The spectroscopy uses the HITRAN 2016 line and cross-section database. The WV continuum MT-CKD, line mixing <> and non-LTE <>.

The TOA radiances computed by SARTA and kCARTA use ECMWF or ERA atmospheric fields for water vapour, temperature and ozone, and where available the sonde water and temperature. Where the soundings do not go up to the uppermost level, the AFGL standard model is spliced on top. The surface temperature is from the ECMWF/ERA model or when available from the field campaign data, this is swapped out. The surface emissivity and land fractions are used in the same way as the standard RTP processing at ASL. The cloud fields are also included and 2-slab SARTA computations are routinely computed along side the clear calculations but are not used in the determination of biases.

During this work the atmospheric profiles for CO2, CH4, N2O and CO from ESRL and related data sources were added to account for spatial and temporal variability including seasonality and long term trends. But note that these were not uniformaly applied to all analyses.

Interpretation of bias between observations and calculations as a measure of model accuracy must be made in the context of both the data set being used and the spectral region of interest. For example, in the window regions the surface properties dominate the uncertainty, and so for these spectral regions well characterized ocean views are preferred. In more opaque channels the uncertainty is dominated by the error in the quantity of absorbing species. In all cases (except the most opaque channels with signal from high in the atmosphere) scenes clear of cloud are preferred, however there is the likelihood that some unknown residual contamination remains. Some insight can be gained from using modelled cloud fields.

The magnitude of unaccountable residual uncertainties are likely dominated by skin temperature and emissivity and cloud and is estimated to be of the order 0.1 to 0.2 K in the LW window. The SW window channels uncertainty is about the same for night time view but worse for day-time.

Validation of model accuracy therefore is the combination of results from the varied strategy of direct kCARTA comparisons, large sample comparisons using ECMWF/ERA fields and the radiosonde matched views.

Before reporting detailed results for each of the three approaches, a brief mention is made of the source data and methods.

The ECMWF or ERA atmospheric fields are used in the same way as standard RTP processing, and include profiles of ozone, H2O, temperature, cloud variables, skin temperature, and the surface emissivity. In much of this work the AFGL standard atmospheric profiles for minor gases are used with CO2 adjusted to nominal current surface values. The capbailbity to include contemporary fields of CO2, CH4, N2O and CO has been added, and when used will be clarified in this report.

There are several sources of atmospheric measurement T(z), WV(z) that have been used to compare calculated TOA radiances with spacecraft observations and include: GRUAN and ARM based sondes, Field (research) campaigns and the assimmilated weather service (NWP) analyses ERA/ECMWF. Each have their own characteristics and issues when used for the RTA validation.

The NWP global fields use the standard RTP processing at ASL and are subset to clear ocean views and restricted to tropical and subtropical latitudes unless otherwise stated. The AIRS FoVs are thus already colocated to the model fields.

Before proceeding with the sonde calculations the sonde data files are screened for bad soundings. In general, the sondes have useful WV information from the surface up to near the tropopause but become inaccurate in this region depending upon condtions. The sonde temperature measurements are generally valid up to the burst height.

The GRUAN sondes that are processed completely and designated -GDP- quality include estimates of the measurement uncertainty. The criteria for using these sonde data are that the sonde burst above (at a lower pressure than) 60 mb, that the humidity and temperature reading are greater than the reported uncertainty and that the relative humidity is greater than 3%, which is an estimate of the lowest valid reading. In practice, the temperature reading is valid up to the burst level, and the humidity reading is valid up to between 60 mb and 100 mb. This is a slightly higher altitude than has been generally used for calibrated ARM sondes in previous SARTA validation studies.

The temperature and WV profiles are extended to the topmost AIRS level by attaching the values from the standard atmospheric profiles. Comparisons of the sonde T(z) and WV(z) to the ECMWF/ERA fields are a useful diagnostic to help interpret the bias obtained between the satellite observations and SARTA calculations.

The Sonde located data sets are identifed for each sonde launch by its location and time, and any AIRS L1C data granules that include the sonde are found. For each granule all fields of view within 1-deg lat/lon of the sonde are selected. When the sonde is at the extreme edge of the granule and the number of nearby FoVs are inadequate the granule is ignored.

Since tropical locations are often not well sampled by AIRS, the nearest preceeding and subsequent sondes are used and interpolated in time to the overpass of AIRS. Otherwise in general the time delay between overpass and sonde launch is kept to within 4 hours unless otherwise stated. The magnitude of the delay was varied in early studies until an optimum was found as a compromise between insufficient samples and bias drift.

For all nearby FoVs two sets of TOA computations are made with SARTA. First with the NWP model fields and then with the sonde fields. Then for each set, the subset of cloud free footprints is determined. The same clear-subsetting algorithm is used as the standard RTP processing.

Histograms of the LW window BT from the original near FoVs and the near and cloud clear subsets are checked.

Source code and scripts are at: /home/chepplew/projects/sarta/validation/ in a git repo <TBD> except for dependent scripts that are referenced by matlab addpath directives.

< perhaps include example processing pipeline >

< Need Sergio to remind me >

The direct comparison of the TOA BT computed for the 49 test and fitting profiles provides a closure test on the fast coefficients and sarta algorithm and is a measure of the limit of accuracy of SARTA. A sample of such a test is shown in figure KA.n. This result is self explanatory.

Before proceeding with direct bias comparisons, we investigate the impact of using contemporary fields of CO2, CH4, N2O and CO for the model atmosphere compared to the AFGL standard atmospheres used by klayers and adjusted to account for nominal growth since the time of those standard profiles.

The CO2 CH4 and N2O fields are from the global monitoring lab. (GML) of the Earth System Research Lab (ESRL) see (http://gml.noaa.gov/), and the CO column is from the MOPITT project, with allowance for observation dates outside the measurement or analysis time frame.

