![](media/image1.png){width="1.8354166666666667in"
height="1.1909722222222223in"} **SC-HIR-1511A**

**HIRDLS**

**High Resolution Dynamics Limb Sounder**

**Earth Observing System (EOS)**

**Data Description and Quality**

**Version 003**

**(HIRDLS Version 2.04.09)**

**January 2008**

**Oxford University University of Colorado National Center for**

**Department of Atmospheric, Center for Limb Atmospheric Research**

**Oceanic, and Planetary Physics Atmospheric Sounding Boulder,
Colorado**

**Oxford, UK Boulder, Colorado, USA USA**

![](media/image2.wmf){width="1.4930555555555556in"
height="1.0243055555555556in"}

**Table of Contents**

[1.0 Introduction 3](#__RefHeading___Toc189293621)

[2.0 The HIRDLS Experiment 3](#__RefHeading___Toc189293622)

[2.1 The Experiment as Designed 3](#__RefHeading___Toc189293623)

[2.2 The Launch-induced Anomaly 4](#__RefHeading___Toc189293624)

[2.2.1 History and Present Status 4](#__RefHeading___Toc189293625)

[2.2.2 Impact of Loss of Azimuth Scan Capability
4](#__RefHeading___Toc189293626)

[3.0 Revised Operational Scan Patterns 7](#__RefHeading___Toc189293627)

[4.0 Method for Processing HIRDLS Data 7](#__RefHeading___Toc189293628)

[4.1 L0-1 Process (L1PP, L1X, L1C) 8](#__RefHeading___Toc189293629)

[4.2 L2 Pre-processor (L2PP) 8](#__RefHeading___Toc189293630)

[4.3 L2 Cloud Detection (L2CLD) 8](#__RefHeading___Toc189293631)

[4.4 L1-2 Processor (L2) 9](#__RefHeading___Toc189293632)

[5.0 HIRDLS Standard Products 9](#__RefHeading___Toc189293633)

[5.1 Temperature 9](#__RefHeading___Toc189293634)

[5.2 Ozone 13](#__RefHeading___Toc189293635)

[5.3 Nitric Acid 19](#__RefHeading___Toc189293636)

[5.4 Cloud Products 28](#__RefHeading___Toc189293637)

[6.0 Data File Structure and Content 30](#__RefHeading___Toc189293638)

[7.0 Algorithm Changes 31](#__RefHeading___Toc189293639)

[8.0 Acronyms 31](#__RefHeading___Toc189293640)

[]{#__RefHeading___Toc189293621 .anchor}1.0 Introduction

As the following sections describe, the entrance aperture of the High
Resolution Dynamics Limb Sounder (HIRDLS) was largely obscured by a
piece of plastic material that came loose during launch. This resulted
in a loss of signal from the atmosphere, and the addition of extraneous
signals from the plastic blockage material. Because of the position of
the blockage, coverage of Antarctica was lost, as well as loss of the
higher longitudinal resolution expected, although latitudinal resolution
has been increased.

The HIRDLS team has been working since the discovery of this anomaly to
understand the nature of the blockage and, to develop four major
correction algorithms to make the resulting radiances as close as
possible to those originally expected. Corrections for some channels,
and therefore the products retrieved from them, have been successful
earlier than others, which has led to this initial group of retrieved
data products. This document provides a description of the first
fully-released version of data for the entire mission, which includes
retrieved temperature, ozone, and nitric acid, as well as cloud top
pressure.

Work is ongoing to improve the radiances, and therefore the retrievals,
for these channels, as well as for those channels whose products are not
included in this release.

These data are scientifically important, but it is recognized that they
will be improved in future versions. Some of the known problems with the
data are described below, but these are almost surely not the only ones.
This work is ongoing, and further improvements are being developed and
implemented. The HIRDLS team is releasing these data for scientific use
and validation, with the expectation that those who look at the data
will provide feedback on deficiencies that need to be addressed in
future versions, as well as strengths of the data. This document will be
updated as additional data products are released, and as other changes
dictate.

We strongly suggest that anyone wishing to work with the data contact
the HIRDLS team. In the first instance, this should be one of the
Principal Investigators (PI's):

John Gille John Barnett

U.S. P.I. U.K. P.I.

<gille@ucar.edu> j.barnett@physics.ox.ac.uk

[]{#__RefHeading___Toc189293622 .anchor}2.0 The HIRDLS Experiment

[]{#__RefHeading___Toc189293623 .anchor}2.1 The Experiment as Designed

HIRDLS is an infrared limb-scanning radiometer designed to sound the
upper troposphere, stratosphere, and mesosphere to determine
temperature; the concentrations of O~3~, H~2~O, CH~4~, N~2~O, NO~2~,
HNO~3~, N~2~O~5~, CFC11, CFC12, ClONO2, and aerosols; and the locations
of polar stratospheric clouds and cloud tops. The goals were to provide
sounding observations with horizontal and vertical resolution superior
to that previously obtained; to observe the lower stratosphere with
improved sensitivity and accuracy; and to improve understanding of
atmospheric processes through data analysis, diagnostics, and use of
two- and three-dimensional models.

HIRDLS performs limb scans in the vertical, measuring infrared emissions
in 21 channels ranging from 6.12 to 17.76 μm. Four channels measure the
emission by CO~2~. Taking advantage of the known mixing ratio of CO~2~,
the transmittance is calculated, and the equation of radiative transfer
is inverted to determine the vertical distribution of the Planck black
body function, from which the temperature is derived as a function of
pressure. Once the temperature profile has been established, it is used
to determine the Planck function profile for the trace gas channels. The
measured radiance and the Planck function profile are then used to
determine the transmittance of each trace species and its mixing ratio
distribution.

The overall science goals of HIRDLS were to observe the global
distributions of temperature, 10 trace species and particulates from the
upper troposphere into the mesosphere at high vertical and horizontal
resolution. Observations of the lower stratosphere are improved through
the use of special narrow and more transparent spectral channels.

