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An investigation of the potential of the high-resolution dynamics limb sounder for upper tropospheric and lower stratospheric ozone and water vapour measurements: a minimum performance scenario
~
Pergamon
www.elsevier.nl/locate/asr
Adv. Space Res. Vol. 27, No. 8, pp. 1479-1482, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 PII: S0273-1177(01)00218-6
AN I N V E S T I G A T I O N O F T H E P O T E N T I A L O F T H E H I G H - R E S O L U T I O N D Y N A M I C S L I M B S O U N D E R F O R U P P E R T R O P O S P H E R I C AND L O W E R S T R A T O S P H E R I C O Z O N E AND WATER VAPOUR MEASUREMENTS: A M I N I M U M P E R F O R M A N C E SCENARIO J.J. Remedios 1, J.C. Gille2 and J.J. Barnetta
1Department of Physics and Astronomy, University of Leicester, UniversityRoad, Leicester LE1 7RH, U.K. e National Center for Atmospheric Research, Boulder, CO 80301, U.S.A. 3AOPP, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K.
ABSTRACT A challenge for the next generation of satellite instruments is to observe the upper troposphere and lower stratosphere. Amongst the many species of significance, ozone and water vapour in the upper troposphere present particular problems due to steep gradients in mixing ratio at the tropopause. The High Resolution Dynamics Limb Sounder (HIRDLS), due to fly on EOS-CHEM in 2003, will observe this atmospheric region with a number of radiometer channels. For both ozone and water vapour, HIRDLS will employ multiple channels to measure their emission signals and will simultaneously determine temperature to a precision of 0.4 K from carbon dioxide radiometers. In this paper, a minimum performance scenario for HIRDLS is examined in which only single ozone and water vapour channels, and a temperature precision of 1.5-2,0 K, are available; the temperature precision adopted is commensurate with performance achieved by instruments on the Upper Atmosphere Research Satellite (UARS). The ozone and water vapour channels at 8.85 #m and 7.09 #m respectively are simulated. Optimal estimation is employed in vector form thus providing averaging kernels which indicate the likely vertical resolution of the measurements. It is shown that even in this degraded performance scenario, HIRDLS should be capable of measuring well into the troposphere with precisions of better than 30% away from the tropical tropopause. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Knowledge of the concentrations of ozone (Oa) and water vapour (H20) is central to radiative, chemical and dynamical studies of the upper troposphere and lower stratosphere. Ozone depletion in the lower stratosphere, due to enhanced chlorine-catalysed destruction, has been the focus of attention for sometime because of effects on UV-B flux at the surface. The vertical profile of 03 has a significant climatic impact through radiative forcing (Lacis et al, 1990) and factors which control the distribution of O3 throughout the upper troposphere and lower stratosphere are therefore the subject of much study. The same is true of tropospheric H20. In addition, both gases can provide much information about the dynamics of this atmospheric region and in particular the role of stratosphere-troposphere exchange in determining constituent concentrations. The High Resolution Dynamics Limb Sounder (HIRDLS) will provide detailed, high spatial resolution observations with which to study these problems (Gille and Barnett, 1996). The HIRDLS is a limb-sounding radiometer due to fly on EOS-CHEM in 2003. It has three designated O3 channels, at 10.00/~m, 9.71 #m and 8.85 #m, and two H20 channels, at 7.09 #m and and 6.74 #m. By defining a degraded performance scenario involving single channel ozone and water vapour operation and temperature precisions of 1.5-2.0 K (compared to the 0.4 K expected cf. HIRDLS Level-2 ATBD), it is possible to demonstrate a minimum performance for the HIRDLS instrument in terms of defined retrieval characteristics. In this study, the target atmospheric region was the upper troposphere and lower stratosphere, 1479
1480
J.J. Remedios et al.
