IASI radiances (pre-) processing
Introduction
Typical IASI radiance data reception consists of a subset of 366 channels out of the full set of 8461. These cover the infrared absorption spectrum from 3.8 to 15.4 micrometers. In the context of NWP, the most useful IASI channels include (i) the temperature-sounding channels in the approximate channel index range 100-450, (ii) the humidity-sounding channels at 2800-3500 and 5000-5500 indices, and (iii) surface-sensing window channels at 500-1000. Most of the NWP impact from IASI is thought to come from group (i) and especially from the upper-tropospheric and lower-stratospheric channels in the range 200-300.
Including IASI radiances in a HARMONIE run
scr/include.ass
scr/include.ass should be edited to "switch on" the use of AMSUA (AMSU-A), AMSUB (AMSU-B/MHS):
export IASI_OBS=1 # IASI
export ATOVS_SOURCE=mars # local: EUMETCast;
export IASI_SOURCE=ears # mars:MARS | else: file in $OBDIR
export IASI_RT_COEF=lblrtm # genln2|kcarta|lblrtm
[[ $IASI_OBS -eq 1 ]] && types_BASE="$types_BASE iasi"Loading the IASI radiances
Data extracted from MARS
elif [ "$base" = iasi ] ; then
# IASI
if [ "$IASI_OBS" -eq 1 ]; then
echo "iasi iasi BUFR iasi ">>batormap
ln -sf "${HM_LIB}"/const/bator_param/param_bator.cfg.iasi param.cfg
if [ "$IASI_SOURCE" = mars ] ; then
ln -sf "$WRK"/splitObs/iasi ./BUFR.iasi
else
ln -sf $OBSDIR/iasi$DTG ./BUFR.iasi
fi
fiLocally received data
Controlling the detection of cloud
IASI radiances are strongly influenced by cloud. Because of inaccurate forward modelling, large background errors in cloud fields, and non-linear effects, success in the use of IASI requires careful screening and removal of cloud-affected data. In the HARMONIE data assimilation system, the screening for cloud follows the method of McNally and Watts (2003). The power of this method lies in the use of a large number of individual channels such that much of the disturbing instrument noise can be smoothed out and the cloud radiative effect is therefore more easily detected.
Even if the active use of IASI is limited to relatively small number of channels (such as the 55-channel subset in MetCoOp in early 2021), it is advisable to include more than 100 channels in the cloud detection channel list. Furthermore, it is important that all these channels are subjected to VarBC. To achieve the latter, one needs to make sure that blacklisting for the cloud detection channels uses the fail(EXPERIMENTAL) syntax rather than fail(CONSTANT). The following excerpt from src/blacklists/hirlam_blacklist.b.data_selection_after_20140601 illustrates the concept:
if (SENSOR = iasi) then
! remove channels that are not used in either cloud detection
! or minimization
if PRESS notin (38, 49, 51, 55, 57, 61, 63, 83, 85, 87,
104, 109, 111, 116, 122, 128, 135, 141, 146, 148,
154, 159, 161, 167, 173, 179, 180, 185, 187, 193,
199, 205, 207, 210, 212, 214, 217, 219, 222, 224,
226, 230, 232, 236, 239, 242, 243, 246, 249, 252,
254, 256, 258, 260, 262, 265, 267, 269, 275, 278,
280, 282, 284, 286, 288, 290, 292, 294, 296, 299,
306, 308, 310, 312, 314, 316, 318, 320, 323, 325,
327, 329, 331, 333, 335, 341, 345, 347, 350, 352,
354, 356, 358, 360, 362, 364, 366, 369, 371, 373,
375, 377, 379, 381, 383, 386, 389, 398, 401, 404,
407, 410, 414, 416, 426, 428, 432, 434, 439, 445,
457, 515, 546, 552, 559, 566, 571, 573, 646, 662,
668, 756, 867, 921, 1027, 1133, 1191, 1194, 1271, 1805,
1884, 1946, 1991, 2094, 2239, 2701, 2819, 2910, 2919, 2991,
2993, 3002, 3008, 3014, 3098, 3207, 3228, 3281, 3309, 3322,
3438, 3442, 3484, 3491, 3499, 3506, 3575, 3582, 3658, 4032)
then fail(CONSTANT); endif;
if PRESS notin (38, 51, 63, 85, 104, 109, 167, 173, 180, 185,
193, 199, 205, 207, 212, 224, 230, 236, 239, 242,
243, 249, 296, 333, 337, 345, 352, 386, 389, 432,
2701, 2819, 2910, 2919, 2991, 2993, 3002, 3008, 3014, 3098,
3207, 3228, 3281, 3309, 3322, 3438, 3442, 3484, 3491, 3499,
3506, 3575, 3582, 3658, 4032)
then fail(EXPERIMENTAL);
endif;
endif; ! SENSOR = IASIHere we provide two lists of IASI channels. The first list includes all those channels that are either used in the cloud detection, or are intended for active assimilation (or both). The second list is a subset of the first and includes just those intended for active assimilation. Only those channels included in the latter list will have a significant weight during the assimilation process.
