Measurements
The TOVS instruments aboard the NOAA Polar Orbiting Environmental Satellites have measured radiation emitted and scattered from
different levels of the atmosphere since 1979.
The TOVS system consists, in particular, of two sounders: the High resolution Infrared Radiation Sounder (HIRS) with 19 IR
spectral channels and the Microwave Sounding Unit (MSU) with 4 microwave channels.
The IR channels are clustered near absorption bands of CO2, H2, and O3.
By assuming that CO2 is mixed uniformly in the atmosphere, absorption and emission due to CO2 can be
used to sense the temperature and the uppermost cloud level in the atmosphere: measured radiances from near the center
of a CO2 absorption band are sensitive only to the upper atmosphere while radiances from the wings of the band
see successively lower levels of the atmosphere. Microwave radiation passes through aerosols and most clouds, since
these wavelengths are much greater than aerosol and cloud particles.
Local Observation Time and Length of Data Record
7:30 AM and 7:30 PM ; 1987–1994, microphysics available for 1987–1990 (NOAA10);
1:30 AM and 1:30 PM ; 1989–1994
Spatial Resolution
17 km (detection), 100 km (retrieval)
Cloud Detection
multi-spectral, MSU clear sky estimation, snow/ice surface from MW
Retrieval Methodology
CP and CEM are determined from 5 CO2 absorbing channels by a weighted χ2 method based on spectral emissivity
coherence.
Therefore spectral emissivities are calculated from measured radiance, clear sky radiance and radiance of an opaque
cloud for 30 pressure levels.
The method takes into account the vertical weigthing of the different channels and the growing uncertainty in the
computation of CEM with increasing pressure.
CT is derived from CP using the retrieved atmospheric temperature profiles.
COD = -2 ln ( 1 - CEM ). Ice clouds are defined by CT < 230 K.
The retrieval of CREIH of semi-transparent cirrus ( 0.3 < CEM < 0.85 ) is based on spectral
cirrus emissivity difference between 8 and 11 μm, applying a Look-Up Table approach.
CIWPH is then deduced from CEMIH and CREIH.
Ancillary Input
References