[]{#__RefHeading___Toc189293624 .anchor}2.2 The Launch-induced Anomaly

[]{#__RefHeading___Toc189293625 .anchor}2.2.1 History and Present Status

HIRDLS was launched on the EOS Aura spacecraft on 15 July 2004. All
steps in the initial activation were nominal until the initialization of
the scanner on 30 July 2004 indicated more drag than anticipated, and a
subsequent health test of the scan mechanism indicated that the damping
of the elevation mechanism was \~ 20% greater than on the ground. After
the cooler was turned on and the detectors reached their operating
temperature (\~62 K), initial scans showed radiances much larger and
more uniform than atmospheric radiances, except for a region of lower
signals at the most negative azimuths. The HIRDLS team immediately
identified this as indicating a probable blockage of a large part of the
optical aperture.

Tests confirmed that the blockage emits a large, nearly uniform
radiance, and covers all of the aperture except a small region 47° from
the orbital plane on the side away from the sun.

A number of scan mirror and door maneuvers were conducted in an attempt
to dislodge the obstruction, now believed to be a piece of plastic film
that was installed to maintain the cleanliness of the optics. None of
these maneuvers were successful in improving HIRDLS' view of Earth's
atmosphere.

However, these studies and subsequent operations of the instrument have
shown that, except for the blockage, HIRDLS is performing extremely well
as a stable, accurate and low noise radiometer. These qualities have
allowed the HIRDLS team to develop methods for extracting the
atmospheric radiance from the unwanted blockage radiance and to retrieve
a significant portion of the original science objectives.

[]{#__RefHeading___Toc189293626 .anchor}2.2.2 Impact of Loss of Azimuth
Scan Capability

In its present configuration, HIRDLS can view past the blockage only at
the extreme anti-sun edge of the aperture. Vertical scans are made at a
single azimuth angle of 47° line of sight (LOS) from the orbital plane,
on the side away from the sun. (This differs from the original design,
in which HIRDLS would have made vertical scans at several azimuth
angles, providing orbit-to-orbit coverage with a spacing of \~ 400-500
km in latitude in longitude.) The inability to make vertical scans at a
range of azimuths is a definite loss in data gathering, but not a major
loss of scientific capability for many of the mission goals. Some of the
impacts of the inability to observe at different azimuth angles are:

[Changes in coverage]{.underline}

The single-azimuth coverage is plotted in Fig. 2.1, which shows that
coverage only extends to 65°S, thus missing all of Antarctica and the S.
Polar cap. In the Northern Hemisphere (N.H.) it reaches 82° N in
mid-latitudes, the descending orbit views nearly the same orbit at
midnight as the ascending orbit at 3:00pm 9 orbits or 15 hours later.

![](media/image3.png){width="6.498611111111111in"
height="4.549305555555556in"}

**Figure 2.1 HIRDLS Daily Coverage**

[Inability to View the Same Air Mass as MLS, TES or OMI Within 15
Minutes]{.underline}

The HIRDLS scan track is compared with the MLS scan track in Fig. 2.2.
HIRDLS views nearly the same volume as MLS at night (descending part of
orbit, left part of figure), but on successive orbits; thus HIRDLS views
the same volume 84 minutes earlier than MLS (one orbit, or 99 minutes,
minus the 15 minutes that separates MLS views ahead of the S/C and
HIRDLS measurements behind the S/C). In the daytime (ascending part of
orbit, right side of plot), HIRDLS observations fall 17° to the east of
the MLS track in the same orbit, or 8° to the west of the MLS track in
the previous orbit. Especially in the daytime, this difference impacts
making comparisons, the planning of correlative measurements, and the
opportunities to do combined science. However, comparisons and science
can easily be done at night where desired.

A corollary feature is that, in the daytime, HIRDLS and MLS combined
observe more longitude at a given latitude, which will improve the
spatial resolution. At night, together they look at the same volume 84
minutes apart, increasing the temporal resolution.

![](media/image4.png){width="5.998611111111111in"
height="4.199305555555555in"}

**Figure 2.2. Comparison of 3 orbits of HIRDLS (red) measurement
locations to MLS (green).** HIRDLS is measuring the atmosphere at one
azimuth angle (i.e., -47° from the orbit plane). The day and night
portion of the orbits are on the right and left side of the figure
respectively. During the day part of the orbit, HIRDLS is trailing MLS
by one orbit (99-minutes). During the night part of the orbit, the
coincidence is much better and HIRDLS is leading MLS by one orbit.

[Some compensating effects]{.underline}

With the azimuthal limitation, the profiles will have closer latitudinal
separation (\~100 km), facilitating gravity wave studies. It has also
been suggested that transects through tropopause folds might be improved
by continuous views at one azimuth.

Since HIRDLS views a long way off the orbital track, as seen above, it
measures at a different local time from MLS, TES and OMI. At the
northern and southern extremes it means that HIRDLS will get data at a
significantly different local time from the other instruments, which
could help constrain data assimilation models.

# 

[]{#__RefHeading___Toc189293627 .anchor}**3.0 Revised Operational Scan
Patterns**

The limited angle at which HIRDLS can see the atmosphere necessitates a
revision to previously planned scan patterns. Scans of the atmosphere
are done in the region in which the view of the atmosphere is the
clearest, at -23.5° azimuth shaft angle, or -47° LOS from the orbital
plane (on the side away from the sun).

Science Modes

Scan Table 30 (21 January 2005-28 April 2005) Initial scan used more
rapid vertical scan speed, which generated larger amplitude oscillations
in the signals. This scan also made vertical scans at an LOS azimuth
angle of -44.8°, which were found to be inferior to those at -47°.

Scan Table 13: (28 April 2005- 24 April 2006). Upper and lower limits of
scans vary around the orbit, following Earth's oblateness. This was
discovered to cause different types of oscillations to be seen in the
signals, complicating attempts to remove these artifacts.

