RAL HIRDLS Validation Technical Note
Source: environment.data.gov.uk (2018-06-19)

HIgh Resolution Global Distributions of Temperature, Clouds, Aerosols, and 10 Trace Species Data from the High Resolution Dynamics Limb Sounder (HIRDLS) instrument measurements. HIRDLS was a joint US-UK development effort, with sponsorship by the British National Space Centre and the Natural Environment Research Council in the UK, and by NASA in the US. HIRDLS was an mid-infrared limb-scanning radiometer (21 channels from 6.12 to 17.76 µm and provides sounding observations to observe the lower stratosphere with improved sensitivity and accuracy. HIRDLS was carried on the Aura mission, part of the A-train procession of polar orbiting satellites forming part of NASA's Earth Observing System (EOS).

This dataset collection contains data of the global distributions of temperature, clouds, aerosols, and 10 trace species O3, H2O, CH4, N2O, NO2, HNO3, N2O5, CFC11, CFC12, and ClONO2 in the stratosphere and upper troposphere at high vertical and horizontal resolution in the Earth's atmosphere between about 8 and 100 km, from the High Resolution Dynamics Limb Sounder (HIRDLS) instrument. 

The instrument provides high vertical resolution information despite the fact that the optical beam is partially obstructed between the scan mirror and the aperture, probably by a piece of inner lining material that became detached during launch. HIRDLS science-team members have produced correction algorithms that make use of the partial view of the atmosphere (vertical scans around azimuth angle of 47 degree line of sight to the orbital plane, on the side away from the sun). In spite of this anomaly, HIRDLS has retained most of its scientific capabilities to support the Aura Mission.

HIRDLS was carried on the Aura mission, part of NASA's Earth Observing System (EOS). Aura was launched on 15th July 2004 at 11:01:59 a.m. BST from Vandenberg Air Force Base, California.

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RAL HIRDLS Validation
Technical Note

W.J. Reburn, A.M. Waterfall, B.J. Kerridge

Remote Sensing Group
SSTD, Rutherford Appleton Laboratory
U.K.

October 2005

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Contents

Introduction....................................................................................................................................................3
HIRDLS Instrument.......................................................................................................................................3
Channels ....................................................................................................................................................3
Data Levels ................................................................................................................................................4
Correlative Data.............................................................................................................................................5
ECMWF Data............................................................................................................................................5
Sonde Data.................................................................................................................................................5
Radiance Comparisons ..................................................................................................................................6
Calculations ...............................................................................................................................................6
ECMWF Comparisons...............................................................................................................................6
Channel 2 – Temperature.......................................................................................................................8
Channel 3 – Temperature.....................................................................................................................12
Channel 4 – Temperature.....................................................................................................................16
Channel 5 - Temperature ....................................................................................................................20
Channel 10 - Ozone .............................................................................................................................24
Channel 11 – Ozone.............................................................................................................................28
Channel 18 – H2O ...............................................................................................................................32
Channel 19 - Aerosol...........................................................................................................................35
Channel 20 – H2O................................................................................................................................38
Pointing Offset.....................................................................................................................................41
Comparison of ECMWF to MIPAS radiances ....................................................................................47
Conclusions from ECMWF radiance comparisons .............................................................................48
Sonde comparisons ..................................................................................................................................49
Conclusions from sonde comparisons .................................................................................................51
Level 2 Comparisons ...................................................................................................................................52
Temperature Sondes ................................................................................................................................52
Ozone sondes...........................................................................................................................................54
ECMWF Analysis Data...........................................................................................................................56
Temperature.........................................................................................................................................56
Ozone...................................................................................................................................................59
Water vapour .......................................................................................................................................62
Conclusions..................................................................................................................................................65
Further Work ...............................................................................................................................................65
References................................................................................................................................................65

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Introduction

The HIRDLS instrument was launched on the NASA EOS Aura spacecraft on 15th July 2004. During
initial operation it was discovered that part of the instrument field of view was obstructed, probably by
material used to insulate and shield the instrument in orbit. As a consequence atmospheric measurements
produced are anomalous. The HIRDLS project team however, have developed new calibration schemes to
convert the anomalous measurements to equivalent standard radiance measurements as would have been
produced by a normally functioning instrument.

The work presented here, as part of the HIRDLS validation effort, contains two parts. The first is a
comparison of HIRDLS corrected radiances for channels designated for temperature, water vapour and
ozone measurement. Comparison with radiances simulated using ECMWF meteorological analysis fields
and also sonde measurements are shown. In the second part HIRDLS Level 2 Temperature and Ozone
profiles have been compared to sonde and ECMWF data.

HIRDLS Instrument
Channels

The HIRDLS spectral channels, as well as the main target species and ranges are shown in Table 1.
In this study channels targeting carbon dioxide (for temperature/pressure retrieval), water vapour and
ozone are examined and the spectral filters for the channels shown in Figure 1.

