# Presentation: 062958

## Validation of the HIRDLS Vertical Resolution using GPS Radio Occultation Data

- John Barnett, Scott Osprey, Chris Hepplewhite,  Oxford University
- John Gille,  R. Khosravi, University of Colorado and National Center for Atmospheric Research
** *****plus the whole of the rest of the HIRDLS Team***

- Aura Science Team Meeting, October 2008, Columbia Maryland.

**Abstract:** The Aura HIRDLS instrument was always intended to provide measurements with a high vertical resolution of order 1 km and horizontal resolution that is commensurate for smaller scale atmospheric phenomena such as gravity waves. The launch problem which partially blocked the HIRDLS optical path resulted in a different placement of vertical profiles, but with the vertical resolution potentially unaffected. Profiles are now approximately 100 km apart along-track and the orbit spacing cross track.  Validation of the vertical resolution is difficult because few other measurements provide that resolution sufficiently closely in time for atmospheric features not to have changed. However, the COSMIC suite of radio occultation satellites that exploit the U.S. GPS transmitters to obtain high resolution (~1 km) temperature profiles in the stratosphere does provide sufficient profiles nearly coincident with those from HIRDLS.  Comparisons show a good degree inter correlation between COSMIC and HIRDLS down to about 2 km resolution, with similar amplitudes for each, implying that HIRDLS and  COSMIC are able to measure the same small scale features. Hence, the optical blockage within HIRDLS does not seem to have affected this capability. This is of crucial importance for gravity wave studies because HIRDLS provides an excellent data set for their quantitative study and measurement.

## HIRDLS Vertical Resolution Objectives

- The aim is to achieve a vertical resolution of order 1 to 1.5 km. Atmospheric features of this size typically have a horizontal scale 100 to 200 times as large.  Hence a profile spacing of a few hundred km is needed to obtain a coherent picture.
- The time scale may be as short as minutes, so ideally the measurements should be made nearly simultaneously.
- The optical blockage within HIRDLS has modified the horizontal placement of profiles: originally there was to be a cross track scan leading to a 2D mesh of profiles at 500 km spacing.  Now we have just a line of profiles at about 100 km spacing – almost as many profiles but organised as a “curtain” along the orbit.
- This situation may actually be better for viewing some small scale structures such as gravity waves.

**HIRDLS Vertical Resolution Objectives**

## FORMOSAT-3/COSMIC Data

- Figure from http://www.cosmic.ucar.edu

- The FORMOSAT-3 COSMIC system has 6 small satellites each carrying a GPS receiver.
- As the satellite set or rises relative to a GPS transmitter, the path through the limb is refracted by the atmosphere, and the extra time of travel is used to deduce a combination of air and water vapour density.  Above about the tropopause where there is almost no water this yields a temperature profile.
- Vertical resolution probably about 1 km above the tropopause (better lower down).
- COSMIC data are generally said to be accurate up to 30-35 km (4-5 scale heights).

**FORMOSAT-3**/**COSMIC Data**

- See paper in March 2008 Bulletin of American Meteorological Society.

## COSMIC profile locations for a typical day.

- 7 December 2007 -  1878 soundings at quasi random times.

- From a presentation by Bill Schreiner of the COSMIC Project Office, HIRDLS Science Team Meeting, January 2008

**COSMIC profile locations for a typical day.**

## Data used 

- This is much tighter than is normally possible with validation, but is necessary because the small scale features change quickly.  Such a tight window is possible because of the large number of HIRDLS and COSMIC profiles each day (~5500 and ~2000).
- Sometimes 2 (possibly 3) HIRDLS profiles matched this criterion, in which case they were averaged together.
- Days 192 2006 to 365 2007 data were used. Very few were missing.
- COSMIC data are from latest (late 2007) reprocessed publicly available version.
- HIRDLS are **Version 4**  (2.04.19 internal to HIRDLS), as available from the NASA DISC.
- NASA GSFC GEOS-5.01 meteorological analyses are used to make a gradient correction in the HIRDLS retrieval and are also compared.

**Data used **

## Subtract a smoothed profile (this is equivalent to a high pass filter) then inter-correlate HIRDLS, COSMIC and GMAO.  Each used its own smoothed profile.  Results are for a 0.5 pressure scale heights* full width at half height filter.

**Two methods of comparison were used:**
- Subtract a smoothed profile (this is equivalent to a high pass filter) then inter-correlate HIRDLS, COSMIC and GMAO.  Each used its own smoothed profile.  Results are for a 0.5 pressure scale heights******* **full width at half height filter.
-  Fourier analysis over the range 2.2-5.7 scale heights*******,** after fitting a parabola to remove the background, and apodizing.

- Full Width Half Height is 0.5 scale heights (approx 3.5 km)

**Method of Comparison**

- [******Pressure Scale Height:*
- Distance over which pressure changes by a factor of e. This is typically 7 km.]

