SABER Temperature and CO₂ Retrieval Accuracies

SABER temperature and pressure profiles are retrieved operationally using two channels observing emission in the vicinity of 15 µm. The measured 15 µm limbpath emission profiles are matched to model calculated values by varying temperature and pressure (hydrostatically constrained) and using WACCM generated CO₂ vertical mixing ratio profiles. Table 1 summarizes the single profile uncertainties of the operational temperture product. These errors are based on studies reported in papers by Remsberg et al. (2008) and Garcia-Comas et al. (2008). The uncertainties discussed in these papers were based on the v1.07 data version but the operational Tk uncertainties used in the tables here include consideration of changes introduced by the v2.0 algorithm. There are no changes in v2.0 relative to v1.07 that would impact precision but overall accuracy can be impacted due to changes in inputs to the NLTE model including:

• v2.0 uses retrieved [O] values for all events whereas v1.07 used retrieved [O] for SZA < 85 degree.
• v2.0 uses CO₂ from an updated WACCM model (version 3.5.48) and is purely WACCM whereas v1.07 uses scaled WACCM results to match a CO₂ trend model.
• Some reaction rates were changed in the CO₂ vibrational temperature model.

Changes to coefficients used for instrument electronic gain switches, the Local Thermodynamic Equilibrium (LTE) algorithm and the procedure used to merge LTE and Non-LTE results, lead to small v2.0 versus v1.07 (<2 K) differences below 80 km. These changes result in a more reliable and more self-consistent product for v2.0. Also, errors in the tables are representative of all environments except for polar summer mesopause events where systematic errors above 80 km are expected to be higher due to higher uncertainty in the vibrational temperatures calculated for these very cold conditions.

Table 1: SABER operational single profile Tk total uncertainty

Operational T Retrieval*	15 km	20 km	30 km	40 km	50 km	60 km	70 km	80 km	90 km	100 km	110 km
Temp. prec. (K)	0.3	0.3	0.3	0.6	0.6	0.7	1.0	1.8	3.6	6.7	15.0
Temp. systematic (K)	1.4	1.2	0.7	1.5	1.9	1.9	1.5	1.4	4.0	5.0	25.0
Temp. accuracy (K)	1.4	1.3	0.8	1.6	2.0	2.0	1.8	2.3	5.4	8.4	29.2

*The WACCM CO₂ results are used at all altitudes for the operational Tk retrieval.

SABER temperature profiles are also retrieved along with CO₂ vertical profiles for daytime data using an independent algorithm developed by HU. This retrieves temperature over the 70 to 110 km range and CO₂ over the 70 to 125 km range using measured 15µm and 4.3 µm CO₂ limb vertical profile emissions. This latter algorithm is applied to daytime measurements to obtain self-consistent T/CO₂ retrievals in the MLT region. The 2-channel T/CO₂ data are available in the Level2C folder in NetCDF format from this website. The methodology used and first results are presented in a paper by Rezac et al. (2015a). Results of a comprehensive CO₂ validation study using the SD-WACCM and ACE-FTS data are presented in a validation paper by Rezac et al. (2015b). Here we summarize important information on using the temperature and CO₂ 2-channel product.

1) The daytime (SZA < 80) measurements have been processed on a continuous basis since 2002 up to near current time. There is typically a month or more lag between operational data release and the 2-channel product.

2) The operational and 2-channel kinetic temperature (Tk) differences are usually within the estimated error bars. Also, note that the operationally retrieved Tk profile has a higher vertical resolution due to application of a retrieval interleaving process, which is not applied for the 2-channel inversion. The 2-channel temperature retrieval is always performed with the currently retrieved CO₂ VMR profile obtained during the global iteration and as a result, the temperature in the polar summer mesopause region is typically slightly colder than the operational temperature.

3) The 2-channel CO₂ retrieval is constrained below 80 km to avoid large CO₂ and hence large Tk variability (See Table 3 in Rezac et al., 2015a).

4) The retrieval for polar summer latitudes (>60^o) below 90 km provides only a mean for the 65-90 km region. Above 90 km the inversion proceeds normally without any constraints. See Rezac et al., (2015a) for a detailed description of the feature and reasoning for the mean retrieval. This constraint does NOT apply to any other season or latitude region.

Table 2 gives the the 2-channel single profile Tk and CO₂ total uncertainties. These errors are based on studies reported in papers by Rezac et al. (2015a,b).

Table 2: SABER 2-channel single profile Tk and CO₂ total uncertainty

2-Channel Tk Retrieval	70 km	80 km	90 km	100 km	110 km	125 km*
Temp. prec. (K)	1	1	3	4	6
Temp. systematic (K)	4	4	5	13	30
RSS Syst. & Prec. Tk (K)	4.1	4.1	5.8	13.6	30.5
2-Channel CO2 Retrieval	70 km	80 km	90 km	100 km	110 km	125 km*
CO2 VMR prec. (%)	1	1	1	1	2	2
CO2 VMR systematic (%)	15	15	12	21	32	22
RSS Syst. & Prec. CO2VMR (%)	15	15	12	21	32	22

* CO₂ VMR above 110 km is based on WACCM T, O, O(¹D). The error at 125 km will become smaller as the non-LTE influence of O(¹D) on the CO₂ populations becomes weaker.

Figure 1 shows typical altitude-latitude cross-section of Tk differences (2-channel - operational) for the month of May as an example.

References:

Garcıa-Comas, M., M. Lopez-Puertas, B.T. Marshall, P. P. Wintersteiner, B. Funke,1 D. Bermejo, Pantaleón, C. J. Mertens, E. E. Remsberg, L. L. Gordley, M.G. Mlynczak, and J.M. Russell III, Errors in SABER kinetic temperature caused by non-LTE model parameters, VOL. 113, D24, doi: 10.1029/2008JD010105, 2008.
Link to Article

Remsberg, E. E., B. T. Marshall, M. Garcia-Comas, D. Krueger, G. S. Lingenfelser, J. Martin-Torres, M. G. Mlynczak, J. M. Russell III, A. K. Smith, Y. Zhao, C. Brown, L. L. Gordley, M. J. Lopez-Gonzalez, M. Lopez-Puertas, C.-Y. She, M. J. Taylor, and R. E. Thompson , Assessment of the quality of the Version 1.07 temperatureversus-pressure profiles of the middle atmosphere from TIMED/SABER, JGR, VOL. 113, D17101, doi:10.1029/2008JD010013, 2008.
Link to Article

Rezac, L., A. Kutepov, J. M. Russell III, A. G. Feofilov, J. Yue, and R. A. Goldberg, Simultaneous retrieval of T(p) and CO₂ VMR from two-channel non-LTE limb radiances and application to daytime SABER/TIMED measurements, J. Atm. And Solar-Terrestrial Physics, 130-131(2015), 23–42, May, 2015(a).
Link to Article

Rezac, L., Y. Jian, J. Yue, J. M. Russell III, A. Kutepov, R. Garcia, K. Walker, P. Bernath, Validation of the global distribution of CO₂ volume mixing ratio in the mesosphere and lower thermosphere from SABER, JGR: Atmospheres, 120, 12,067–12,081. doi: 10.1002/2015JD023955, 2015(b).