect over 98% of radiatively significant ice clouds and 40% of all accuracy in pointing alignment is one of the stratocumulus, for the 10km horizontal critical design items, because EarthCARE integration case [2]. relies on synergistic measurements among on- board sensors. The co-alignment between CPR will have the capability to measure the CPR and ATLID footprints is given the highest Doppler signal from the vertical motion of priority. cloud particles to an accuracy of 1m/s (for a horizontal signal integration over 10km, or possibly even shorter), in order to measure 3.3 Passive Instruments: MSI convection and ice sedimentation rates. A principle of pulse-pair Doppler radar The Multi-Spectral Imager, MSI, is designed to measurements is based on the detection of provide images in the visible and infra-red spectral regions in support of the active the phase difference between echo signals instruments. It will provide scientific products from two consecutive radar pulses provided for clouds and aerosols as well as the that the correlation between them is sufficiently information of the cloud and aerosol layers. high. The MSI will also be used for the calibration of the BBR (broad-band radiometer, see below) The radar will operate at a frequency of and for supporting the conversion of BBR 94.05±0.0035GHz, and cover the atmosphere measured broad-band radiances into fluxes. in the vertical domain from -0.5km up to 20km. The instrument will look at nadir with a spatial Since the Doppler performance will very resolution of 500m and a swath width of strongly depend on the correlation of 150km. The swath will be tilted in across-track consecutive pulses (i.e., the density of vertical