Table 1: HIRDLS spectral channels. The letters L, M and H indicate low, middle and high altitude sounding ranges. The
species code A indicates aerosol. (Taken from HIRDLS L2 ATBD document)

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Figure 1: HIRLDS channel peak normalised filter responses for the main temperature, ozone and water vapour sounding
channels.
Data Levels

The basic HIRDLS instrument telemetry data received at the ground is classified as Level 0. These data
are processed using the Level 0 to 1 (L01) processor to convert the raw signal counts to radiance and to
determine geo-location (latitude, longitude, time) and tangent height (bore-sight) for the measurements.
This procedure produces a Level 1B file.

Due to the instrument anomaly the next step in the processing chain is a “correction” module. This
converts the L1 radiances as seen by the instrument into an equivalent radiance as if the obstruction were
not present. The output of this step is a L1C data file. There is some data reduction in terms of
housekeeping and auxiliary data, making the L1C file significantly smaller than the L1B input.

Next is the Level 2 pre-processor, L2PP, which prepares the data by selecting out legitimate profiles that
can be used for retrieval. This produces a so-called HIRRAD file, which gives radiance versus bore-sight
tangent height as well as individual channel pointing vectors and other data needed by the retrieval.

These data are the most useful for comparison in the radiance domain since it is this data that feeds
directly into the retrieval processor to provide the geophysical products as HIRLDS Level 2 files.

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Correlative Data
The correlative data used to compare with HIRLDS measurements is of two basic types: meteorological
analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for temperature,
water vapour and ozone, and atmospheric balloon sonde data for temperature and some ozone
measurements.

Descriptions of data sources and attributes and the handling of the data is discussed in the following
sections.

ECMWF Data

ECMWF operational data was obtained through the British Atmospheric Data Centre (BADC). The
temperature and humidity data used was supplied interpolated onto a regular 1.125˚ x 1.125˚ grid in
latitude and longitude and vertically on model levels (60 levels up to 0.1 hPa). Ozone data was provided
on an N80 Reduced Gaussian Grid, which was then interpolated to the same latitudes and longitudes as
the rest of the data. Data was available at 4 time throughout the day (00.00, 06.00, 12.00 and18.00 UTC).

The ECMWF data was then interpolated spatially and in time to the locations of each of the HIRDLS
measurements. The profiles were extended above 0.1 hPa using a set of reference atmospheres compiled
for use with MIPAS on ENVISAT.

Sonde Data

Sonde data was obtained from three sources. The temperature profiles were obtained from the UK Met
Office Global Radiosonde Data Set, via the BADC, which contains soundings from stations worldwide,
up to four times a day depending on the station. Profiles typically extend up to 20 or 30 km in altitude.
Ozonesonde data, containing both ozone and temperature measurements, were obtained both from the
World Ozone and Ultraviolet Radiation Data Centre (WOUDC) and from the SHADOZ data archive.

In order to simulate radiance profiles, the dataset was extended above the top level of the profile using
ECMWF data. Above the limit of the ECMWF data, reference profiles were used as described in the
previous section. The HIRDLS measurements within a specified criterion (e.g. 500 km and 6 hours) were
extracted and used in further comparisons.

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Radiance Comparisons
Calculations

In order to compare the HIRDLS radiances to correlative data, simulated radiances have been generated.
The radiative transfer model used is the Oxford Reference Forward Model (RFM). One-dimensional
monochromatic, pencil-beam calculations were performed, using the RFM, over the spectral regions
covered by the HIRDLS channels. These were then spectrally integrated to provide simulated
measurements for comparison. The calculations have been performed for geometric tangent heights to
correspond to the altitudes given in the HIRDLS data files. Cloud and aerosol have not been included in
the simulations, which will result in a tropospheric deviation compared to HIRDLS unless the channels
black out above the tropopause.

ECMWF Comparisons

The HIRDLS data used for the ECMWF comparison is indicated in Table 2. Several versions of the data
for the selected days became available during this analysis, and have been compared. In addition to data
from the standard processing scheme (SIPS), data was also provided which had been through an
additional radiance adjustment scheme (shaded blue in Table 2). In general, on each day the comparison
was carried out for 1 profile in 10 (numbers 1, 11, 21 …), although in some cases others are also
included. For each profile radiances were simulated at 50 levels, for 0–30 km at 1 km and for 30–68 km
at 2 km vertical intervals. Currently, nearly 5000 radiance profiles have been calculated for channels 2 – 5
(temperature), 10 – 11 (ozone) and 18 – 20 (water vapour).

Filename
Day /Year
Date
HIRRAD_v3.9.4-aIrix-c1_2005d028.he5
28 / 2005
28-Jan-2005
HIRRAD_V3.9.9-aIrix-c1_2005d028.he5
28 / 2005
28-Jan-2005
HIRRAD_V4.0.0-aIrix-c1_2005d028.he5
28 / 2005
28-Jan-2005
HIRRAD_V4.0.0-aIrix-c4_2005d028.he5
28 / 2005
28-Jan-2005
HIRRAD-rs6c3a6d040_2005d028.he5
28 / 2005
28-Jan-2005
HIRRAD_v3.9.9-aIrix-c1_2005d125.he5
125 / 2005
05-May-2005
HIRRAD_v4.0.0-aIrix-c1_2005d125.he5
125 / 2005
05-May-2005
HIRRAD-rs6c3a6d040_2005d125.he5
125 / 2005
05-May-2005
HIRRAD_v3.9.9-aIrix-c1_2005d126.he5
126 / 2005
06-May-2005
HIRRAD_v4.0.0-aIrix-c1_2005d126.he5
126 / 2005
06-May-2005
HIRRAD-rs6c3a6d040_2005d126.he5
126 / 2005
06-May-2005
Table 2: HIRDLS Radiance files used in ECMWF comparison. The coloured files are those which are shown in the cross-
section plots. Files shaded blue are the most recent version of the data available to us, in which the temperature (ch2-5) and
ozone (10-12) channels have been additionally adjusted outside of the standard processing scheme

