High Resolution Dynamics Limb Sounder (HIRDLS)

Group Members

• John Gille
• Charles Cavanaugh
• James Craft
• Vince Dean
• Tom Eden
• Chris Halvorson
• Rashid Khosravi
• Charles Krinsky
• Joanne Loh
• Joe McInerney
• Bruno Nardi
• Brent Petersen
• Brendan Torpy
• Greg Young

The High Resolution Dynamics Limb Sounder (HIRDLS) is a 21 channel infrared limb scanning radiometer, jointly developed by ACD, the University of Colorado, and the Physics Department of Oxford University. It is designed to make observations of temperature, O 3 , H 2 O, and 8 other trace species, as well as PSC's, aerosols and cirrus clouds, from the upper troposphere to the mesosphere, with higher vertical resolution than has previously been available from space observations. NASA funded the U.S. share of the HIRDLS development. When HIRDLS was launched on the Aura spacecraft in July 2004, a thin plastic film from inside HIRDLS came loose and obstructed most of the instrument's aperture, limiting the view to the atmosphere to a small fraction of the width of the optical beam. Attempts to shake it off were unsuccessful. However, enough had been learned for the team, led by John Gille, the U.S. PI, and John Barnett (Oxford), the U.K. PI, to propose that they be allowed to show they could make use of the signals that could be seen through the partial aperture. These efforts have demonstrated that there is recoverable atmospheric information in the signals, and initial results for temperature, ozone, PSC's and cirrus were very encouraging. Based on these results, NASA approved the continuation of the HIRDLS effort.

The next steps were to correct the measured signals to make them as close as possible to the expected radiances. The major efforts this year were to improve and refine the correction algorithms, make them more robust, and if possible develop a physical basis for them that would assure that they would work under all conditions. The 3 required corrections are the removal of spurious oscillations from the signal (due to mechanical oscillation of the plastic), correction for the reduced viewing area, and subtraction of the infrared signal from the plastic itself.

A key method for determining these corrections is to have the spacecraft pitch by ~5°, so that HIRDLS looks above the atmosphere and measures signals only from the plastic film, thus allowing their characteristics to be determined. This year the ACD HIRDLS team coordinated 3 of these pitch maneuvers. The initial development of these algorithms was described last year, but continued work to improve the estimate of the reduced viewing area, and especially subtraction of the signal from the plastic film. The latter is the biggest difficulty at this time. In addition, the calibration coefficients, radiance sample geolocation, and improved cloud location were incorporated in the operational processing code. This has resulted in the production of Version 2.04.09 data, and release of global temperature, ozone, nitric acid and aerosol/cirrus data to the community. These data have been demonstrated to be of high quality by an extensive validation process. For example, the HIRDLS temperatures were compared to U.K. Meteorological Office high resolution data from radiosondes for 9 widely distributed stations, and for all data available from January 2005 until August 2007. The 2 HIRDLS retrievals for two of the stations, Gibralter and St. Helena show very good agreement with the sonde and with each other. These also illustrate the fidelity with which the HIRDLS retrievals track the smaller scale variations of the sondes above 200 hPa. Five papers have been submitted for an Aura special issue. A presentation on stratosphere-troposphere exchange was given at the AMS Middle Atmosphere meeting.

The NCAR part of the team hosted a meeting of the HIRDLS science team in January, and attended one hosted by Oxford in June. The team reviewed the correction algorithms and approaches, presented results of HIRDLS validation comparisons with other data, and discussed future plans.

In the next year the algorithms to correct the radiances will be improved to allow additional species to be recovered. Emphasis will be on improved estimation of the partial view area and the signal from the plastic. In parallel, emphasis will be placed on the use of the released data for scientific studies, especially of gravity waves, and UT/LS processes, notably strat-trop exchange. In particular, we expect to be heavily involved in the START08 experiment.

The HIRDLS observations of temperature, ozone and nitric acid during the tropopause folding events in the UT/LS region

Episodes of stratospheric air intrusion in the troposphere seen by HIRDLS and MLS instruments during NASA/Aura space-borne mission have been analyzed by HIRDLS/ACD group. Figure 1 illustrates the bright episode of intrusions of air masses in the upper troposphere/lower stratosphere (UT/LS) from space utilizing the high vertical probing of atmosphere by HIRDLS instrument. Indications of UT air intrusions from low latitudes into mid-latitude LS and indications of mid-latitude LS ozone and nitric acid transport into the tropical UT are remarkably seen along the selected HIRDLS orbit (141 ° - 143 ° W) on Jan 23 2006 (Figure 2). In January of 2006, effect of a sudden strong polar warming on distribution of LS chemical species in mid- and high latitudes amplifies intrusion of HNO3- and O3-rich air in the subtropics and mid-latitudes. In the NH hemisphere sub-tropics and mid-latitudes the remarkable double tropopause structures have been observed by HIRDLS, SABER and MLS instruments and reproduced by the meteorological analyses in the UT/LS. The HIRDLS science team begins to provide analysis of dynamics and chemistry during this and other episodes highlighting importance of the high vertical resolution of satellite data in order to predict and resolve such phenomena in the UT/LS. Other observed episodes of stratospheric air mass intrusions into the UT are localized in their latitudinal and longitudinal spreads. However, the unique spatial resolution of HIRDLS temperature, ozone and HNO3 retrievals (~ 1 km in vertical and ~1 degree along the orbit) helps to study these events with consistent vertical and horizontal sampling along the orbits. It is expected that the assimilation of HIRDLS temperature and constituents retrievals in the global models will bring the new level for data analysis in the UT/LS transport and chemistry studies.

Figure 1. Pairs of HIRDLS profiles compared to high resolution sondes at St. Helena and Gibralter, illustrating agreement between HIRDLS retrievals, and their ability to track small scale structure in profiles above 200 hPa.

Figure 2. HIRDLS/Aura-EOS observations of temperature (top), ozone (middle) and nitric acid (bottom) during January 23, 2006 along the single satellite orbit (141 ° - 143 ° W). On the color plates the white contours represent contours of PV (potential vorticity, 1.25 and 1.75 PV-unit) indicating the position of the dynamical tropopause (1.75 PV-unit), and active areas of the air-mass exchanges as seen in the O3 and HNO3 spatial distributions. Several HIRDLS temperature vertical profiles with features of the double tropopause phenomenon are observed along this and adjacent HIRDLS orbits.