Scan Table 22: (25 April 2006 to 3 May 2006) Similar to ST 23, but with
lower space-ward limit on the scans.

Scan Table 23: (Used since 4 May 2006) It makes slower vertical scans;
27 vertical up and down scans of \~ 15.5 sec. duration each, followed by
a 1-2 second space view before the next 27 scans. To facilitate removal
of the oscillations, the space-ward and earth-ward limits of the scans
are at fixed elevation scan angles.

[]{#__RefHeading___Toc189293628 .anchor}4.0 Method for Processing HIRDLS
Data

The modified science scans described in Sec. 3 and the need to account
for blockage of the scene and radiance from the blockage require
substantial modifications to the operational data processing. A diagram
of the flow of data in the HIRDLS processing is shown in Figure 4.1.

**Figure 4.1 HIRDLS Processing Flow**

[]{#__RefHeading___Toc189293629 .anchor}4.1 L0-1 Process (L1PP, L1X,
L1C)

In the L0-1 suite of processor, L1PP corrects an occasional problem with
the time in L0 data (raw data counts). L1X carries out the modified
calibration, and geolocation, while L1C applies the 3 main correction
algorithms to remove the effect of the blockage. Overall, the L0-1
processor creates a time series of calibrated radiances blocked into
profiles as well as housekeeping data necessary to the further data
processing.

[]{#__RefHeading___Toc189293630 .anchor}4.2 L2 Pre-processor (L2PP)

The L2PP process takes the time series of radiance profiles from L1,
separates it into individual geolocated vertical scans, determines the
vertical registration in altitude, and performs low-pass filtering to
condition the radiances for retrieval by the L1-2 software.

[]{#__RefHeading___Toc189293631 .anchor}4.3 L2 Cloud Detection (L2CLD)

The L2CLD routine screens for clouds based on detection of radiance
perturbations from the average clear sky case. Cloud tops are located
and identified.

[]{#__RefHeading___Toc189293632 .anchor}4.4 L1-2 Processor (L2)

The L2 step accepts the conditioned radiance data from the L2CLD, and
performs the retrievals through a series of iterations. This code is
designed to be flexible in handling combinations of radiance channels to
retrieve the HIRDLS target species in a user-defined sequence. One of
the major features is the use of ancillary GMAO data to determine
temperature gradients along the line of sight, which are incorporated to
yield an improved retrieval. This processor is described in the L1-2
ATBD.

[]{#__RefHeading___Toc189293633 .anchor}5.0 HIRDLS Standard Products

[]{#__RefHeading___Toc189293634 .anchor}5.1 Temperature

**Species:** Temperature

**Data Field Name** Temperature

**Useful Range:** 400 -- 1hPa

**Vertical Resolution** 1.2 Km

**Contact:** John Gille

**Email:** <gille@ucar.edu>

**Validation paper** Gille *et al*., The High Resolution Dynamics Limb
Sounder (HIRDLS): Experiment Overview, Results and Validation of Initial
Temperature Data -- First results, in review, *J. Geophys. Res*., 2008

Systematic Biases

HIRDLS temperatures have been compared to several data sets in an effort
to determine the extent and magnitude of any biases. The results shown
here summarize results described by Gille et al. (2008). An important
comparison is between radiosondes and nearby HIRDLS temperatures. Fig.
5.1.1 shows comparisons among high resolution radiosonde profiles and 2
nearby HIRDLS retrievals at St. Helena (15.9°S in the Atlantic) and
Gibralter. The differences in space and time are given. Points to note,
in addition to the good agreement, are the way the HIRDLS retrievals
follow the small scale vertical structure in the radiosonde data, and
HIRDLS' apparent small warm bias at the tropical tropopause. Statistics
of such comparisons at Gibralter, a representative site, are shown in
Figure 5.1.2. The green line indicates that HIRDLS is between 1-2 K
warmer than the sondes from 100 to above 10 hPa, but becoming as much as
2K cooler down to \~ 400 hPa. The red and blue dotted lines are
discussed below. Separate analyses show HIRDLS is sensitive to
temperature variations with wavelengths as small as 2 km

A comparison with the more extensive ECMWF global analyses are shown in
Figure 5.1.3. From this we see that HIRDLS V003 temperatures are between
1-2K warmer than ECMWF temperatures from 100 to 6 hPa. Above and below
these levels HIRDLS temperatures become lower, and overall are within ±
2K from 400-1 hPa.

Temperature precision

One definition of precision is that it is the standard deviation (s.d.)
of repeated measurements of the same quantity. Following this, one
approach to determining the precision of HIRDLS temperatures is to
compare repeated views of the same atmosphere, and derive an estimate of
the noise from the statistics of their differences. At 80°N and 63°S,
successive scan tracks pass over the same points on the Earths' surface
one orbit (99 minutes) apart.

> ![](media/image5.png){width="2.6465277777777776in"
> height="3.7576388888888888in"}
> ![](media/image6.png){width="2.5590277777777777in"
> height="3.7576388888888888in"}

**Figure 5.1.1.** Temperature comparisons between radiosonde profiles at
St. Helena (left) and Gibralter. Black lines are high resolution
radiosondes, blue and magenta are two nearby HIRDLS retrievals.
Differences in distances and times are shown.

Results of this type of analysis are shown in Figure 5.1.4 for 63S for 3
days each season. The pairs of HIRDLS profiles are separated on average
by 33 km. The short dashed line shows the precision derived from the
differences between the 2 paired HIRDLS profiles after removing the
small meteorological changes shown by changes in the GMAO data (RMS
value ≤ 0.1K up to 40 km, increasing to 0.8K at 58 km) for December. The
thick solid line shows the precision calculated from the parameters in
the retrieval code. The agreement between the predicted and observed

precision at the minimum near 20 km, \~ 0.6 K, is reasonably good for
this summer season, when a minimum of small-scale dynamical activity is
expected. The curves are similar in shape, although the empirical curve
increase more rapidly with altitude, presumably because it includes the
effects of small-scale motions (e.g. gravity waves) with rapid time
scales that have changed in the 99 minute interval between observations.
Northern Hemisphere summer values at 80°N are very similar. The larger
values of the paired precisions, especially at high altitudes during
southern winter and spring, when small scale motions should be larger,
support this interpretation.