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Figure 1: Retrieved mixing ratios for a Northern hemisphere October simulation: (a) O3 (ppmv) from H12; (b) H20 (ppmv) from H18. Retrieved values are very close to the "true" climatogical fields employed in the radiance simulations (see Figure 2(b) and (d) respectively). and the 8.85 #m channel (HIRDLS channel 12 or H12) and 7.09 #m channel (HIRDLS channel 18 of H18) were chosen for simulation of cloud-free, aerosol-free views. RETRIEVAL CHARACTERISTICS The forward model and retrieval codes employed in the simulations were those described by Marks and Rodgers (1993) and extensively employed in the processing of data for the Improved Stratospheric and Mesospheric Sounder. The forward model, based on tabulations of channel-integrated transmittance, is particularly suitable for the current study since it is designed for fast forward calculation of radiances and analytic weighting functions. Spectroscopic data from the HITRAN 1996 database were employed to calculate the transmittance tables. Simulated radiances were calculated at a vertical spacing of 1.18 km in altitude from zonal mean constituent fields (the "true" data) for the Northern hemisphere in the month of October. The vertical sampling implicitly accounts for the resolution limits imposed by the field-of-view in the HIRDLS channels, which is about 1.0 km wide. Approximately 1500 simulated radiance profiles were calculated for pressures between 500 mb and 0.1 mb and latitudes between 0 ° to 80°N allowing good statistical sampling at the retrieval stage. Non-local thermodynamic equilibrium effects (nonLTE) were not included but are relevant between 1 mb and 0.1 mb which is well above the target region discussed here. The simulations in this high altitude range are however characteristic of nighttime data that might be obtained. Errors on the radiances were estimated to be 3 x 10-4/3(290 K) and 3 x 10-4/3(290 K) for H12 and H18 respectively, based on detector noise, calibration accuracy, atmospheric decorrelation and the HIRDLS fine sampling within a fieldof-view (five samples). The term/3(290 K) is defined as the radiance that would be observed from a blackbody at 290 K. Temperature, pressure and constituent mixing ratio datasets were constructed from various zonal mean reference
HIRDLSMinimumPerformancefor 03 and H20
1481
datasets. For H12, Oa, H20 and CO2 mixing ratios were required and for H18, those for H20, CH4, CO2, 03 and 02. The temperature and pressure data were integrated hydrostatically with CIRA 1986 (Barnett and Corney, 1985). The 03 data were extended from a dataset produced by Fortuin and Kelder (1998) merging tropospheric ozonesonde data with SAGE II climatology (see Figure l(a)). The H20 data were extended from a SAGE II seasonal climatology of tropospheric and stratospheric measurements (Chiou et al., 1997) and are shown in Figure l(b). This climatology is implicitly biased towards cloud-free conditions. The CH4 data consisted of long-term means calculated by Randel et al. (1998) merged with an average tropospheric value of 1730 ppbv from NOAA CMDL measurements. The CO2 and O2 data were set to 351 ppmv and 0.209 ppv over the range of these retrievals. Expected HIRDLS temperature precisions are of the order of 0.4 K (HIRDLS Level-2 ATBD). Degradation of performance in the HIRDLS retrieval of temperature from the CO2 channels was introduced by assuming that the instrument could only achieve temperatures precisions of 1.5 K to 2.0 K commensurate with those achieved by the Upper Atmosphere Research Satellite (UARS), e.g. Dudhia and Livesey (1996). Errors of 1.5 K were assumed in the lower stratosphere under this scenario and errors of 2 K in the upper troposphere. A further error for retrievals from limb sounders arises from pointing jitter which for HIRDLS is expected to be of the order of 10 m r.m.s, through use of gyros on the optical bench of the instrument. Pointing errors tend to be less important for O3 due to its rather fiat radiance gradient with height but are more important for H 2 0 . Sensitivity tests performed during this work have shown that the H20 error at mid-latitudes due to 100 m uncertainty pointing is of the order of 10% at 30 km rising to 20% at 10 km. The effect for HIRDLS pointing of 10 m is much smaller and was therefore not included directly in these simulations. Contaminant errors are a small influence for HIRDLS O3 and H20 because of the frequency passband chosen. In addition, all the principal contaminants are directly measured by HIRDLS itself or are well mixed. Retrievals of 03 from H12 and H20 from H18 were performed between 500 mb and 0.1 mb using a 'vector-vector' form of optimal