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Aura MLS THz observations of global cirrus near the tropopause
Author(s) -
Wu D. L.,
Pickett H. M.,
Livesey N. J.
Publication year - 2008
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2008gl034233
Subject(s) - cirrus , radiance , microwave limb sounder , remote sensing , ice cloud , terahertz radiation , ice crystals , environmental science , tropopause , satellite , radiative transfer , optical depth , physics , geology , atmospheric sciences , troposphere , optics , meteorology , astronomy , aerosol
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 ( T cir ), 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 T cir by averaging the data spectrally and vertically, removing systematic error in the radiances, and screening cloudy radiances from clear‐sky variability. The derived THz T cir 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 T cir 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 T cir can be converted to a partial ice water path (pIWP) above ∼15 km, and the estimated 0.7 g/m 2 /K sensitivity for THz T cir is consistent with the radiative transfer model calculation and MLS 240 GHz measurements.