> ![](media/image7.png){width="3.325in" height="4.608333333333333in"}

**Figure 5.1.2**. Statistics of HIRDLS minus sonde differences for
Gibralter. Green line shows mean differences, blue dots show ± 1
standard deviation of the differences, while red dots show ± 1 standard
deviation predicted by retrieval algorithm.

The s.d.'s of GMAO and HIRDLS retrieved temperatures for 3 days in July
2006 are also shown, indicated by the thin solid and dotted lines
respectively. The HIRDLS and GMAO variations are similar up to about 40
km, above which the GEOS5 data show larger variations, perhaps
reflecting a lack of observational data at those levels. This shows the
magnitude of the meteorological variations. These curves show that the
noise on the HIRDLS temperatures is much less than the atmospheric
variation, making clear that HIRDLS is able to track the meteorological
variations to this kind of fidelity.

These values were indicated by the red dotted lines in Figure 5.1.2,
where they were seen to be smaller than the blue dotted lines. Inclusion
of the stated precision of the radiosondes does not explain the
differences. It is believed that most of the difference comes from
differences in time and space between the radiosondes and the HIRDLS
profiles, as well as effects of gradients along the HIRDLS line of sight
which are not completely corrected. On the other hand, the differences
between the ECMWF and HIRDLS data are also larger than the quadrature
sum of HIRDLS and ECMWF precisions, although the latter are interpolated
in time and space to the HIRDLS locations. In this case, the errors of
the interpolations, and the coarser vertical resolution, as well as
gradient effects, are apparently the source of the differences.

> ![](media/image8.png){width="4.159722222222222in"
> height="6.004166666666666in"}

Figure 5.1.3 Results of comparison of HIRDLS temperatures with analyzed
ECMWF temperatures interpolated to HIRDLS measurement locations over the
full latitude range of HIRDLS observations. HIRDLS minus ECMWF
differences ± 1 standard deviation of the differences.

![](media/image9.png){width="5.557638888888889in"
height="4.343055555555556in"}

Figure 5.1.4. Empirically determined precision for HIRDLS determined
from differences of paired profiles (dashed line) at latitude cross-over
points at 63°S for 3 days each in in March, July, September and
December, compared to total precision calculated by the retrieval code
(solid line). The standard deviations of the GMAO and HIRDLS data for
July are also shown. The similarity between the shapes of the summer
curve and the predicted values gives us confidence that the predicted
values are a reasonable indication the true repeatability of HIRDLS
temperatures.

[]{#__RefHeading___Toc189293635 .anchor}5.2 Ozone

**Species** Ozone (O3)

**Data Field Name:** O3

**Useful Range:** 1hPa -- 100+ hPa

**Vertical Resolution:** 1.2 - 2 km

**Contact:** Bruno Nardi,

**Email:** [nardi@ucar.edu](../nardi@ucar.edu)

**Validation Paper:** Nardi *et al*., Initial Validation of Ozone
Measurements from the High Resolution Dynamic Limb Sounder (HIRDLS), *J.
Geophys. Res*., in press 2008.

**Early Results and Validation**

[Validation and Accuracy]{.underline}

Comparisons with ground based, balloon-borne and satellite correlative
measurements indicate that HIRDLS ozone shows good accuracy between 1
hPa - 100 hPa with accuracy of 5% to 10% between 1 hPa - 20 hPa (Figures
**5.2.1-5**). Comparisons with ground-based lidar indicate agreement as
good as \<5% in this region.

![](media/image10.png){width="3.0in"
height="2.591666666666667in"}![](media/image11.png){width="3.0in"
height="2.591666666666667in"}

**Figure 5.2.1**. Ozone difference between 97 SHADOZ Network (low
latitude) ozonesonde profiles and 1042 coincident HIRDLS profiles, in
terms of mixing ratio, left and in terms of percent (of sonde values),
right. Shown are the mean difference (solid blue), the standard
deviation (dashed blue) and the individual differences (black dots) from
which these are derived.

![](media/image12.png){width="3.0in"
height="2.591666666666667in"}![](media/image13.png){width="3.0in"
height="2.591666666666667in"}

**Figure 5.2.2**. Ozone difference between 73 MLO lidar profiles and the
659 coincident HIRDLS profiles, in terms of mixing ratio, left, and
percent (of sonde values), right.

![](media/image14.jpeg){width="2.9902777777777776in"
height="2.4743055555555555in"}![](media/image15.jpeg){width="2.7472222222222222in"
height="2.466666666666667in"}

![](media/image16.jpeg){width="2.9902777777777776in"
height="2.466666666666667in"}![](media/image17.jpeg){width="2.751388888888889in"
height="2.466666666666667in"}

**Figure 5.2.3**. ACE satellite solar occultation comparisons since May
2006 show agreement to within \~10% (bottom) between 3 - 25 hPa in the
Northern Hemisphere (left) and between 1 - 100 hPa in the Southern
Hemisphere (right), with HIRDLS biased generally low .

![](media/image18.jpeg){width="6.0in" height="1.9722222222222223in"}

**Figure 5.2.4**. A single orbit curtain plot comparison with MLS (Left:
HIRDLS; Middle: MLS; Right: Difference) indicates HIRDLS is tends to be
10% lower earthward of \~30 hPa and is generally 10% high above \~1hPa
and rapidly increases. HIRDLS ozone tends to be slightly lower than MLS,
especially in the (night-time) descending node (RHS of panels).

