Aura MLS is a passive instrument with seven radiometers at frequencies near 118 (R1), 190 (R2), 240 (R3), 640 GHz (R4), and 2.5 THz (R5) [Waters et al., 2006]. The 118 GHz and 2.5 THz radiometers have dual polarizations. Except for the 118 GHz, all radiometers are double-sideband receivers, which means that the measured radiance is a sum of radiation from two different frequencies. The Aura MLS antenna views forwards in the satellite flying direction and the cross-track FOV (field-of-view) width is frequency-dependent, varying from 12 km at 118 GHz to 2.9 km at 640 GHz. Likewise, the vertical FOV also depends on frequency, varying from 5.8 km at 118 GHz to 1.4 km at 640 GHz [Cofield et al., 2006].
The Aura satellite has a 705-km sun-synchronous (~1:40 p.m. ascending crossing time) orbit with 98? inclination. MLS limb scans are synchronized to the orbital period such that nominal operation will have 240 limb scans (called the major frames or MAFs) per orbit, producing daily latitude coverage from 82S to 82N. Unlike step-wise scans in UARS MLS, Aura MLS scans are continuously across tangent height (ht) from the surface to ~92 km in 24.7s. The integration time for each measurement is 1/6 second, called a minor frame (MIF). For GHz measurements, each scan has 40-50 MIFs dedicated to tropospheric measurements with a 300 m sampling resolution in tangent height. These low-ht measurements are particularly useful for cloud observations. More on the MLS operation, onboard radiometric calibration, and radiance noise can be found in Jarnot et al. [2006].
The MLS cloud flags and cloud ice retrievals are made from radiance measurements at the window channels of each radiometer [Figure 1]. MLS radiances can be affected by clouds when the instrument FOV is near or below cloud top. The fundamental quantity for MLS cloud measurements is the so-called cloud-induced radiance (Tcir), which is defined as the difference between the measured and expected clear-sky radiances. In v2.2 algorithm, the expected clear-sky radiance is calculated for each profile by the clear-sky radiative-transfer (RT) model using the retrieved atmospheric state. The clear-sky RT model is described by Read et al. [2006].
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| Figure 1. Radiance spectra targeted by MLS and the window channels (indicated by arrows) used for cloud flags and cloud ice retrievals. Except for the 118 GHz radiometer, all the MLS receivers are double-sideband radiometers with the local oscillator (LO) frequency indicated by the dashed line, and MLS radiances represent a sum of the lower and upper sideband radiances. The horizontal long (short) grey bars indicate MLS filterbank (wideband-filter) locations. The radiance spectra are calculated at 25 km (dark line) and 15 km (grey line) tangent heights. |