## Example comparison – day 336 2006

- Day 336 2006
- COSMIC profile **atmPrf_C002.2006.336.14.42.G17**
- 57.6 o S, 39.2 oE, 14:40:15
- HIRDLS
- 57.0o S, 39.6 oE, 14:40:18

- Black –
- COSMIC profile
- Magenta –
- nearest HIRDLS
- Blue –
- adjacent HIRDLS profiles to south
- Gold –
- adjacent HIRDLS profiles to north
- Blue and gold are offset by 4 K per profile.

**Example comparison – day 336 2006**

## Example comparison – day 344 2007

- Day 344 2007
- COSMIC profile **atmPrf_C006.2007.344.01.15.G25**
- 19.8 o N, 8.6 oW, 01:13:05
- HIRDLS
- 19.8o N, 8.44 oW, 01:11:35

- Black –
- COSMIC profile
- Magenta –
- nearest HIRDLS
- Blue –
- adjacent HIRDLS profiles to south
- Gold –
- adjacent HIRDLS profiles to north
- Blue and gold are offset by 4 K per profile.

**Example comparison – day 344 2007**

## Example Comparisons, before and after high 

- Initial profiles

**Example Comparisons, before and after high **
**pass filtering**

- After subtracting smoothed profile.

## COSMIC vs HIRDLS correlation and standard deviations from high pass filtered data

**COSMIC vs HIRDLS correlation and standard deviations from high pass filtered data**

## COSMIC vs GMAO model correlation and standard deviations

**COSMIC vs GMAO model correlation and standard deviations**

- COSMIC standard deviations are much bigger than those of GMAO but correlation still tends to be positive

## HIRDLS vs GMAO  correlation and standard deviations

**HIRDLS vs GMAO  correlation and standard deviations**

## Fourier analysis approach.

- The range of 2.0 to 5.7 pressure scale heights was used (~16-40 km). Profiles had to be present over the whole of this range.  In most cases the COSMIC profile was the cause for not filling the domain, in which none was used. This range was a compromise between maximising the range and maximising the number of profile comparisons.
- The data were interpolated to 1/48 intervals in log10(pressure), which is half of the HIRDLS interval, giving 72 levels.  A classical Fourier analysis was used, i.e. each sine and cosine component was evaluated separately, rather than an FFT to avoid having to pad out to a power of 2.
- The data were apodised with a triangular function, but this gave essentially the same result as a cosine bell apodisation.
- A background profile was subtracted from each. After much experimentation, mainly with polynomials of different order, a parabolic fit was used (a separate fit for each of COSMIC, HIRDLS and GMAO).  This would be expected to attenuate the lowest wavenumber. Higher order polynomials gave attenuation to progressively higher frequencies but seemed to leave the still higher ones about the same.

**Fourier analysis approach.**

## Fourier analysis results.

**Fourier analysis results.**

## How do sequences of profiles compare?

- Successive profiles are offset by 5 K
- (-7.5, -2.5, 2.5, 7.5 from absolute).

**COSMIC**
**284 2006**
**Time in day **
**13694-14030 secs**

**How do sequences of profiles compare?**

## How do sequences of profiles compare?

- Successive profiles are offset by 5 K
- (centre profile not offset).

**COSMIC**
**284 2006**
**Time in day **
**13694-14030 secs**

**How do sequences of profiles compare?**

**HIRDLS**
**284 2006**
**Time in day**
**24856-24949 secs**

## How do sequences of profiles compare?

**COSMIC**
**225 2006**
**Time in day**
**39259-39685 secs**

**HIRDLS**
**225 2006**
**Time in day**
**80260-80353 secs**

**How do sequences of profiles compare?**

## What do radiosondes observe?

- U.K. Met Office high resolution high altitude radiosonde data, launched from Mount Pleasant, Falkland Islands (52 deg S, 58 deg W) , provided at 2 second intervals during the ascent.

- Data are for 1 - 19 January 2008 in time order from left to right.
- Temperature is correct for first profile and offset +7 K for each subsequent one.

**What do radiosondes observe?**

## What do radiosondes observe?

- U.K. Met Office high resolution high altitude radiosonde data, launched from Mount Pleasant, Falkland Islands (52 deg S, 58 deg W) , provided at 2 second intervals.

- Data are for 26 May to 13 June 2008 in time order from left to right.
- Temperature is correct for first profile and offset 7 K for each subsequent one.

- As before but for a different time of the year.

**What do radiosondes observe?**

## Conclusions

- HIRDLS agrees well with COSMIC data down to 2 km resolution (4 km wavelength) or better.
- COSMIC shows similar amplitudes to HIRDLS suggesting that either they have similar vertical resolutions or they are both are adequate to resolves the scales that are present
- Agreement on finer vertical scales is difficult to verify because of small amplitudes in both HIRDLS and COSMIC data – the atmospheric waves seem to have a decreasing amplitude. This may be due to the along-line-of-sight averaging inherent in both HIRDLS and COSMIC measurements with smaller vertical wavelengths being associated with smaller horizontal wavelengths.

**Conclusions**