Observing Ice Clouds with Microwave-IR Polarimetric Radiometry: SWIRP Development

Clouds remain as a major source of uncertainty in climate models. Ice clouds, in particular, are poorly constrained and have been used as a tuning parameter in models to balance the radiation budget at the top of atmosphere and precipitation at the surface. Lack of accurate measurements of cloud ice and its microphysical properties has led to large uncertainty about global clouds and their processes within the atmosphere. NASA’s Aerosol, Cloud and Ecosystems (ACE), an Earth Science Decadal Survey (DS) mission, recommended an advanced science payload with submm-wave and longwave infrared (LWIR) radiometers for such cloud ice measurements. In a recent community white paper, Cloud and Precipitation Process Measurements (CaPPM), dynamics and microphysical properties are identified as the key links between the cloud-precipitation processes and need more accurate measurements.

The Submm-Wave and LWIR Polarimeter (SWIRP) project, funded by NASA ESTO, is developing a compact (20x20x40 cm) conical-scan instrument to measure the polarimetric radiation from ice cloud scattering at mm-submm (220 and 680 GHz) and IR (8.6, 11, and 12 mm) bands. Accurate radiometric and polarimetric measurements at these bands will provide the needed sensitivity to cloud ice and its microphysical properties (particle size and shape) over a wide dynamic range of cloud ice. The conical-scan configuration with SWIRP allows horizontal (H) and vertical (V) polarization information in each scan from space such that cloud ice and microphysical properties can be simultaneously retrieved. In the SWIRP project, miniaturization of a conical-scan observing system is enabled by new technologies in low-power, direct-detection receivers at mm/submm wavelengths, a multi-channel longwave IR (LWIR) spectro-polarimeter, and a compact Bearing and Power Transfer Assembly (BAPTA). The project goal is to advance the instrument system TRL from 3 to 5 in three years.