![](media/image19.png){width="2.173611111111111in"
height="1.820138888888889in"}![](media/image20.png){width="1.9180555555555556in"
height="1.820138888888889in"}![](media/image21.png){width="1.9729166666666667in"
height="1.820138888888889in"}

![](media/image22.png){width="2.173611111111111in"
height="2.7111111111111112in"}![](media/image23.png){width="1.9180555555555556in"
height="2.7111111111111112in"}![](media/image24.png){width="1.9729166666666667in"
height="2.7111111111111112in"}

**Figure 5.2.5.** Mercator representations of the ozone percent
difference between HIRDLS and collocated MLS (v2.2) for 2006-July-15, at
pressure levels, 1 hPa, 3 hPa, 10 hPa, 31 hPa, 51 hPa and 100 hPa, as
indicated over each sub-plot. The ozone fields for this day are
relatively quiescent. Clearly illustrated in this and the previous
figure is that in the tropics earthward of \~60 hPa HIRDLS ozone tends
to be very high with respect to MLS. This is probably related to the
presence of high cloud-tops.

**Vertical Resolution**

Comparisons with ozonesondes and lidar give strong indication that
HIRDLS is capable of resolving fine vertical ozone features (1.2 km to 2
km) in region 1hPa to \>100+hPa. Numerous coincident profiles with
better than 500 km coincidence show similar layered features. Figure
**5.2.6** shows comparisons with ozonesonde and lidar profiles.

![](media/image25.png){width="3.0in"
height="2.501388888888889in"}![](media/image26.png){width="3.0in"
height="2.501388888888889in"}

**Figure 5.2.6** Comparisons of HIRDLS profiles with: ozonesonde profile
from La Reunion Island (left), and with MLO lidar profiles (right).

**Precision**

The variability of the ozone data is plotted on in equivalent latitude-
potential temperature bins for NH and SH summer in figure **5.2.7**.
This variability includes both the atmospheric variability and the
random error of the HIRDLS data.  The atmospheric variability is a
minimum In the summer seasons, so that the variability shown in the high
latitude summer is an upper bound to the HIRDLS precision. This is
estimated to be 5-10% between 1 and 50 hPa (500-2000K in potential
temperature.

**Data Caveats**

The data are most reliable over the pressure range 1 to \> 100 hPa at
latitudes poleward of ± 30 degrees, and from 1 to 51 hPa at low
latitudes (-30° \< lat \< 30°).

For p \>100 hPa: HIRDLS ozone may be useable sometimes, to several
hundred hPa, particularly at mid-latitude and in the absence of local
cloud features. Vertical features may be captured, but systematic bias
is unknown due to lack of statistics; HIRDLS ozone here is expected to
be low in the absence of clouds, and very high where high clouds are
present.

![](media/image27.png){width="3.1972222222222224in"
height="2.2215277777777778in"}![](media/image28.png){width="3.073611111111111in"
height="2.2680555555555557in"}

![](media/image29.png){width="3.1972222222222224in"
height="2.2680555555555557in"}![](media/image30.png){width="3.073611111111111in"
height="2.2680555555555557in"}

**Figure 5.2.7**. An estimate of the HIRDLS ozone precision based on
ozone variability in equivalent latitude bins. High precision estimates
at winter-hemisphere high latitudes (LHS of top plots; RHS of bottom
plots) are indicative of large atmospheric variability rather than of an
actual deterioration of HIRDLS precision.

Precision: [Negative]{.underline} values of calculated retrieval total
error (\"O3Precision\") indicate a \>50% contribution to the error from
a priori, and thus a strong *a priori* influence in the result. Ozone
values containing negative precision should not be used, or used only
with caution.

Clouds: Any profile for which the \'CloudTopPressure\' parameter is a
non-zero number should be used with caution. Ozone values may be
severely effected by the presence of clouds at pressures greater than or
equal to the \'CloudTopPressure\', and possibly also at lower pressures.
Ozone corrupted by clouds appear as ozone spikes which can be variable
in magnitude and thickness. These are especially prevalent at pressure
\> \~60 hPa at low latitudes (-30\<Lat\<30).

**Artifacts**

Lower Stratosphere: HIRDLS ozone has a systematic generally low bias,
between roughly 30 hPa to 100 hPa, with values of up to 10%. The region
of roughly positive 5% HIRDLS bias exists in a limited pressure range
within 10-30 hPa at nearly all latitudes; this is observed by
comparisons with sondes (SHADOZ, WAVES), lidars (MLO, TMF) and
satellites (ACE, MLS).

Upper Troposphere / Lower Stratosphere: Two of the three ozone channels,
10 and 12, are very sensitive to clouds in the upper troposphere / lower
stratosphere. Even when the cloud algorithm is turned on, spikes are
present in some of the ozone profiles, primarily between 80-200 hPa but
also at pressures as low as 60 hPa; these are probably related to the
presence of clouds detected at these or greater pressure levels. This is
particularly true of low latitude profiles.

Priorities for future data improvements are the reduction of the
systematic low bias of ozone at pressures greater than \~50 hPa, and
improvement of removal of the cloud effect, especially at low latitudes.

[]{#__RefHeading___Toc189293636 .anchor}5.3 Nitric Acid

**Species:** Nitric Acid (HNO~3~)

**Data Field Name:** HNO~3~

**Useful Range:** 161 hPa -- 10 hPa

**Vertical Resolution:** \~ 1 km

**Contact:** Douglas E. Kinnison, NCAR-HIRDLS

**Email**: <dkin@ucar.edu>

**Validation Paper** Kinnison *et al*., Global observations of HNO~3~
from the High Resolution Dynamics Limb Sounder (HIRDLS) -- First
results, *J. Geophys. Res*., in press 2008.

