Search

We report on the progress of a far-infrared/submillimeter radiometer being developed in Cardiff for the measurement of cirrus clouds. Remote sensing of cirrus clouds is known to be of great importance to the long-term accuracy of current General Circulation Models (GCM) and climate prediction but with greater measurement coverage needed. The instrument reported here is an aircraft deployed, 5 channel fixed band radiometer capable of retrieving cirrus Ice Water Path (IWP) and mean particle diameter (Dme) using a spectral range of between 10 cm-1 and 55 cm-1. The radiometer will capitalise on ongoing measurements from the Fourier transform interferometer based, Far-infrared Sensor for Cirrus (FIRSC), an instrument for which Cardiff has been closely associated. Initial results of channel selection simulations are presented here with comparisons between different combinations of channel frequency and bandwidths, along with the number of channels used and cloud particle shape. Also demonstrated is the effect of instrument noise on retrieval performance which is shown to be the dominant source of retrieval error.

We introduce a low cost, lightweight and compact polarisation sensitive radiometer for the measurement of Cirrus clouds in the submilimeter and far-infrared region (10-50 cm-1). It is widely recognised that enhanced global measurements of cirrus properties are essential to the development of General Circulation and Climate Prediction Models (GCMs) since cirrus clouds have a strong effect on the Earths Global Radiation Budget. We introduce a project currently under development in Cardiff, to design and build a novel instrument suitable for aircraft deployment in order to measure Ice Water Path (IWP) along with cirrus particle size and shape. The radiometer will capitalise on the on going measurements of the NASA led, Fourier Transform interferometer based, Far-Infrared Sensor for Cirrus (FIRSC) instrument for which Cardiff has been closely associated. Data from FIRSC campaigns is being used to select optimum radiometer channels that exhibit good sensitivity to specific cirrus. This new multi-channel radiometer will however have some key advantages over similar spectroscopic instruments for example: portability, increased optical efficiency, a multi-angle field of view and a reduced integration period leading to an improved spatial resolution. The radiometer will benefit from the application of state-of-the-art submm/FIR polariser and solid filter technology currently being developed in Cardiff.

Remote sensing measurements of cirrus clouds are crucial for improving global climate models. Spectral measurements of the far-infrared region provide especially useful information to retrieve cirrus ice water path and particle size properties. Earth radiance spectra of this region have been obtained for a range of cloud conditions using the airborne instrument FIRSC (Far-InfraRed Sensor for Cirrus). The instrument flew on board the high altitude Proteus aircraft in the ARM-FIRE Water Vapor Experiment (AFWEX) during the Nov - Dec 2000 Intensive Operations Period. FIRSC is a Martin-Puplett type Fourier transform spectrometer with two channels covering the ranges 10 - 33 cm -1 and 80 -140 cm -1 at a resolution of 0.1 cm -1 . It has achieved a noise equivalent temperature of approximately 1K at 30 cm -1 using a scan duration of 4 seconds. For the first time this far-infrared data has been compared to data from the thermal IR NAST-I (NPOESS Airborne Sounder Testbed - Interferometer), which was part of the same payload during the AFWEX flights. Retrievals of cirrus ice water path and particle size from the FIRSC data are presented.

Improved techniques for remote sensing of cirrus are needed to obtain global data for assessing the effect of cirrus in climate change models. Model calculations show that the far infrared/sub-millimeter spectral region is well suited for retrieving cirrus Ice Water Path and particle size parameters. Especially useful cirrus information is obtained at frequencies below 60 cm-1 where single particle scattering dominates over thermal emission for ice particles larger than about 50 micrometer. Earth radiance spectra have been obtained for a range of cloud conditions using an aircraft-based Fourier transform spectrometer. The Far InfraRed Sensor for Cirrus (FIRSC) is a Martin-Puplett interferometer which incorporates a polarizer for the beamsplitter and can be operated in either intensity or linear polarization measurement mode. Two detector channels span 10 to 140 cm-1 with a spectral resolution of 0.1 cm-1; achieving a Noise Equivalent Temperature of approximately 1K at 30 cm-1 in a 4 sec scan. Examples are shown of measured and modeled Earth radiance for a range of cloud conditions from 1998 and 1999 flights.

The lack of a global cirrus cloud database is a major deficiency in validating Global Circulation Models which are the basis for estimating long term climate change. Cirrus clouds being composed of ice particles act to reflect the incoming solar radiation and to block infrared energy radiation loss from the earth. The net effect can be either warming or cooling. The essential data are global distributions of the Ice Water Path (IWP) and effective particle size. We have modeled the outgoing earth radiance spectrum between 8 and 1000 micrometer as a function of IWP and effective particle size. The results are used to estimate cirrus retrieval accuracies for cirrus from far infrared/submm measurements by Fourier transform spectroscopy. We describe the aircraft-based Far Infrared Sensor for Cirrus (FIRSC) instrument which is currently under construction. We also discuss the potential contribution of far infrared/submm measurements for the validation of cirrus products anticipated in the MTPE MODIS program.