The control for the cloud detection happens via namelist file at nam/IASI_CLDDET.NL. The format of the namelist file is illustrated below:
&NAMCLDDET
N__Band_Size(1)=145
N__Bands(1:145,1) =
38, 49, 51, 55, 57, 61, 63, 83, 85, 87,
104, 109, 111, 116, 122, 128, 135, 141, 146, 148,
154, 159, 161, 167, 173, 179, 180, 185, 187, 193,
199, 205, 207, 210, 212, 214, 217, 219, 222, 224,
226, 230, 232, 236, 239, 242, 243, 246, 249, 252,
254, 256, 258, 260, 262, 265, 267, 269, 275, 278,
280, 282, 284, 286, 288, 290, 292, 294, 296, 299,
306, 308, 310, 312, 314, 316, 318, 320, 323, 325,
327, 329, 331, 333, 335, 341, 345, 347, 350, 352,
354, 356, 358, 360, 362, 364, 366, 369, 371, 373,
375, 377, 379, 381, 383, 386, 389, 398, 401, 404,
407, 410, 414, 416, 426, 428, 432, 434, 439, 445,
457, 515, 546, 552, 559, 566, 571, 573, 646, 662,
668, 756, 867, 921, 1027, 1133, 1191, 1194, 1271, 1805,
1884, 1946, 1991, 2094, 2239
/Here we specify a list of 145 channels to be included in "band 1" of the cloud detection, i.e., in the main cloud detection channel band. The setup of the cloud detection involves not just the channel list but several additional tuning parameters that can be modified to make the screening more or less conservative. The default settings are specified in src/arpifs/obs_preproc/cloud_detect_setup.F90. A comprehensive description of the cloud detection scheme, including explanations of the various tuning parameter values, is given at the NWPSAF web site https://nwp-saf.eumetsat.int/site/software/aerosol-and-cloud-detection/documentation/.
Log file of the Screening task will indicate whether the formatting of the namelist file is appropriate:
READING CLOUD DETECTION FILE FOR IASI
IASI CLOUD DETECTION FILE READ OKIn case of an error, the following is printed instead:
READING CLOUD DETECTION FILE FOR IASI
PROBLEM READING IASI CLOUD DETECTION FILE: Using Default ValuesThe third possibility is that the namelist file does not appear in the working directory, in which case the printout statement is this:
READING CLOUD DETECTION FILE FOR IASI
NO IASI CLOUD DETECTION FILE : Using Default ValuesPlease note that the use of the "Default Values" is generally not a desired outcome. This is because many of the cloud detection channels in the default list (see src/arpifs/obs_preproc/cloud_detect_setup.F90) are sensitive to higher stratosphere and therefore may be severely affected by the relatively low model top of limited-area HARMONIE systems.
References:
McNally, AP, and PD Watts, 2003: A cloud detection algorithm for high-spectral-resolution infrared sounders. Quarterly Journal of the Royal Meteorological Society, 129, 3411-3423, doi:10.1256/qj.02.208.