**Early Results and Validation:**

HIRDLS HNO~3~ data are generally good between 28 April 2005 to the
present over the latitude range of 64ºS to 80ºN and pressure range \~100
hPa to 10 hPa, with some profiles, depending on latitude, having useful
information between 100 hPa to 161 hPa. HIRDLS observations before 28
April 2005 are marginal in quality and not recommended for science
applications. The HIRDLS HNO~3~ vertical resolution is approximately 1
km, but does vary with altitude and latitude (Figure 5.3.1). The HIRDLS
HNO~3~ precision was estimated in two ways. The first approach examined
the precision as derived by the HIRDLS level-2 retrieval algorithm. We
called this the "theoretical" precision. The second approach derived the
precision by examining HIRDLS HNO~3~ data in regions of low atmospheric
variability. We call this the "measured" precision. The components of
the theoretical precision (forward model, measurement, and *a priori*
errors) are shown in Figure 5.3.2. This figure shows that for the 100
hPa - 10 hPa (18 - 32 km) region, the contribution of the *a priori* is
minimal. However, in the deep tropics, the altitude region where the *a
priori* contribution is minimal is smaller (\~40 hPa - 10 hPa; \~22 km -
32 km). The HIRDLS HNO~3~ at pressures \<10 hPa (\>32 km) is
characterized by large uncertainties and should be used with caution.
The individual profile "measured" precision is between 10 - 15%, but can
be much larger if the HNO~3~ abundance is low or outside the \~100 hPa
to 10 hPa range (Figure 5.3.3). Global results are compared with the
HNO~3~ observations from version 2.2 of the EOS Aura Microwave Limb
Sounder (MLS) and it is found that large-scale features are consistent
between the two instruments (Figures 5.3.4 and Figures 5.3.5). HIRDLS
HNO~3~ is biased 0-20% low relative to Aura MLS in the mid-to-high
latitudes and biased 50% high in the tropical stratosphere (Figure
5.3.6). More work will be needed to see whether this high bias in the
tropics is an issue with the HIRDLS or Aura MLS HNO~3~ observations.
HIRDLS HNO~3~ is also compared with Atmospheric Chemistry Experiment
Fourier Transform Spectrometer (ACE-FTS) (Figures 5.3.7 and 5.3.8). In
these, high latitude comparisons, the HIRDLS HNO~3~ data are biased
10-30% low, depending on altitude.

![](media/image31.wmf){width="5.997222222222222in"
height="7.501388888888889in"}

**Figure** **5.3.1**: HIRDLS HNO~3~ averaging kernels and full width
half maximum (FWHM) vertical resolution profiles are shown on 21 June
2006 at 1.5°N, 45.3°N, and 63°S. The left column shows the averaging
kernels (green lines) and the integrated area under each kernel (red
line). Where values of unity indicate that all of the information for
that vertical region is coming from the measurements and not the *a
priori* estimate. The right column represents the vertical resolution as
derived from the FWHM of each kernel (blue line).

> ![](media/image32.wmf){width="4.277777777777778in"
> height="8.006944444444445in"}

**Figure 5.3.2**: Shown is the HIRDLS HNO~3~ "theoretical precision"
displayed as the fractional error contribution for 21 June 2006. The
top, middle, and lower panels are for latitudes 1.5°N, 45.3°N and 63°S
respectively (same regions as Figure 5.3.1). The forward model error
(blue line), the *a priori* error (green line), and the measurement
error (red line), along with the total retrieval error (black line) are
shown.

![](media/image33.wmf){width="5.683333333333334in" height="4.125in"}

**Figure 5.3.3**: Shown is the standard deviation of HIRDLS HNO~3~
retrievals in a potential temperature and equivalent latitude coordinate
system. Results are given for 20 June 2006. Standard deviation units are
given in terms of percentage of HNO~3~ mixing ratio. The black lines
highlight the 10% contour. In this figure we call this estimate of the
precision the "measured precision". To examine the observed variability,
24-hours of HIRDLS HNO~3~ were interpolated onto a potential temperature
grid and then assembled into 4° equivalent latitude bins centered on 1°
increments (essentially 4° wide box-car smoothing in equivalent
latitude). The equivalent latitude was derived from Met Office potential
vorticity data. In addition, in order to compare air parcels with
similar insolation, an additional criterion was imposed. This criterion
limits measurements to within 5 degrees of the average geographic
latitude in each equivalent latitude bin. The potential temperature
range is from 300 to 1000 K or approximately 10 to 35 km. The minimum
observed standard deviation is approximately 10 % in this figure. This
occurs in the summer hemisphere, where atmospheric variability is known
to be a minimum and HNO~3~ is abundant (typically \>4 ppbv). The
theoretical precision (not shown) from the HIRDLS level-2 processor
approaches 5% in this region. This figure supports the derivation of the
theoretical precision obtained from the level-2 retrieval. Based on this
analysis, the theoretical precision may be an underestimate of the true
precision, but the error would be on the order of 5% in regions of
relatively high abundance of HNO~3~.

![](media/image34.wmf){width="5.995138888888889in" height="6.625in"}
**Figure 5.3.4.** Latitude-height cross sections are shown for HIRDLS
and MLS HNO~3~ (ppbv). The top panel is a zonal mean average of four
days (in May 2006). The middle panel is a zonal mean average of eight
days (in October 2006). The bottom panel is a zonal mean average of 27
days (in March 2007). The percentage difference of (HIRDLS -- MLS / MLS)
is also shown. The zero percentage difference contour line is the
transition from white to blue.

![](media/image35.wmf){width="5.996527777777778in"
height="5.392361111111111in"}

**Figure 5.3.5**. Longitude-latitude cross sections of HIRDLS and MLS
HNO~3~ (ppbv) on 28 October 2006. The top and bottom rows are for 51.1
hPa and 31.6 hPa respectively. The percentage difference of (HIRDLS --
MLS / MLS) is also shown. The zero percentage difference contour line is
the transition from white to blue.

