Daytime SABER/TIMED observations of water vapor in the mesosphere: retrieval approach and first results

Abstract

Abstract. This paper describes a methodology for water vapor retrieval in the mesosphere-lower thermosphere (MLT) using 6.6 µm daytime broadband emissions measured by SABER, the limb scanning infrared radiometer on board the TIMED satellite. Particular attention is given to accounting for the non-local thermodynamic equilibrium (non-LTE) nature of the H 2 O 6.6 µm emission in the MLT. The non-LTE H 2 O(ν 2 ) vibrational level populations responsible for this emission depend on energy exchange processes within the H 2 O vibrational system as well as on interactions with vibrationally excited states of the O 2 , N 2 , and CO 2 molecules. The rate coefficients of these processes are known with large uncertainties that undermines the reliability of the H 2 O retrieval procedure. We developed a methodology of finding the optimal set of rate coefficients using the nearly coincidental solar occultation H 2 O density measurements by the ACE-FTS satellite and relying on the better signal-to-noise ratio of SABER daytime 6.6 µm measurements. From this comparison we derived an update to the rate coefficients of the three most important processes that affect the H 2 O(ν 2 ) populations in the MLT: a) the vibrational-vibrational (V-V) exchange between the H 2 O and O 2 molecules; b) the vibrationaltranslational (V-T) process of the O 2 (1) level quenching by collisions with atomic oxygen, and c) the V-T process of the H 2 O(010) level quenching by collisions with N 2 , O 2 , and O. Using the advantages of the daytime retrievals in the MLT, which are more stable and less susceptible to uncertainties Correspondence to: A. G. Feofilov (artem-feofilov@cua-nasa-gsfc.info) of the radiance coming from below, we demonstrate that applying the updated H 2 O non-LTE model to the SABER daytime radiances makes the retrieved H 2 O vertical profiles in 50-85 km region consistent with climatological data and model predictions. The H 2 O retrieval uncertainties in this approach are about 10% at and below 70 km, 20% at 80 km, and 30% at 85 km altitude.

Simultaneous retrieval of T(p) and CO2 VMR from two-channel non-LTE limb radiances and application to daytime SABER/TIMED measurements

“…We tested higher end (3%) of this uncertainty on the Einstein coefficient of FH, SH bands, and found less than 1% radiance deviation in both channels, however, Ch7 increases (2.0%) above 115 km for the tested case. The radiative transfer module used in this work is the same as applied by Feofilov et al (2009), which has been validated against the radiative transfer code of GATS Inc. (Marshall et al, 1994) and the KOPRA codes (Stiller, 2002) with differences less than 1%. In comparison, the effects of these uncertainties are very small relative to the previously described uncertainties in non-LTE quenching rates, atomic O and/or O( 1 D) VMR.…”

Extending MGS-TES temperature retrievals in the martian atmosphere up to 90km: Retrieval approach and results

“…Kutepov et al (2006) used ALI-ARMS to validate temperature retrievals from the 15 lm CO 2 emissions measured by the SABER instrument onboard the terrestrial TIMED satellite. This code was also used for the H 2 O non-LTE model validation (Feofilov et al, 2009) and for the k VT {CO 2 -O} rate coefficient estimate from the atmospheric observations (Feofilov et al, 2012).…”

Tomographic retrieval of water vapour and temperature around polar mesospheric clouds using Odin-SMR

“…Emission limb sounders can, unlike solar occulting instruments, provide global maps of water vapour and temperature across the entire PMC region within a day. And, unlike infrared emission sounders Feofilov et al, 2009), instruments operating in the microwave region do not have to account for non-LTE emissions. Accordingly, the microwave limb sounder (MLS) on board Aura has been used to study the latitudinal variations in cloud formation (Rong et al, 2014).…”