estimation (Marks and Rodgers, 1993). The a priori data employed for both retrievals were single profiles from averages of the zonal mean October data described above. The errors on the a priori were set to 75%, based on the maximum variance observed within the climatological dataset, with a 1.0 km vertical correlation scale consistent with field-of-view effects. The forward model extended above the retrieval range to 0.01 mb. RESULTS AND CONCLUSIONS The results for the O3 retrieval from H12 are shown in Figure 2(a) and Figure 2(b) illustrating single profile errors and differences between retrieved data and the input "true" climatology data as a function of latitude They demonstrate the ability of HIRDLS to measure from 500 mb to 0.1 mb with very good single profile precision (better than 30% away from the tropical tropopause) from a single channel. In particular, the troposphere is well covered apart from the tropical tropopause region which represents a particular difficulty because of the cold temperatures and steep gradients in ozone mixing ratios. Encouragingly, high vertical resolution is demonstrated by the averaging kernels (not shown) even though the fine sampling of the HIRDLS instruments (five samples per field-of-view) is not directly included here but is accounted for in the noise. Below the cold tropopause, the ozone sensitivity improves again at all latitudes as expected from the increasing temperature. From single channel retrievals alone, HIRDLS will therefore be able to measure Oa well into the upper troposphere. Similar results are seen for the H20 retrieval from H18 shown in Figure 2(c) and Figure 2(d) except that the high H20 mixing ratios in the troposphere maintain the vertical resolution until the channel loses sensitivity in the middle troposphere. This typically occurs at 300 mb or around 10 km in these simulations due to the combination of rapidly increasing H20 mixing ratio and temperature. The simulations for both gases suggest that HIRDLS will achieve very good vertical resolution and precision in the upper troposphere and lower stratosphere from single channels even when a degraded performance scenario with single channels and pessimistic temperature errors are employed. The actual HIRDLS performance with respect to temperature will improve the retrieval errors for 03 and H20 considerably in these single channel retrievals; the final temperature errors will depend on channel characteristics and line-of-sight temperature gradient errors. Finally, with multiple channels for 03 and H20 operating as planned for HIRDLS, the instrument will produce very useful data in the upper troposphere and lower stratosphere.
1482
J.J. Remedios
et al.
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Figure 2: Single profile point error (%) and ratios to 'true' climatological field: (a) H12 Oa single profile error; (b) H12 Oa, ratio of retrieval to true climatological field; (c) H18 H20 single profile error; (d) H18 H20, ratio of retrieval to true climatological field. These single channel simulations are for degraded temperature precisions of 1.5-2.0 K compared to 0.4 K expected for HIRDLS. ACKNOWLEDGEMENTS John Remedios would like to acknowledge the financial support of the National Center for Atmospheric Research, Boulder, Colorado, for work described in this study. The authors would like to thank Paul Fortuin, Bill Randel, and NOAA CMDL for providing reference atmosphere values. REFERENCES
Barnett, J.J, and M. Corney, MAP handbook, 16, 47-85, 1985. Chiou, E.W., M.P. McCormick and W.P. Chu, J. Geophys. Res., 102, 19105-19118, 1997. Dudhia, A., and N. Livesey, Validation of temperature measurements from the Improved Stratospheric and Mesospheric Sounder, J. Geophys. Res., 101, 9795-9809, 1996. Fortuin, J.P.F. and Kelder H, An ozone climatology based on ozonesonde and satellite measurements, J. Geophys. Res., 103, 31709-31734, 1998. Gille, J.C., and J.J. Barnett, Conceptual design of the High Resolution Dynamics Limb Sounder (HIRDLS) for the EOS Chemistry Mission, SP1E, 2830, 190-201, 1996. HIRDLS Level-2 ATBD, Lambert A., P.L. Bailey, D.P. Edwards, J.C. Gille, B.R. Johnson et al., http://eospso.gsfc.nasa.gov/atbd/pgl.html. Lacis, A., D.J. Wuebbles, and J.A. Logan, Radiative forcing of climate by changes in the vertical distribution of ozone, J. Geophys. Res., 95, 9971-9981, 1990. Marks, C.J., and C.D. Rodgers, A retrieval method for atmospheric composition from limb emission methods, J. Geophys. Res., 98, 14939-14953, 1993. Randel, W.J., F. Wu, J.M Russell HI, A.E. Roche and J. Waters, Seasonal Cycles and QBO variations in stratospheric CI-I4 and H20 observed in UARS HALOE data, J. Atmos. Sci., 55, 163-185, 1998.