Comparisons for each channel are shown both as along orbit cross-sections and mean differences. The
cross-section figures contain four panels, showing simulated radiances, HIRDLS radiances, the radiance
difference (as HIRDLS – simulated radiance) and latitude for the five HIRRAD files shown above. These
have been calculated for all of the above files, but are shown here for only the most recent versions of the
files indicated by the shaded lines in Table 2. Mean differences in absolute and percentage terms, relative
to the simulated radiance, have then been calculated and plotted for each of five latitude bands. These
show a comparison of each of the different versions of the data for each day.

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The plots shown in this section are a simple comparison between the HIRDLS and ECMWF radiances,
assuming the true HIRDLS altitudes are those given in the HIRRAD files. However, it is expected that
there is a shift in the given HIRDLS altitudes compared to reality. Improvements in the radiance
differences are achieved when the HIRDLS altitudes are shifted downwards by a few kilometres. This is
discussed further in a later section.

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Channel 2 – Temperature

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Figure 2 ECMWF / HIRDLS radiance and difference plots for Channel 2. The 6 sets of plots show the results for each
of the shaded HIRRAD files given in Table 2. Each plots shows a) simulated radiances from ECMWF for HIRDLS
measurement locations, b) HIRDLS radiances, c) Difference between HIRDLS and simulated data, d) Variation in
latitude along the cross-section.

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Figure 3 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 2 for 28 January
2005, for 5 latitude bands. Solid coloured lines give the average difference for each the versions of the data given in
Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 4 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 2 for 5/6 May
2005, for 5 latitude bands. Solid coloured lines give the average difference for each the versions of the data given in
Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 3 – Temperature

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Figure 5 ECMWF / HIRDLS radiance and difference plots for Channel 3. The 6 sets of plots show the results for each
of the shaded HIRRAD files given in Table 2. Each plots shows a) simulated radiances from ECMWF for HIRDLS
measurement locations, b) HIRDLS radiances, c) Difference between HIRDLS and simulated data, d) Variation in
latitude along the cross-section.

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Figure 6 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 3 for the 28th
January 2005 divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the
data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 7 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 3 for 5/6 May
2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the data
given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 4 – Temperature

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Figure 8 ECMWF / HIRDLS radiance and difference plots for Channel 4. The 6 sets of plots show the results for each
of the shaded HIRRAD files given in Table 2. Each plots shows a) simulated radiances from ECMWF for HIRDLS
measurement locations, b) HIRDLS radiances, c) Difference between HIRDLS and simulated data, d) Variation in
latitude along the cross-section.

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Figure 9 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 4 for the 28th
January 2005 divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the
data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 10 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 4 for 5/6 May
2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the data
given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 5 - Temperature

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Figure 11 ECMWF / HIRDLS radiance and difference plots for Channel 5. The 6 sets of plots show the results for each
of the shaded HIRRAD files given in Table 2. Each plots shows a) simulated radiances from ECMWF for HIRDLS
measurement locations, b) HIRDLS radiances, c) Difference between HIRDLS and simulated data, d) Variation in
latitude along the cross-section.

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Figure 12 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 5 for the 28th
January 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the
data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 13 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 5 for 5/6 May
2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the data
given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 10 - Ozone

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Figure 14 ECMWF / HIRDLS radiance and difference plots for Channel 10. The 6 sets of plots show the results for
each of the shaded HIRRAD files given in Table 2. Each plots shows a) simulated radiances from ECMWF for
HIRDLS measurement locations, b) HIRDLS radiances, c) Difference between HIRDLS and simulated data, d)
Variation in latitude along the cross-section.

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Figure 15 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 10 for the 28th
January 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the
data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 16 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 10 for 5/6 May
2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each the versions of the data
given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 11 – Ozone

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Figure 17 ECMWF / HIRDLS radiance and difference plots for Channel 11. The 6 sets of plots show the results for
each of the shaded HIRRAD files given in Table 2. Each plots shows a) simulated radiances from ECMWF for
HIRDLS measurement locations, b) HIRDLS radiances, c) Difference between HIRDLS and simulated data, d)
Variation in latitude along the cross-section.

