Observing ice water path and crystal size from space

Clouds are the atmospheric link between the hydrological and energy cycles. Improving weather and climate forecasts requires accurate satellite measurements of various cloud properties at appropriate temporal and spatial scales. These properties include cloud horizontal and vertical structure, cloud water content and some measure of particle sizes and shapes.

While there are many satellite observations that have sensed the tops of clouds, we do not yet quantitatively understand how the properties vary the in the vertical nor the processes that control the water budget and precipitation processes in the upper and middle troposphere. This knowledge is essential for the successful modeling of the Earth¹s climate. This uncertainty is reflected in 1) large discrepancies in model simulations of the upper tropospheric water budget, and 2) a sensitivity of the model simulations to the parameterization of ice clouds and ice processes.

One barrier to achieving accurate global ice cloud properties is the lack of adequate observations at submillimeter wavelengths. Recent advances in instrumentation show a technological breakthrough with submillimeter-wave receiver design, as implemented in an airborne submillimeter-wave radiometer.

This presentation will describe a proposed satellite mission designed to acquire global Earth radiance measurements in the infrared and submillimeter-wave region (183 -874 GHz). This mission bridges the measurement gap between microwave sounders and shorter-wavelength infrared and visible sensors. The brightness temperatures at submillimeter-wave frequencies are especially sensitive to cirrus ice particle sizes (because they are comparable to the wavelength). This allows for more accurate ice water path estimates when multiple channels are used to probe into the cloud layers. Further, submillimeter wavelengths offer simplicity in the retrieval algorithms because they do not probe into the liquid and near surface portions of clouds, thus requiring only one term of the radiative transfer equation (ice scattering) to relate brightness temperatures to ice. Scientific justification and an approach to measuring ice water path and particle size spanning a range encompassing both the hydrologically active and radiatively active components of cloud systems will be presented.