The EOS MLS Experiment

On board the NASA Aura satellite, the EOS MLS instrument has been measuring cloud ice since August 2004, IWC at pressures < 215 hPa and ice water paths (IWP) at > 5 km, along MLS line-of-light (LOS). MLS IWC in the upper troposphere (UT) are the average quantity over the instrument field-of-view (FOV), which is a volume of 200km long, 15km wide and 3km high at 240 GHz. Because of its tangental view and long path length, the Aura MLS can measure clouds of IWC from as low as ~1 mg/m3 to ~80 mg/m3 where it becomes saturated. Details on the measurement technique and radiative transfer model for cloudy-sky atmospheres can found in Wu et al. [2006]. The limb view also gives MLS the best angle to measure polarized scattering differences due to inhomogeneous cloud ice particles, which was found to be ~10% of cloud-induced radiances at 118 GHz Davis et al. [2005].

The EOS MLS is a passive instrument with seven radiometers at frequencies near 118, 190, 240, 640 GHz, and 2.5 THz [Waters et al., 1999] with dual polarizations at 118 GHz and 2.5 THz. Except for the 118 GHz radiometer, MLS radiometers are double-sideband receivers with key parameters specified in Table 1. There are a total of 19 25-channel filter bank spectrometers, 5 11-channel mid-band filter bank spectrometers, 12 wide-band filters, and 4 digital autocorrelator spectrometers (DACS). The GHz and THz systems have separate antennas but synchronize their scans to produce 240 profiles per orbit. Unlike step-scanning with the UARS MLS, the EOS MLS scans continuously in tangent height from the surface to the mesopause (~90 km) in 24.7s. Each scan is referred as to a major frame (MAF) and is divided to 148 minor frames (MIFs). Two adjacent scans are separated by ~165 km in distance. Excluding instrument calibration, each MAF devotes ~120 MIFs for atmospheric measurements with MIF integration time of 1/6 second. For the nominal operation the GHz radiometers have ~42 MIFs dedicated to tropospheric measurements (separated by ~300 m in h_t), whereas the THz scan has only ~7 MIFs at h_t < 18 km.

Table 1. Characteristics of the EOS MLS radiometers. Both vertical and horizontal FOVs are estimated at h_t = 1 km.

a) Two frequency ranges in R2, R3, R4, R5H and R5V represent the double-sideband coverage although the radiances from two sidebands are inseparable in the radiometric measurements. R1A and R1B are single sideband radiometers with different polarizations.

b) The MLS radiance error contains frequency-coherent and random components. The latter can be reduced by radiances from several channels within each radiometer but the radiance uncertainty will be limited by the coherent component.  The values in this column reflect the minimum radiance error for each radiometer in single MIF after possible channel averaging is applied.

A similar IWC retrieval technique has been applied earlier to the UARS (Upper Atmosphere Research Satellite) MLS 203 GHz measurements at 100 hPa [ Wu et al., 2005] MLS hIWPs are retrieved at multiple frequencies and multiple tangent heights (h_t). Each hIWP represents a column in the LOS direction, which is tilted nearly horizontally with an elevation angle of ~3º. In most situations the hIWP column does not reach the surface at the MLS frequencies, and the penetration depth associated with each hIWP depends on frequency and h_t.