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Figure 18 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 11 for the 28th
January 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of
the data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 19 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 11 for the 5/6 th
May 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of the
data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 18 – H2O

Figure 20 ECMWF / HIRDLS radiance and difference plots for Channel 18. The 3 sets of plots show the results for
each of the shaded HIRRAD files given in Table 2. (Channel 18 hasn’t been adjusted outside the SIPS processor as was
the case for earlier channels in the blue files. so plots from the two shaded files for each day are identical). Each plots
shows a) simulated radiances from ECMWF for HIRDLS measurement locations, b) HIRDLS radiances, c) Difference
between HIRDLS and simulated data, d) Variation in latitude along the cross-section.

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Figure 21 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 18 for the 28th
January 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of
the data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 22 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 18 for the 5/6th
May 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of the
data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Channel 19 - Aerosol

Figure 23 ECMWF / HIRDLS radiance and difference plots for Channel 19. The 3 sets of plots show the results for
each of the HIRRAD files given in Table 2. . (Channel 19 hasn’t been adjusted outside the SIPS processor as was the
case for earlier channels in the blue files. so plots from the two shaded files for each day are identical). Each plots
shows a) simulated radiances from ECMWF for HIRDLS measurement locations, b) HIRDLS radiances, c) Difference
between HIRDLS and simulated data, d) Variation in latitude along the cross-section.

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Figure 24 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 19 for 28th
January 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of
the data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 25 Radiance difference (HIRDLS – simulated) and fractional radiance difference for channel 19 for 5/6th May
2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of the data
given in Table 2, whilst dashed lines show the standard deviations of the differences.

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Channel 20 – H2O

Figure 26 ECMWF / HIRDLS radiance and difference plots for Channel 20. The 3 sets of plots show the results for
each of the HIRRAD files given in Table 2. . (Channel 20 hasn’t been adjusted outside the SIPS processor as was the
case for earlier channels in the blue files. so plots from the two shaded files for each day are identical). Each plots
shows a) simulated radiances from ECMWF for HIRDLS measurement locations, b) HIRDLS radiances, c) Difference
between HIRDLS and simulated data, d) Variation in latitude along the cross-section.

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Figure 27 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 20 for 28th
January 2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of
the data given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Figure 28 Radiance differences (HIRDLS- simulated) and fractional radiance differences for channel 20 for 5/6th May
2005, divided into 5 latitude bands. Solid coloured lines give the average difference for each of the versions of the data
given in Table 2, whilst dashed lines show the standard deviation of the differences.

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Pointing Offset

In the above comparisons, vertical structure in the apparently large radiance differences between most
versions of the HIRDLS data and ECMWF data can be significantly reduced by the introduction of a
pointing offset to the HIRDLS altitudes. The HIRDLS tangent-altitudes in L1 data appear to be
consistently higher by a couple of kilometres in comparison to those from ECMWF, although the best
values estimated from this comparison vary between channels and also between latitude bands. The
exceptions to this are the latest data sets for temperature and water vapour (shaded blue in Table 2), which
have been through a radiance adjustment scheme after the standard SIPS processing, and no longer show
evidence for the large altitude shift seen previously. However, recent data which has not been through
this additional adjustment still appear to show a similar altitude effect.

As a first approximation to determining the pointing offset, radiance differences assuming HIRDLS
altitude shifts varying from +1km to -5km were calculated for the HIRRAD data (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5)for the 28th January 2005. From this, a preliminary pointing offset could be deduced
(by eye). For channels 2-5, 10, and 11 this was typically in the range of 2 to 3km. Channels 18 – 20
seem to suggest a larger altitude shift (4-5km); however the method used to estimate this tended to focus
on a single feature around the tropopause, which could potentially be influenced by other factors. In
addition, a similar analysis was carried out for the latest version of the HIRDLS data, which had also been
through the external radiance adjustment scheme (HIRRAD-rs6c3a6d040_2005d028.he5). In this case,
there was no evidence for an altitude shift of a couple of kilometres magnitude, although some
improvement in the radiance differences could be obtained when a smaller (<1km) altitude offset was
used. Further work would be required to confirm and quantify this apparent small shift.

The pointing offset needs to be corrected for in order to estimate the residual errors in radiometric gain
and radiometric offset present in the data. The following figures show the improvement to the radiance
comparisons if an altitude shift is considered. Note that here we are comparing data (HIRRAD_V4.0.0-
aIrix-c1_2005d028.he5), which hasn’t been through the external radiance adjustment scheme, and hence
suffers the larger altitude shift described above. For each channel the HIRDLS data has been shifted in
altitude by a constant amount determined by eye; in reality there may be variations in the pointing offset
that vary with latitude and altitude that would improve the comparison further.

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Figure 29 Channel 2 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 2km.

Figure 30 Channel 3 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 2km.

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Figure 31 Channel 4 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 2.2km.

Figure 32 Channel 5 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5) showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 2.2km.

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Figure 33 Channel 10 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 2.5km. Difference plots use a restricted colour scale to highlight differences in the stratosphere and
above.

Figure 34 Channel 11 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 2.5km.

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Figure 35 Channel 18 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 5km. Difference plots use a restricted colour scale to highlight differences in the stratosphere and above

Figure 36 Channel 19 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5) , showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 5km. Difference plots use a restricted colour scale to highlight differences in the stratosphere and above.