![](media/image36.wmf){width="5.999305555555556in"
height="4.649305555555555in"}

**Figure 5.3.6.** Latitude-height cross sections of HIRDLS-MLS
coincidences for two different HIRDLS scan tables. The left column is
for scan table 13; the right column is for scan table 23. There were
165,513 and 67,595 coincidences used to create the panels for scan
tables 13 and 23 respectively. The top row shows the percentage
difference of (HIRDLS -- MLS / MLS). The bottom row shows the absolute
differences in volume mixing ratio units (ppbv).

![](media/image37.wmf){width="5.999305555555556in"
height="4.179861111111111in"}

**Figure 5.3.7**. Latitude-longitude cross sections showing ACE-FTS
HNO~3~ measurement locations (blue) and HIRDLS HNO~3~ measurement
locations (red). Coincidences are defined as occurring within 2-hours in
time and 500 km. The coincidences shown here are for scan table 23.
There were a total of 150 coincidences between 19 May 2006 and 31
October 2006.

![](media/image38.wmf){width="5.995833333333334in"
height="4.667361111111111in"}

**Figure 5.3.8.** Profile differences of HIRDLS and ACE-FTS for the scan
table 23 measurements. Comparisons shown here were between 19 May 2006
and 31 October 2006 (see Figure 5.3.5). Coincident HIRDLS profiles were
averaged together and then subtracted from a single ACE-FTS profile. The
top row shows the percentage difference of (HIRDLS -- ACE-FTS /
ACE-FTS). The bottom row shows absolute differences in volume mixing
ratio (ppbv). The mean (solid red) and standard deviation (dashed red)
of the differences for all coincidences are shown. The individual
differences from which these are derived are the horizontally
distributed layers of small black dots. The thin red lines bracketing
the mean are the uncertainty in the mean (standard deviation divided by
the square root of the number of points).

**Acknowledgements:** The HIRDLS team would like to thank both the EOS
Aura MLS science and the ACE science teams for making their data
available for HIRDLS validation.

[]{#__RefHeading___Toc189293637 .anchor}5.4 Cloud Products

**Species** Cloud Top Pressure

**Data Field Name:** CloudTopPressure

**Useful Range:** 422-10 hPa

**Vertical Resolution**: 1km

**Contact:** Steven Massie

**Email:** <massie@ucar.edu>

**Validation Paper** Massie *et al*., High Resolution Dynamics Limb
Sounder observations of polar stratospheric clouds and subvisible
cirrus, *J. Geophys. Res*., VOL. 112, D24S31, doi:10.1029/2007JD008788,
2007.

HIRDLS data files contain cloud flags and cloud top pressures. Details
of the determination of cloud top pressures and cloud flags are
discussed in Massie et al \[2007\].

Cloud flag data is contained in the "12.1MicronCloudAerosolFlag" data
variable. Cloud flags are stated at each pressure level when pressures
correspond to altitudes between 5 and 30 km altitude. Cloud flag values
are 0 (no clouds), 1 (unknown cloud type), 2 (cirrus layer), 3
(extensive Polar Stratospheric Cloud), and 4 (opaque). If the cloud flag
is nonzero, then this indicates that the radiance at that pressure is
measurably different from the clear sky radiance profile. Note that the
total number of PSCs is equal to the number of cloud flags with values
of either 1 or 4.

Comparisons of clear sky and individual radiance profiles of the various
cloud types are presented in Figure 5.4.1. Note that radiance
perturbations are substantial for several cloud types, since gas opacity
in HIRDLS Channel 6, the 12 µm "infrared window" channel, is very low.
Any cloud opacity along the HIRDLS limb-view tangent ray path produces a
substantial 12 µm radiance signal.

The cloud top pressure (i.e. the 'CloudTopPressure' variable in the
archived data file) is determined in the following manner. For a single
day's set of radiance profiles, the clear sky radiance profile for
HIRDLS channel 6 is calculated by an iterative technique for several
latitude bands. For the first iteration, the average profile, its
standard deviation, and associated gradients from 5 to 30 km altitude,
are calculated summing over all profiles. For the second iteration,
profiles greater than the mean are removed from the ensemble average
(based on the fact that a cloudy radiance profile deviates from the
average curve). New standard deviations and associated gradients are
recalculated. The iterative process continues for five iterations.

Once the clear sky radiance profile is calculated, we determine the
altitude level at which cloud radiance perturbations are first noted.
The cloud top pressure (in hPa) is the pressure derived by the
operational retrieval that corresponds to the cloud top altitude. Since
the v2.04.09 cloud top altitudes are on an altitude grid with one
kilometer spacing, the cloud top pressure has a granularity reflective
of the altitude grid spacing, e.g. for pressure level P, the cloud top
pressure could be large by ∆P∼ P ( exp (1 km/ 7 km) -- 1.0) \~ 0.15 P.

![](media/image39.wmf){width="5.814583333333333in"
height="6.197222222222222in"}

**Figure 5.4.1**. Four representative Channel 6 (12.1 µm) radiance
(single squares) and clear sky average profiles (dotted curves) on
January 27 2005, a) clear sky (15.57º N, 216.20º E), b) PSC (68.31º N,
343.41º E), c) tropical cirrus layer (4.32º N, 220.00º E), and d) opaque
tropical cloud (16.79º S, 223.72º E) cases. Panels a, b, c, and d
correspond to cloud flags equal to 0, 3, 2, and 4, respectively.

The distributions of time averaged cloud statistics, as determined by
the HIRDLS cloud- detection algorithm, compare very well to
distributions derived from correlative data. HALOE and HIRDLS time
averaged cloud top pressures have a correlation coefficient of 0.87 and
0.93 in the tropics and mid-latitudes, respectively. SAGE III and HIRDLS
cloud top pressure values in 2005 have a correlation coefficient of 0.85
when the distance between observations is less than 100 km and the time
difference is less than six hours. HIRDLS cloud top pressures are 20%
lower and 15% higher than SAGE III and HALOE cloud top pressures,
respectively. Time series of the T \< 195 K hemispherical area, on the
450 K potential temperature surface, and the total number of PSCs
observed by the HIRDLS experiment in January and February 2005 have a
correlation coefficient of 0.92. Finally, HALOE and HIRDLS normalized
distributions of cloud counts, expressed as a function of outgoing
longwave radiation (OLR), have a correlation coefficient of 0.99.