Pergamon
www.elsevier.nl/locate/asr
Adv. Space Res. Vol. 27, No. 8, pp. 1479-1482, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 PII: S0273-1177(01)00218-6
AN I N V E S T I G A T I O N O F T H E P O T E N T I A L O F T H E H I G H - R E S O L U T I O N D Y N A M I C S L I M B S O U N D E R F O R U P P E R T R O P O S P H E R I C AND L O W E R S T R A T O S P H E R I C O Z O N E AND WATER VAPOUR MEASUREMENTS: A M I N I M U M P E R F O R M A N C E SCENARIO J.J. Remedios 1, J.C. Gille2 and J.J. Barnetta
1Department of Physics and Astronomy, University of Leicester, UniversityRoad, Leicester LE1 7RH, U.K. e National Center for Atmospheric Research, Boulder, CO 80301, U.S.A. 3AOPP, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K.
ABSTRACT A challenge for the next generation of satellite instruments is to observe the upper troposphere and lower stratosphere. Amongst the many species of significance, ozone and water vapour in the upper troposphere present particular problems due to steep gradients in mixing ratio at the tropopause. The High Resolution Dynamics Limb Sounder (HIRDLS), due to fly on EOS-CHEM in 2003, will observe this atmospheric region with a number of radiometer channels. For both ozone and water vapour, HIRDLS will employ multiple channels to measure their emission signals and will simultaneously determine temperature to a precision of 0.4 K from carbon dioxide radiometers. In this paper, a minimum performance scenario for HIRDLS is examined in which only single ozone and water vapour channels, and a temperature precision of 1.5-2,0 K, are available; the temperature precision adopted is commensurate with performance achieved by instruments on the Upper Atmosphere Research Satellite (UARS). The ozone and water vapour channels at 8.85 #m and 7.09 #m respectively are simulated. Optimal estimation is employed in vector form thus providing averaging kernels which indicate the likely vertical resolution of the measurements. It is shown that even in this degraded performance scenario, HIRDLS should be capable of measuring well into the troposphere with precisions of better than 30% away from the tropical tropopause. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Knowledge of the concentrations of ozone (Oa) and water vapour (H20) is central to radiative, chemical and dynamical studies of the upper troposphere and lower stratosphere. Ozone depletion in the lower stratosphere, due to enhanced chlorine-catalysed destruction, has been the focus of attention for sometime because of effects on UV-B flux at the surface. The vertical profile of 03 has a significant climatic impact through radiative forcing (Lacis et al, 1990) and factors which control the distribution of O3 throughout the upper troposphere and lower stratosphere are therefore the subject of much study. The same is true of tropospheric H20. In addition, both gases can provide much information about the dynamics of this atmospheric region and in particular the role of stratosphere-troposphere exchange in determining constituent concentrations. The High Resolution Dynamics Limb Sounder (HIRDLS) will provide detailed, high spatial resolution observations with which to study these problems (Gille and Barnett, 1996). The HIRDLS is a limb-sounding radiometer due to fly on EOS-CHEM in 2003. It has three designated O3 channels, at 10.00/~m, 9.71 #m and 8.85 #m, and two H20 channels, at 7.09 #m and and 6.74 #m. By defining a degraded performance scenario involving single channel ozone and water vapour operation and temperature precisions of 1.5-2.0 K (compared to the 0.4 K expected cf. HIRDLS Level-2 ATBD), it is possible to demonstrate a minimum performance for the HIRDLS instrument in terms of defined retrieval characteristics. In this study, the target atmospheric region was the upper troposphere and lower stratosphere, 1479
1480
J.J. Remedios et al.
(a) 0.1 2.00 - -
2.00
1.0
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:
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60
80
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7
t
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5 ~ 5-100.0 I 1000.0 0
20
40 Latitude (deg.)
60
80
Figure 1: Retrieved mixing ratios for a Northern hemisphere October simulation: (a) O3 (ppmv) from H12; (b) H20 (ppmv) from H18. Retrieved values are very close to the "true" climatogical fields employed in the radiance simulations (see Figure 2(b) and (d) respectively). and the 8.85 #m channel (HIRDLS channel 12 or H12) and 7.09 #m channel (HIRDLS channel 18 of H18) were chosen for simulation of cloud-free, aerosol-free views. RETRIEVAL CHARACTERISTICS The forward model and retrieval codes employed in the simulations were those described by Marks and Rodgers (1993) and extensively employed in the processing of data for the Improved Stratospheric and Mesospheric Sounder. The forward model, based on tabulations of channel-integrated transmittance, is particularly suitable for the current study since it is designed for fast forward calculation of radiances and analytic weighting functions. Spectroscopic data from the HITRAN 1996 database were employed to calculate the transmittance tables. Simulated radiances were calculated at a vertical spacing of 1.18 km in altitude from zonal mean constituent fields (the "true" data) for the Northern hemisphere in the month of October. The vertical sampling implicitly accounts for the resolution limits imposed by the field-of-view in the HIRDLS channels, which is about 1.0 km wide. Approximately 1500 simulated radiance profiles were calculated for pressures between 500 mb and 0.1 mb and latitudes between 0 ° to 80°N allowing good statistical sampling at the retrieval stage. Non-local thermodynamic equilibrium effects (nonLTE) were not included but are relevant between 1 mb and 0.1 mb which is well above the target region discussed here. The simulations in this high altitude range are however characteristic of nighttime data that might be obtained. Errors on the radiances were estimated to be 3 x 10-4/3(290 K) and 3 x 10-4/3(290 K) for H12 and H18 respectively, based on detector noise, calibration accuracy, atmospheric decorrelation and the HIRDLS fine sampling within a fieldof-view (five samples). The term/3(290 K) is defined as the radiance that would be observed from a blackbody at 290 K. Temperature, pressure and constituent mixing ratio datasets were constructed from various zonal mean reference
HIRDLSMinimumPerformancefor 03 and H20
1481
datasets. For H12, Oa, H20 and CO2 mixing ratios were required and for H18, those for H20, CH4, CO2, 03 and 02. The temperature and pressure data were integrated hydrostatically with CIRA 1986 (Barnett and Corney, 1985). The 03 data were extended from a dataset produced by Fortuin and Kelder (1998) merging tropospheric ozonesonde data with SAGE II climatology (see Figure l(a)). The H20 data were extended from a SAGE II seasonal climatology of tropospheric and stratospheric measurements (Chiou et al., 1997) and are shown in Figure l(b). This climatology is implicitly biased towards cloud-free conditions. The CH4 data consisted of long-term means calculated by Randel et al. (1998) merged with an average tropospheric value of 1730 ppbv from NOAA CMDL measurements. The CO2 and O2 data were set to 351 ppmv and 0.209 ppv over the range of these retrievals. Expected HIRDLS temperature precisions are of the order of 0.4 K (HIRDLS Level-2 ATBD). Degradation of performance in the HIRDLS retrieval of temperature from the CO2 channels was introduced by assuming that the instrument could only achieve temperatures precisions of 1.5 K to 2.0 K commensurate with those achieved by the Upper Atmosphere Research Satellite (UARS), e.g. Dudhia and Livesey (1996). Errors of 1.5 K were assumed in the lower stratosphere under this scenario and errors of 2 K in the upper troposphere. A further error for retrievals from limb sounders arises from pointing jitter which for HIRDLS is expected to be of the order of 10 m r.m.s, through use of gyros on the optical bench of the instrument. Pointing errors tend to be less important for O3 due to its rather fiat radiance gradient with height but are more important for H 2 0 . Sensitivity tests performed during this work have shown that the H20 error at mid-latitudes due to 100 m uncertainty pointing is of the order of 10% at 30 km rising to 20% at 10 km. The effect for HIRDLS pointing of 10 m is much smaller and was therefore not included directly in these simulations. Contaminant errors are a small influence for HIRDLS O3 and H20 because of the frequency passband chosen. In addition, all the principal contaminants are directly measured by HIRDLS itself or are well mixed. Retrievals of 03 from H12 and H20 from H18 were