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Figure 37 Channel 20 ECMWF / HIRDLS radiance and difference plots for 28th Jan 2005, (HIRRAD_V4.0.0-aIrix-
c1_2005d028.he5), showing the effect of shifting the altitude. (Sections of plots are defined as in Figure 2.) The first
plot shows the original comparison, whilst the second shows the effect of shifting the HIRDLS radiance profiles
downwards by 5km. Difference plots use a restricted colour scale to highlight differences in the stratosphere and above.

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Comparison of ECMWF to MIPAS radiances

In order to validate the ECMWF calculated radiances, a comparison has been made to MIPAS radiances
which have been spectrally integrated using the HIRDLS filter functions (by Clare Waymark, University
of Oxford). The results, showing a comparison between the average radiances from MIPAS and ECMWF
along with the differences and ratio with HIRDLS radiances are shown in Figure 38 and Figure 39. The
MIPAS average profile is the average of all MIPAS data in the selected latitude band on the 28th January,
compared to the average of all the HIRDLS data, whilst the ECMWF analysis is for the subset of
HIRDLS data for which simulated radiances have been performed. However, the difference between the
mean HIRDLS profile in either case is very small above 10km and should not affect the results. The
comparison is restricted to Channels 18 and 19, as the other channels considered in this report are not
fully covered by the MIPAS spectra. It should be noted that this comparison has not been performed with
the most recent version of the data (HIRRAD_V4.0.0-aIrix-c1_2005d028.he5), so conclusions in respect
to HIRDLS data may have changed.

It can be seen that above approximately 30 km, there is very good agreement between the ECMWF and
MIPAS radiances, and a consistent offset and gain would be derived from either data set. However,
lower down there is a larger discrepancy. The most likely cause of this is the lack of any clouds or
aerosols in the ECMWF simulations, a factor which should be particularly important in Channels 18 and
19, as they are more sensitive to aerosols than many of the other channels. At these altitudes the MIPAS
data are in better agreement than the ECMWF profiles with the radiances observed by HIRDLS, and
show no evidence for the 4-5 km altitude shift that was tentatively derived from the HIRDLS – ECMWF
comparison in the previous section. A smaller altitude shift of a couple of kilometres, as was derived
from the other channels of this version of the data, is more consistent with the MIPAS results. This
would agree with the suspicion that the ECMWF data is overestimating the altitude shift in Channels 18 –
20, and is instead compensating for a lack of clouds and aerosols in the simulation.

Figure 38 Comparison between MIPAS, ECMWF and HIRDLS radiances for Channel 18 on the 28th January 2005. a)
The average profiles from HIRDLS (blue), MIPAS (red) and ECMWF (black). The light blue HIRDLS radiances are
the average of all profiles in the selected latitude band, whilst the dark blue is the average of the subset of profiles used
in the ECMWF analysis. b) Average radiance difference profiles: HIRDLS – ECMWF (Black), HIRDLS – MIPAS
(Red). C) Average radiance ratios: HIRDLS / ECMWF (Black), HIRDLS / MIPAS (Red).

- 48 -

Figure 39 Comparison between MIPAS, ECMWF and HIRDLS radiances for Channel 19 on the 28th January 2005. a)
The average profiles from HIRDLS (blue), MIPAS (red) and ECMWF (black). The light blue HIRDLS radiances are
the average of all profiles in the selected latitude band, whilst the dark blue is the average of the subset of profiles used
in the ECMWF analysis. b) Average radiance difference profiles: HIRDLS – ECMWF (Black), HIRDLS – MIPAS
(Red). C) Average radiance ratios: HIRDLS / ECMWF (Black), HIRDLS / MIPAS (Red).

Conclusions from ECMWF radiance comparisons
• The HIRDLS radiances are in qualitative agreement with the atmospheric structure seen in
radiance fields simulated from ECMWF data.
• There is evidence for a pointing offset in the HIRDLS data processed through SIPS. Shifting the
HIRDLS tangent-altitudes downwards by 2-3 kilometres can significantly improve the
comparison between HIRDLS and ECMWF radiances. There appears to be some channel and
latitude dependence to this offset.
• Data which has been through an additional radiance adjustment scheme does not show evidence
for such large pointing offsets. However it is possible that there is still a small altitude shift.
• Once the pointing offset has been applied, values can be inferred for the radiometric offset (from
the height-independent (i.e. constant) offset at the top of the profile) and the gain (from the %
difference profile). The latter could then be interpreted in terms of the error in the estimate for
each channel of the unobstructed area.
• Differences are seen between the different versions of the data, and between the 28th Jan and the
5th / 6th May. In general, the more recent versions of the data are an improvement over older
versions. A significant latitudinal structure can also be seen in the radiance difference plots.
• On the 28th Januray, (data version HIRRAD_V4.0.0-aIrix-c1_2005d028.he5), most temperature
channels were seen to be in agreement within 10% once an altitude offset had been removed .
Channel 3 appears to show larger differences than the other three CO2 channels. In the later
version of data which had been through the radiance adjustment scheme Channel 2 does not seem
to agree quite so well.
• The H2O and aerosol channels appear to show larger pointing offsets (>4km) than the temperature
and ozone channels, although this conclusion is mainly based on a feature around the tropopause,
and it seems probable that this is falsely influencing the results. Gain errors are also larger, from
which a much larger overestimate of clear area for these channels (~25 – 40%) can be inferred.