[]{#__RefHeading___Toc189293638 .anchor}6.0 Data File Structure and
Content

**Contact:** Cheryl Craig

**Email:** <cacraig@ucar.edu>

HIRDLS Level 2 data are stored in the HDF-EOS5 format and the fields are
as described in the \_HDF-EOS Aura File Format Guidelines\_ document \^1
.  These data files can be read via C/C++ or Fortran using either the
HDF-EOS5 or HDF5 library. HIRDLS has developed both an IDL routine
\"get_aura\" and a set of Fortran90 routines to access the HIRDLS Level
2 data.  Both of these routines will be available for download via the
HIRDLS web site,
[www.eos.ucar.edu/hirdls/](http://www.eos.ucar.edu/hirdls/), starting in
2008.  The routines can also be supplied via email upon request.

Users should obtain the pre-compiled HDF5 library for their operating
system, if possible, otherwise source code is also available (see
[http://hdf.ncsa.uiuc.edu](http://hdf.ncsa.uiuc.edu/)). These are
prerequisite in order to compile the HDF-EOS5 library (see
<http://www.hdfeos.org/>). Both libraries are needed to fully access the
Aura HIRDLS data files. For additional help contact the GES DISC at

[help-disc@listserv.gsfc.nasa.gov](../help-disc@listserv.gsfc.nasa.gov%20)
or telephone 301-614-5224.

Each HIRDLS Level 2 file contains one day\'s worth of data and contains
all species that HIRDLS measures. A number of the fields are filled
completely with missing values until correction algorithms are refined
for these species. For users who require only a subset of the HIRDLS
species, the Goddard DISC has the ability to subset data before
distributing it to users. Contact the DISC directly for more information
on this service.

Individual HIRDLS data values for a product are stored in fields labeled
with the species name (see the appropriate section above for the exact
Data Field Name). The estimated precision of each data point is a
corresponding field named *Species*Precision (for instance, Temperature
and TemperaturePrecision). Two additional fields for each species,
*Species*NormChiSq and *Species*Quality, are both filled with missing
for V003. CloudTopPressure does not have Precision, NormChiSq or Quality
fields.

There are two time fields in the HIRDLS data file, *Time* and
*SecondsInDay*. *Time* is stored in TAI time (seconds since the epoch of
UTC 12 AM 1-1-1993). This time includes leap seconds and can cause
problems with simplistic conversions. For this reason, HIRDLS is also
storing *SecondsInDay* which is seconds since midnight of the data day.
Leap seconds do not pose a problem when using this field. Note that the
first data point may be negative which indicates a time stamp before
midnight. This is the case for scans which span a day boundary.

^1^<http://www.eos.ucar.edu/hirdls/HDFEOS_Aura_File_Format_Guidelines.pdf>

[]{#__RefHeading___Toc189293639 .anchor}7.0 Algorithm Changes

[HIRDLS Version]{.underline} [DISC Version]{.underline}
[Changes]{.underline}

2.00 001 \[Baseline\]

2.01 Modified to process Scan Table 22

2.02.07 002 Modified to process Scan Tables 30, 13, 22 and 23

2.04.09 003 Modified to include more precise geo-

location, updated cloud detection, updated

calibration constants, and bug fixes.

[]{#__RefHeading___Toc189293640 .anchor}8.0 Acronyms

  ---------- ------------------------------------------------------------
  ACE        Atmospheric Chemistry Experiment

  ACD-FTS    Atmospheric Chemistry Experiment Fourier Transform
             Spectrometer

  ATBD       Algorithm Theoretical Basis Document

  DISC       Data and Information Services Center

  ECMWF      European Center for Medium range Weather Forcasting

  EOS        Earth Observing System

  FWHM       Full Width Half Maximum

  GES DISC   Goddard Earth Sciences Data and Information Services Center

  GMAO       Goddard Modeling and Assimilation Office

  HALOE      Halogen Occultation Experiment

  HDF5       Hierarchical Data Format Version 5

  HDF-EOS5   HDF for EOS Version 5

  HIRDLS     HIgh Resolution Dynamics Limb Sounder

  HIRDLS1C   HIgh Resolution Dynamics Limb Sounder Level 1Data

  IDL        Interactive Data Language

  L0         Level 0

  L0-1       Level 0-1

  L1         Level 1

  L1-2       Level 1-2

  L1C        Level 1 Corrector

  L1PP       Level 1 Pre-processor

  L1X        Level 1 Excellorator

  L2         Level 2

  L2CLD      Level 2 Cloud Detector

  L2PP       Level 2 Pre-processor

  LOS        Line Of Sight

  MLO        Mauna Loa Observatory

  MLS        Microwave Limb Sounder

  NASA       National Aeronautics and Space Administration

  NCAR       National Center for Atmospheric Research

  OMI        Ozone Monitoring Instrument

  PIs        Principal Investigators

  PSC's      Polar Stratospheric Clouds

  RMS        Root Mean Square

  SAGE III   Stratospheric Aerosol and Gas Experiment III

  s.d.       Standard Deviation

  SHADOZ     Southern Hemisphere ADditional OZonesondes

  ST         Scan Table

  TAI        International Atomic Time

  TES        Tropospheric Emission Spectrometer

  TMF        Table Mountain Facility

  UK         United Kingdom

  USA        United States of America

  UTC        Coordinated Universal Time

  WAVES      Water Vapor Validation Experiment Satellite/Sondes
  ---------- ------------------------------------------------------------