An example of the difference using the standard RTP approach and the enhanced minor gas modelling is shown in figure NWP.n. The data sample is based on 10 days of clear subset RTP for IASI.2 on MetOp-B. Notice that this uses the IASI version of SARTA, where the short-wave CH4 optical depth fast coefficients are not included. < note to add them! >.

Tenerife is a GRUAN certified site with RS92 sondes periodically available in the years 2008, 2009, 2010, 2016 and 2017, and RS41 from 2018 to 2022 (current date).

Tenerife is situated in the Eastern North Atlantic off the NW coast of Africa which means that it tends to have relatively low incidence of cloud cover, tends to be rather low humidity, has many nearby ocean scenes but can be subject to airborne (mineral) dust and some unresolved cirrus clouds. This attributes makes it more useful for studying the atmospheric window bias and hence for tuning the water continuum absorption model, however, the BT defecit due to WV absorption in the window is similar to the uncertainty due to skin temperature and residual clouds. Example bias plots are shown in figures F.TEN.

Notice that both of the sondes tend to be slightly drier in total and drier in the upper troposphere compared to ECMWF. This is reflected in the Obs:calc bias in the MW band. The mean bias in the LW window is of the order 0.2+/-0.05 K. It is hard to justofy tuning WV continuum model based on this difference. Note that only nominal CO2, O3 and CH4 are included in the RTA.

Tateno is a GRUAN certified site with with RS92 from 2011 to 2014 and a very few thereafter, and RS11 from 2013 to Jan 2018, and RS41 from 2020 to 2022 (current date). Some are fully processed, others partially processed with calibration.

Tateno is situated north of Tokyo about 20 km from the ocean, so there are some clear ocean views within the specified proximity but the sample size is rather small. There is a strong seasonal cycle with summer months being relatively humid.

Figures F.TAT show the results for subset clear ocean views for two radiosondes RS92 and RS-11G for the years 2013 and 2017. Notice the slight differences in the temperature and water vaopur profiles and how these are reflected in the spectral bias plots. The clear ocean sample size is very small, and of these there appears to be some cloud contamination in the LW window. The suggestion is that both the sondes and the ECMWF have too much water vapour in the UT compared to the observations.

The Singapore site is GRUAN certified, about 3 miles from ocean with other islands. Available sondes include RS41 from 2017 to current period. Most sondes data are calibrated but don't yet include the GDP uncertainties. The profiles are generally very humid.

Two years of sonde data are matched to AIRS, in 2019 there are 637 valid sonde launches and after subsetting to clear ocean and night views 425 matched FoVs remain. In 2020 the number of launches is 591 with 245 clear match FoVs. A summary of results is plotted in figures F.SNG.

Paramaribo is a GRUAN site currently being certified, near the capital city of Suriname about 8 miles from the Ocean. Soundings used in this study were provided by the Royal Dutch Met Institute (KNMI). There are soundings from 1999 to 2020 using RS92-SGP about once a week. KNMI apply some T and WV corrections, have QC applied but don't have individual reading uncertainty,

Plots to follow

Reunion is a GRUAN site in the Indian Ocean East of Madagacar, very close to the ocean. Plots to follow …

Tropical Western Pacific ARM soundings …

Southern Great Plains ARM soundings …

Lindenberg ARM soundings …

There are pre-matched AIRS observations for sondes in the TWP, NSA, SGP and Magic ARM campaigns. The data files are available in /asl/val/IrionSondes/{TWP,NSA,SGP,MAGIC}_RTP/ sub-directories, with the T(z) and WV(z) already incorporated.

The TWP collection includes 199 matched sonde launches from late 2002 to early 2003. The profiles are processed through klayers and sarta to obtain computed spectra for comparison to the observations. A sample of the bias after subsetting for clear ocean views is shown in figures IS.n. Note that the atmospheric model only includes T(z), WV(z) and skin temperature, with the remaining gases using the default AFGL standard atmosphere from klayers. The TWP clear ocean bias subset sample size is only about 30 footprint matchups. The results are not very useful. There is too much uncertainty in the skin temperature and residual cloudiness.

The Magic Campaign includes 295 matched sonde launches from about September 2012 to October 2013. There are originally about 74k matchups reducing to about 24k clear ocean subset. The mean bias and std.dev of the bias is shown in figure IS.n. The results are not very useful, with too much uncertainty in the skin temperature and residual cloudiness.

The Southerm Great Plains (SGP) collection includes 99 matched sonde launches from August 2002 to February 2003. The original set of 31k samples is subset to 8k clear ones. The mean bias and std.dev of the bias is shown in figure IS.n. These are similarly not very useful.

From work in 2009, collections in: /asl/val/airs_2009/{'ARMTWP/Phase2_corr3/', 'ARMTWP/Phase1_corr3_hannon/'}

for Phase1_corr3 there are 220 sondes, 148 semi-clear AIRS FoV matchups w/ AIRIBRAD L1b data (2378 channels). Sonde T(z) WV(z) have corrections applied. Proximity: 1-deg lat/lon, max delay 1.2 hours.

Code: calc_hannon_airs_2009_bias.m child proc: initialize_sonde_airs_bias.m sarta_exec: /airs_l1c_2834_may19_prod_v2

As example shown in figures HO.n the original obs:calc bias from the Hannon file and the new sarta calcs. There appears to be no added value in using those data.

The computation of tuning parameters uses the method developed by Scott and uses the fractional change in radiance computed from the bias of the observed to calculated radiances. The fractional change is then applied to the SARTA optical depths at that point during the SARTA algorithm.

Scripts are found in /home/chepplew/projects/sarta/tuning/

/asl/val/airs_2009/ARMTWP/Phase1_corr3_hannon/