- 49 -
Sonde comparisons

In addition to the ECMWF data, comparisons were performed with correlative sonde data. A large
number of matches between HIRDLS data and temperature profiles obtained from radiosondes were
found, using a selection criterion of within 500 km and 6 (or 12) hrs. Generally these sondes were found
to be a very good match to the ECMWF temperature data used in the comparisons above, and simulations
of radiances would have added very little further information. Simulations of radiance profiles were
therefore only performed for co-located ozonesondes, where greater differences are seen in the sonde data
compared to the matching ECMWF profiles. Unfortunately far fewer ozonesondes were available, and
only a small number of matches could be found for each day.

The results shown here are from comparisons to HIRDLS data from the 23rd February 2005
(HIRRAD_v3.9.12-aIrix-c1_2005d054.he5). It should be noted that this is for a different day to the
ECMWF calculations above and that the latest versions of the data using the radiance adjustment scheme
were not available Simulations were performed for the same channels as were used in the ECMWF
calculations described above, with the addition of Channel 12. (Channels 2-5, 10-12, 18-20). In general
the results are similar to those derived from the ECMWF data, and we have restricted the discussion to a
few select channels.

In Figure 40, profiles of temperature and ozone from three European mid-latitude sondes are shown. In
all three of these profiles, similar structure can be seen in the ozone profile. Figure 41 and Figure 42
show the radiance profiles calculated for two of these sondes in comparison to those measured by
HIRDLS, and the difference (HIRDLS – simulation) between the two. Although there is an obvious large
radiance offset between the HIRDLS and ECMWF radiance profiles, it is clear that both profiles have a
similar shape, with a minimum in the profile around 25 km, corresponding to similar minima in the ozone
profiles. Differences in the altitudes of these features would be consistent with the HIRDLS altitudes
being too high by a couple of kilometres, as was found with the ECMWF data.

Figure 40 Data from sondes from three European stations on the 23rd February 2005, showing a persistent shape to
the ozone field. Sondes were located at (47.8N,11.02E),(50.01N,14.45E) and (52.21E,14.12E).

- 50 -

Figure 41 The left plot shows (black line) a radiance profiles calculated from temperature and ozone measured by an
ozonesonde, and (coloured line) co-located HIRDLS data. The HIRDLS radiance profile is within 500 km, and 6 hours
of the sonde. The right plot shows the radiance difference between the two.

Figure 42 The left plot shows (black line) a radiance profiles calculated from temperature and ozone measured by an
ozonesonde, and (coloured lines) co-located HIRDLS data. The HIRDLS radiance profile is within 500 km, and 12
hours of the sonde. The time has been increased relative to the other sonde to allow profiles closer in distance to be
shown, The right plot shows the radiance difference between HIRDLS and the sonde radiances.

Simulations of Channel 12 radiances have not yet been performed with the ECMWF data, so results for
comparisons with radiances from sonde data are shown below. These calculations (Figures Figure 43
and Figure 44) show smaller radiances differences than are observed with Channel 10, with HIRDLS
being approximately 0.01 lower in the stratosphere for the midlatitude sondes. Larger differences occur
below about 12 km, probably due to the lack of any clouds or aerosols in our simulations. The lack of
features above the tropopause means the pointing offset is difficult to estimate from these three sonde
comparisons alone.

- 51 -

Figure 43 Comparison between HIRDLS Channel 12 radiance profiles (coloured lines) and radiances calculated from
sonde data of ozone and temperature (black). Sondes and HIRDLS data are collocated within 500km and 6 hrs in the
1st case, and 500km and 12 hrs in the 2nd case (to allow profiles close in distance to be shown).

Figure 44 Comparison between HIRDLS Channel 12 radiance profiles (coloured lines) and radiances calculated from
sonde data of ozone and temperature (black). Sondes and HIRDLS data are collocated within 500km and 6 hrs. the
right hand plot shows the difference (HIRDLS – simulation) between the radiance profiles.

Conclusions from sonde comparisons
The analysis of the sonde data supports the general results seen with the ECMWF data with reference to
the gain and offset. There is also evidence for a pointing offset in the data, although most of the
comparisons in Channels 18 – 20 would suggest smaller offsets than suggested by our preliminary
ECMWF analysis. Calculations have also been performed for Channel 12, which has not yet been
simulated from ECMWF data.

Some correspondence can be seen between features in a number of midlatitude ozone profiles, the
corresponding calculated radiance profile and the HIRDLS radiance data. This observed structure in the
ozone profile shows similar altitude offsets to those derived from the mean HIRDLS-ECMWF
differences.

- 52 -
Level 2 Comparisons
Temperature Sondes
Comparisons of the HIRDLS retrieved temperature profiles to nearby radiosondes have been performed
for files from the 7th and 28th February, the 22nd March and 11th May 2005. All files are from the latest
version of the processor and have been corrected using the radiance adjustment scheme (Table 3).
Examples of the best results for the 11th May are shown in Figure 45 below. All sondes shown are within
3 hours and 200km of the HIRDLS data, and the HIRDLS profiles have been colour-coded according to
distance from the sonde. It can be seen that the HIRDLS profiles show the same features as are seen in the
sonde data. The comparisons for the other days and the remainder of the sondes are not shown here, but
are available if desired. .

