Comparison of upper tropospheric water vapor observations from the Microwave Limb Sounder and Atmospheric Infrared Sounder

Abstract

[1] We compare matched retrievals of upper tropospheric water vapor (UTWV) mixing ratios from the Microwave Limb Sounder (MLS) instrument on the Aura satellite, and the Atmospheric Infrared Sounder (AIRS) instrument on the Aqua satellite. Because each instrument's sampling is affected by tropical conditions, about half of mutually observed scenes in the tropics yield simultaneous successful retrievals from both systems. The fraction of mutually retrieved scenes drops to 30% at higher latitudes where clouds significantly inhibit AIRS sounding. Essentially all scenes observed by MLS in extratropical and polar regions yield successful retrievals. At 250 hPa in the tropics, measurements from the two instruments are highly correlated, the differences of their means ( D q ) are smaller than 10%, and the standard deviations of their differences (s q ) are 30% or less. At 300 hPa, MLS means are drier by 10-15%, and s q is 40-60%, indicating that responses of MLS and AIRS to UTWV perturbations are not one-to-one. Root mean square agreement is also poorer over the poles at 300 hPa and at 200 and 150 hPa at lower latitudes. In these regions, j D q j = 10% or more, and s q = 40-70%. Correlations between the two data sets are 0.7-0.9 at 300 and 250 hPa globally and at 200 hPa in the tropics. This high correlation indicates that s q of 50% or greater comes mainly from systematic differences in sensitivity of the two instruments, especially for small and large UTWV amounts; larger values of s q are generally not due to large random errors from either instrument. An AIRS low-end sensitivity threshold of 15-20 ppmv leads to poorer agreement under the driest conditions. Disagreement at 300 hPa likely comes from overestimation by MLS for the wettest conditions of >400 ppmv. While MLS is biased slightly dry overall at 300 hPa, it is biased wet in the wettest regions, particularly those associated with deep convection. These sensitivity differences explain nonunity slopes of linear fits to the two data sets. MLS everywhere has a greater dynamic range than AIRS, with larger maxima and smaller minima. Good agreement at 250 hPa suggests AIRS uncertainties of 25% up to the reported 250-200 hPa layer in the tropics and extratropics, consistent with previous comparisons with balloon-and aircraft-borne instruments. The agreement at 250 hPa also indicates that MLS is reliable from its reported 215-hPa level upward in altitude.

Validation of MIPAS IMK/IAA temperature, water vapor, and ozone profiles with MOHAVE-2009 campaign measurements

“…The retrieval of the AIRS water vapor profile uses a large set of channels associated with the strong 6-µm water band, while temperature information is derived from the 15 and 4.3-µm CO 2 bands, and ozone is retrieved from the 9.6-µm ozone band (Susskind et al, 2003. Water vapor amount is retrieved at twelve standard pressure levels between the surface and 100 hPa, though sensitivity is low for mixing ratios of about 10 ppmv or less (Gettelman et al, 2004;Fetzer et al, 2008). AIRS water vapor retrievals have been validated versus aircraft and balloon in situ measurements (Hagan et al, 2004;Gettelman et al, 2004;Tobin et al, 2006) and versus MLS .…”

“…The estimated combined precisions again are higher than the bias-corrected rms differences and confirm the finding of an overly pessimistic precision estimate in the stratosphere. AIRS retrieves water vapor between the surface and 100 hPa, though sensitivity is low for mixing ratios of about 10 ppmv or less (Gettelman et al, 2004;Fetzer et al, 2008). Since this is the lower part of the MIPAS observations, comparison is somewhat difficult, despite the high number of coincidences found.…”

Comparisons of the tropospheric specific humidity from GPS radio occultations with ERA-Interim, NASA MERRA, and AIRS data

“…In general, the MERRA SH retrievals are accurate to ∼ 20 % (Rienecker et al, 2011). AIRS estimated SH product accuracies are typically ∼ 25 % at p > 200 hPa (Fetzer et al, 2008), and ERA-Interim SH products have an estimated accuracy of ∼ 7-20 % in the tropical lower-to-middle troposphere (Dee et al, 2011). The RO retrievals seem to have better accuracy than the AIRS retrievals, which could be attributed to the fact that the RO observations are based on precise time measurements and have very low sensitivity to clouds (unlike the IR observations).…”

“…In particular, infrared (IR) space-based platforms have a relatively coarse vertical resolution (e.g., 2.0-3.0 km), are prone to cloud contamination (Fetzer et al, 2006), and tend to be biased low over wet and dry humidity extremes (Fetzer et al, 2008;Chou et al, 2009). The use of IR observations in the lower troposphere still remains a challenge due to the decreasing information content and the difficulty of detecting low-cloud contamination (Schreier et al, 2014).…”

“…Heavy cloudiness, especially in the middle-to-upper troposphere can also introduce biases in the upwelling MW radiation from water vapor due to the presence of ice particles that can contaminate the MW retrievals (Fetzer et al, 2008). Global circulation models do not properly represent the middle troposphere moist convection (Sherwood et al, 2010;Holloway and Neelin, 2009;Frenkel et al, 2012), and large discrepancies in the tropospheric humidity among different reanalyses and among reanalyses, models, and satellite observations (Chuang et al, 2010;Jiang et al, 2012;Tian et al, 2013;Wang and Su, 2013) still persist.…”