Vivienne H. Payne1, Jennifer S. Delamere1, Karen E. Cady-Pereira1, Robert R. Gamache2, Eli J. Mlawer1,Jean-Luc Moncet1, Shepard. A. Clough3

vpayne@aer.com1Atmospheric and Environmental Research Inc., 131 Hartwell Ave., Lexington, MA, 02421-3626, USA

2University of Massachusetts, Lowell, MA, USA3Clough Associates, Lexington, MA, USA

Air-broadened half-widths of the 22 GHz and 183 GHz water vapor lines

Model: MonoRTM

Microwave radiative transfer model, developed at AER, Inc.

Summary

• Air-broadened half-widths of the 22 GHz and 183 GHz water vaporlines and associated uncertainties have been determined usingcomparisons between ground-based radiometric measurements anda radiative transfer model.

• Width values and associated temperature dependences are alsopresented from new state-of-the-art spectroscopic calculations.

• Width values derived from radiometer measurements are in goodagreement with the new calculations.

• Values for the half-width (γ) of the 22 and 183 GHz lines at 296 Kadopted in the latest version of AER’s line-by-line models:

Introduction

The 22.24 GHz and 183.31 GHz water vapor lines are two of themost important spectroscopic lines for remote sensing of the Earth’satmosphere. These lines are utilized by a wide variety of instrumentsthat measure water vapor for atmospheric studies and weatherprediction, including instruments at the surface, on high-altituderesearch aircraft and on satellites. These measurements are used forretrievals of atmospheric water vapor and as input to dataassimilation schemes. The accuracy of the retrievals and theforecasts from the assimilation schemes directly depend on theaccuracy of the line parameters used as input to the relevantradiative transfer models.

Air-broadened half-widths of(a) the 22 GHz and (b) the183 GHz lines: laboratorymeasurements, calculationsand values from recentversions of the HITRANdatabase, displayed in orderof publication. Vertical dottedlines represent valuesobtained in this work usingthe MWRP (22 GHz) andGVR (183 GHz)measurements. Shaded barsshow error bounds for thesevalues (Payne et al., 2008and references therein).

22 GHz• ARM Southern Great Plains (SGP) site• Data used: March 2000 to February 2002• MicroWave Radiometer Profiler (MWRP)

• 12 channels from 22-60 GHz• Built by Radiometrics• (http://www.radiometrics.com)

• RS-80 radiosondes

1. Model simulations with clear-sky radiosondes2. Removal of dry bias from RS-80s3. Estimation of channel-dependent offsets4. Scaling of PWV using 23.835 GHz channel5. Retrieval of 22GHz line width

183 GHz• ARM North Slope of Alaska (NSA) site• Data used: January to October 2007

• Includes data from RHUBC campaign• G-band Vapor Radiometer (GVR)

• 4 channels around 183 GHz (Pazmany et al. 2007)• Built by Prosensing (http://www.prosensing.com)

• RS-92 radiosondes

1. Model simulations with clear-sky radiosondes2. Estimation of channel offsets for +/-1, +/-3 GHz channels3. Scaling of PWV using +/-3 GHz channel4. Retrieval of 183 GHz line width5. Scaling of PWV using +/-7 GHz channel in linear regime6. Second iteration of line width retrieval

References• Payne, V. H. et al., Air-broadened half-widths of the 22 GHz and 183 GHz watervapor lines, IEEE TGRS, in review, 2008• Pazmany, A. L., et al., A compact 183 GHz radiometer for water vapor and liquidsensing, IEEE TGRS, vol. 45, No. 7, pp2202-2207, July 2007

Acknowledgements

This research was supported by the Office of Biological and Environmental Research of theU.S. Department of Energy as part of the Atmospheric Radiation Measurement Programand by the Joint Center for Satellite Data Assimilation. One of the authors (R. R. Gamache)is pleased to acknowledge support of this research by the National Science Foundation(NSF) through Grant No. ATM- 0242537. Any opinions, findings and conclusions orrecommendations expressed in this material are those of the authors and do not necessarilyreflect the views of the National Science Foundation.

Complex Robert-Bonamy (CRB) calculations

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Error source Contribution

Random retrieval error 0.1 %

1 K temperature uncertainty 0.3 %

3 % continuum uncertainty 0.6 %

10 % column ozone uncertainty 0.02%

1.5 % PWV uncertainty 1.5 %

0.5% intensity uncertainty 0.25%

Water vapor profile shape uncertainty 0.4 %

1 K instrument calibration uncertainty 1.6%

Total 2.4 %

Error source Contribution

Random retrieval error Less than 0.001 %

1 K temperature uncertainty Less than 0.1 %

3 % continuum uncertainty 0.2 %

1.5 % PWV uncertainty 1.5 %

0.5% intensity uncertainty 0.5%

5% self-broadened halfwidth uncertainty 0.2%

Total 1.6 %

22 GHz183 GHz

Transition (cm-1 atm-1) n (cm-1 atm-

1) 22.24 GHz

6 1 6 5 2 3 0.0913 0.755 -0.000882

183.31 GHz 3 1 3 2 2 0

0.0997 0.769 -0.00269

Half-width (γ), temperaturedependence of the half-width(n), and line shift (δ) for the 22and 183 GHz lines at 296 Kwith air as the buffer gas fromnew CRB calculations by R. R.Gamache.

2.4%0.0997183.31 GHz1.6%0.090022.24 GHz

Uncertaintyγ (cm-1/atm)Water line