Filename
Day /Year
Date
HIRDLS2-PATH0abcegv19-rs6c3a5d040_b072-
forSCF_2005d038.he5
xx-xxxx-xxxx
07-Feb-2005
HIRDLS2-PATH0abcegv19-rs6c3a5d040_b072-
forSCF_2005d054.he5
54 / 2005
23-Feb-2005
HIRDLS2-PATH0abcegv19-rs6c3a5d040_b072-
forSCF_2005d081.he5
81 / 2005
22-Mar-2005
HIRDLS2-PATH0abcegv19-rs6c3a5d040_b072-
forSCF_2005d131.he5
131 / 2005
11-May-2005
Table 3 HIRDLS L2 files used in the analysis shown here. All files have been corrected using a radiance adjustment
scheme

Figure 45 Examples of the comparison between radiosonde data (Black dashed line) and HIRDLS
temperatures (coloured lines) for the 11th May 2005. HIRDLS profiles are colour-coded according
to distance from the sonde. (All profiles are within 200 km and 3 hrs of sonde.)

- 53 -
In addition, the mean difference between the sonde data and the HIRDLS Level 2 profiles for temperature
are shown below for four latitude bands, for each of the four days . These show generally reasonable
agreement, although mean differences of up to 2 K are seen in the stratosphere. In addition there appears
to be an oscillation with altitude within the differences. In the Northern Hemisphere mid-latitudes where
the largest number of sondes are available for comparison, this structure in the difference profile is
consistent for all four days.

Figure 46 Mean difference (solid lines) between temperature sondes and HIRDLS data for the 4 files in Table 3
Dashed lines give the standard deviation of the differences.

- 54 -
Ozone sondes

Comparisons of HIRDLS retrieved ozone with profiles from ozone sondes have been performed for the
four files in Table 3. Using a matching criterion of 12 hours and 200 km, only a few ozone sondes were
found to be a match. The coincident profiles for the 23rd February and the 22nd March are shown in
Figure 47 and Figure 48 below. It can be seen that the retrieved ozone profile shows many of the features
seen in the sonde data, particularly between 100hPa and 10 hPa. The profile below 100 mb is not
shown, and agreement is not so good, probably due to the presence of clouds in the HIRDLS data.
However differences can also be seen and HIRDLS ozone concentrations are often lower than those
measured by the sonde. In a number of the profiles, the variability in the HIRDLS ozone is much greater
than that in the sonde profile, and seems unrealistic.

Figure 47 Comparison between HIRDLS ozone and ozone sondes for 23rd February 2005. The ozone sonde data is
shown by the black crossed line, whilst the HIRDLS data is shown by the smooth coloured lines. Profiles are colour-
coded according to distance from the sonde . All profiles shown are within 12 hours and 200 km of the sonde data.
Ozone sonde data for the three comparisons shown above were obtained from the WOUDC.

- 55 -

Figure 48 Comparison between HIRDLS ozone and ozone sondes for 22 March 2005. The ozone sonde data is shown
by the black crossed line, whilst the HIRDLS data is shown by the smooth coloured lines. Profiles are colour-coded
according to distance from the sonde. All profiles shown are within 12 hours and 200 km of the sonde data. Ozone
sonde data was obtained from the WOUDC.

- 56 -

ECMWF Analysis Data

HIRDLS level 2 data have been compared directly with ECMWF analysis data interpolated spatially and
temporally to HIRLDS profile positions for the 7th and 23rd February. Results for 7th February are shown
in the following sections. A similar result was obtained for the 23rd February.

Temperature

The structure seen in the HIRDLS temperature fields agrees well with that predicted by ECMWF data.
Within the stratosphere the mean profiles are typically within approximately 2 K of each other. However,
a clear oscillation with altitude can be seen in this difference, with similar features in all latitude bands.
Above 1 hPa HIRDLS is generally several degrees lower than the ECMWF data.

Figure 49 Comparison between HIRDLS temperatures and ECMWF temperatures for 7th February 2005. a) HIRDLS
Level 2 retrieved temperatures. b) ECMWF temperatures interpolated to the location of the HIRDLS profiles. c) The
difference between HIRDLS and ECMWF data(HIRDLS – ECMWF) d) Latitude variation along the cross-section

- 57 -

Figure 50 Mean HIRDLS and ECMWF temperature profiles for the 7th February 2005 for the global average, and
divided into 5 latitude bands

- 58 -

Figure 51The mean difference between HIRDLS and ECMWF temperature profiles (solid lines) and the standard
deviation of the difference (dotted lines).

