MLS Wave Filtering in Limb-Scan Observations

IIn the limb-scan observations, only MLS radiances at the bottom of each scan are saturated and the weighting functions at these tangent height change little in the vertical direction but are slightly displaced from each other horizontally. Depending on its orientation with respect to  wave fronts, MLS variance may have maximum response to wave perturbation in case A and minimum response in case B due to the weighting function averaging.

When projected onto the horizontal plane, the weighting function layout depends on whether it is an upward or downward scan and the displacement will shift to different sides from the satellite velocity. The averaging on wave amplitudes also depends on horizontal wavelength and viewing angle (that is defined as the angle between wave propagation and line-of-sight directions).

Therefore, the instrument filtering on 3D wave propagation is somewhat complicated in the limb-scan situation. Simulations with a realistic instrument weighting function are carried out to characterize such filtering effect. In the upward scan case, the instrument wave filter shows an asymmetric characteristic with respect to the viewing angle. The asymmetry is more prominant at long vertical wavelengths with the peak response at ~110o. At vertical wavelengths less than 10km, the peak responses are found around 150-180o. In the case of 6-point truncation at 110o viewing angle, MLS wave filter shows that most waves observed are of vertical wavelengths greater than ~10km and horizontal wavelengths around ~100km. The smaller vertical scales are filtered out by MLS antanna pattern in the vertical dimension, which has 9.6km half-beam power width (HBPW). The smaller horizontal scales are filtered out by the combined effects of the 2-second integration time (~15km) and the antanna pattern in the horizontal dimension (~20km HBPW). The longer horizontal scales are filtered out as a result of the 6-point truncation.
 
 

Vertical cross-section
Case A
graphic of MLS vertical cross section of limb scan - case a
Case B
graphic of MLS vertical cross section of limb scan - case b
Horizontal cross-section
Case Downward-Scan
graphic of MLS horizontal cross section of limb scan - case downward scan
Case Upward-Scan
graphic of MLS horizontal cross section of limb scan - case upward scan
 

Instrument wave filters

as a function of vertical wavelength and viewing angle

plot of vertical instrument wave filters

as a function of vertical and horizontal wavelengths
plot of vertical and horizontal wave lengths