Aura MLS THz observations of global cirrus near the tropopause

Abstract

The first global cirrus observation from spaceborne THz (terahertz) sensors is presented. The 2.5 THz channels of the Microwave Limb Sounder (MLS) instrument on NASA's Aura satellite, measuring atmospheric thermal emission at the limb, has sensitivity to cloud ice particle scattering above ∼15 km. The magnitude of cloud-induced radiance (Tcir), as high as 40 K, is observed in the THz radiances at the lowermost tangent heights of limb viewing. A special algorithm is developed to extract the THz Tcir by averaging the data spectrally and vertically, removing systematic error in the radiances, and screening cloudy radiances from clear-sky variability. The derived THz Tcir has typical precision of 1.5-1.7 K, and a threshold of -6 K is used to determine significant cloud measurements. Monthly maps show that morphologies of the THz Tcir are generally consistent with those of cloud IWC (ice water content) at 121 hPa retrieved simultaneously from the MLS 240 GHz channel. However, the spatial distribution of THz clouds often spreads more broadly in latitude than the GHz clouds, as expected for the better sensitivity of the THz channels to small ice particle scattering. The THz Tcir can be converted to a partial ice water path (pIWP) above ∼15 km, and the estimated 0.7 g/m2/K sensitivity for THz Tcir is consistent with the radiative transfer model calculation and MLS 240 GHz measurments. Copyright 2008 by the American Geophysical Union.