- 59 -
Ozone

The HIRDLS retrieved ozone fields show similar latitudinal structure to that from ECMWF. In general
the peak of the mean ozone profile at all latitudes is higher in the ECMWF data than in the HIRDLS data.
This leads to higher HIRDLS mixing ratios compared to ECMWF below 10 hPa, and lower HIRDLS
mixing ratios above 10 hPa. There is a tendancy for HIRDLS to retrieve very high (unrealistic ) ozone
concentrations below 100 hPa. This could be partly due to the presence of clouds in the data.

Figure 52 Comparison between HIRDLS ozone and ECMWF ozone for 7th February 2005. a) HIRDLS Level 2
retrieved ozone. b) ECMWF ozone interpolated to the location of the HIRDLS profiles. c) The difference between
HIRDLS and ECMWF data(HIRDLS – ECMWF) d) Latitude variation along the cross-section

- 60 -

Figure 53 Mean HIRDLS and ECMWF ozone profiles for the 7th February 2005 for the global average, and divided
into 5 latitude bands

- 61 -

Figure 54The mean difference (HIRDLS – ECMWF) between HIRDLS and ECMWF ozone profiles (solid lines) and
the standard deviation of the difference (dotted lines).

- 62 -
Water vapour

Comparison between H2O and ECMWF data show that in the stratosphere the average HIRDLS water
vapour profile is approximately 2ppmv higher than that from ECMWF in most latitude bands. The
exception to this is below 60 S where the mean HIRDLS and ECMWF profiles agree relatively well.
Below 100 hPa and above 1hPa differences become much larger, and there is large variability in the
values of water vapour retrieved.

Figure 55 Comparison between HIRDLS water vapour and ECMWF water vapour for 7th February 2005. a) HIRDLS
Level 2 retrieved water vapour. b) ECMWF water vapour interpolated to the location of the HIRDLS profiles. c) The
difference between HIRDLS and ECMWF data(HIRDLS – ECMWF) d) Latitude variation along the cross-section

- 63 -

Figure 56 Mean HIRDLS and ECMWF water vapour profiles for the 7th February 2005 for the global average, and
divided into 5 latitude bands

- 64 -

Figure 57 The mean difference (HIRDLS – ECMWF) between HIRDLS and ECMWF water vapour profiles (solid
lines) and the standard deviation of the difference (dotted lines).

- 65 -
Conclusions

In order to validate HIRDLS radiance profiles, radiance profiles have been simulated using both ECMWF
and sonde data. It was found that qualitatively the HIRDLS radiances show the atmospheric structure
predicted from the ECMWF data. The radiance difference profiles for those files processed only by SIPS
were better behaved (i.e. more readily interpreted by residual radiometric offset and gain errors) after
applying pointing offsets in the HIRDLS data. The true HIRDLS geometric tangent heights in Ch2, 3, 4,
5, 10 & 11 appear to be a couple of kilometres lower than are given in L1 files. However, data which has
been corrected through the additional radiance adjustment scheme does not show these large pointing
offsets.

Once pointing offsets have been applied, values can be inferred for the radiometric offset and the gain.
These were seen to vary with latitude and with channel. Certain differences are also seen between the
different versions of the calibration correction algorithm applied to the data studied here. Once an
altitude offset had been removed, in the region of the radiance profile not affected by residual radiometric
offset errors, radiances in CO2 (i.e. T channels) were found to generally differ from the simulated
radiance profiles by ~10%, although Channel 3 appears to show larger differences than the other
temperature channels. In the latest radiance adjusted version of the data, the differences in Channel 2 now
appear to be greater than 10 %.

HIRDLS level 2 temperature and ozone results have been compared to data from sondes and ECMWF.
The temperature data shows good agreement with the sonde and ECMWF profiles and is typically within
2K in the stratosphere. There appears to be an oscillation with altitude in the difference profile which is
seen in the comparison with both ECMWF and sonde data, and is consistent between all days considered
here. Comparisons to a smaller number of ozone sondes show reasonable agreement between sondes and
HIRDLS, with structure in the sondes replicated in the HIRDLS data. Comparison to ECMWF data
indicates that peak in the mean ozone profile is at a higher altitude in the ECMWF data than in the
HIRDLS data. The HIRDLS water vapour was seen to be a approximately 2 ppbv higher in the
stratosphere than the ECMWF field.

Further Work
• Undertake more extensive analysis of HIRDLS L1 & L2 T, O3 & H2O data generated by the
current UCB processor* and future versions.
• Investigate potential to generate improved T, O3 & H2O products through use of correlative data
(eg ECMWF) to derive instrument parameters† and also to extrapolate to other HIRDLS channels

References
•
Lambert et al. HIRDLS L2 Algorithm Theoretical Baseline Document,
•
ECMWF: see http: //www.ecmwf.int/research/ifsdocs/
•
UKMO global radiosonde data: see http: //www.badc.nerc.ac.uk/data/radiosglobe/radhelp.html
•
Thomson et al, Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998 – 2000 tropical
ozone climatology 1. Comparison to total Ozone Mapping Spectrometer (TOMS) and ground
based measurements , J. Geophys. Res, Vol 108, No. D2, 8238, 30 January 2003.
•
WOUDC: http://www.woudc.org

* Incorporating state-of-the-art correction schemes for radiometry and pointing
† eg corrections for radiometric offset and gain and pointing offset