performed between 500 mb and 0.1 mb using a 'vector-vector' form of optimal estimation (Marks and Rodgers, 1993). The a priori data employed for both retrievals were single profiles from averages of the zonal mean October data described above. The errors on the a priori were set to 75%, based on the maximum variance observed within the climatological dataset, with a 1.0 km vertical correlation scale consistent with field-of-view effects. The forward model extended above the retrieval range to 0.01 mb. RESULTS AND CONCLUSIONS The results for the O3 retrieval from H12 are shown in Figure 2(a) and Figure 2(b) illustrating single profile errors and differences between retrieved data and the input "true" climatology data as a function of latitude They demonstrate the ability of HIRDLS to measure from 500 mb to 0.1 mb with very good single profile precision (better than 30% away from the tropical tropopause) from a single channel. In particular, the troposphere is well covered apart from the tropical tropopause region which represents a particular difficulty because of the cold temperatures and steep gradients in ozone mixing ratios. Encouragingly, high vertical resolution is demonstrated by the averaging kernels (not shown) even though the fine sampling of the HIRDLS instruments (five samples per field-of-view) is not directly included here but is accounted for in the noise. Below the cold tropopause, the ozone sensitivity improves again at all latitudes as expected from the increasing temperature. From single channel retrievals alone, HIRDLS will therefore be able to measure Oa well into the upper troposphere. Similar results are seen for the H20 retrieval from H18 shown in Figure 2(c) and Figure 2(d) except that the high H20 mixing ratios in the troposphere maintain the vertical resolution until the channel loses sensitivity in the middle troposphere. This typically occurs at 300 mb or around 10 km in these simulations due to the combination of rapidly increasing H20 mixing ratio and temperature. The simulations for both gases suggest that HIRDLS will achieve very good vertical resolution and precision in the upper troposphere and lower stratosphere from single channels even when a degraded performance scenario with single channels and pessimistic temperature errors are employed. The actual HIRDLS performance with respect to temperature will improve the retrieval errors for 03 and H20 considerably in these single channel retrievals; the final temperature errors will depend on channel characteristics and line-of-sight temperature gradient errors. Finally, with multiple channels for 03 and H20 operating as planned for HIRDLS, the instrument will produce very useful data in the upper troposphere and lower stratosphere.
1482
J.J. Remedios
et al.
(a) 0.I
. . . . . . . . . .
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Figure 2: Single profile point error (%) and ratios to 'true' climatological field: (a) H12 Oa single profile error; (b) H12 Oa, ratio of retrieval to true climatological field; (c) H18 H20 single profile error; (d) H18 H20, ratio of retrieval to true climatological field. These single channel simulations are for degraded temperature precisions of 1.5-2.0 K compared to 0.4 K expected for HIRDLS. ACKNOWLEDGEMENTS John Remedios would like to acknowledge the financial support of the National Center for Atmospheric Research, Boulder, Colorado, for work described in this study. The authors would like to thank Paul Fortuin, Bill Randel, and NOAA CMDL for providing reference atmosphere values. REFERENCES
Barnett, J.J, and M. Corney, MAP handbook, 16, 47-85, 1985. Chiou, E.W., M.P. McCormick and W.P. Chu, J. Geophys. Res., 102, 19105-19118, 1997. Dudhia, A., and N. Livesey, Validation of temperature measurements from the Improved Stratospheric and Mesospheric Sounder, J. Geophys. Res., 101, 9795-9809, 1996. Fortuin, J.P.F. and Kelder H, An ozone climatology based on ozonesonde and satellite measurements, J. Geophys. Res., 103, 31709-31734, 1998. Gille, J.C., and J.J. Barnett, Conceptual design of the High Resolution Dynamics Limb Sounder (HIRDLS) for the EOS Chemistry Mission, SP1E, 2830, 190-201, 1996. HIRDLS Level-2 ATBD, Lambert A., P.L. Bailey, D.P. Edwards, J.C. Gille, B.R. Johnson et al., http://eospso.gsfc.nasa.gov/atbd/pgl.html. Lacis, A., D.J. Wuebbles, and J.A. Logan, Radiative forcing of climate by changes in the vertical distribution of ozone, J. Geophys. Res., 95, 9971-9981, 1990. Marks, C.J., and C.D. Rodgers, A retrieval method for atmospheric composition from limb emission methods, J. Geophys. Res., 98, 14939-14953, 1993. Randel, W.J., F. Wu, J.M Russell HI, A.E. Roche and J. Waters, Seasonal Cycles and QBO variations in stratospheric CI-I4 and H20 observed in UARS HALOE data, J. Atmos. Sci., 55, 163-185, 1998.