Emission from Supercooled Clouds and Dielectric Models of Water Christian Mätzler, Phil Rosenkranz (MIT), Jan Cermak (ETH) URSI Commission-F Microwave Signatures 2010 Florence, Italy, 4-8 Oct 2010

2 Outline Motivation Surface-Based Radiometry Method Results Conclusions References

3 Motivation Supercooled liquid clouds are very common in the troposphere, especially in winter. They influence atmospheric processes: radiation, precipitation... They can be a hazard for air traffic. Large uncertainties exist on their radiative properties. Microwave radiometers (20 to 100 GHz) are able to distinguish liquid clouds from ice clouds (which are transparent in this frequency range). Continuous microwave observations of the sky above Bern, Switzerland, using a tropospheric radiometer system (TROWARA), provided information on liquid clouds (T = 246 – 279 K). Here we present results on supercooled clouds with a view on the dielectric properties of supercooled water. Paper in press: 4283

4 Surface-Based Radiometry SPIRA VarioCamASMUWARA TROWARA The Tropospheric-Water Radiometer (TROWARA) Located in Bern, Switzerland Channels MW: 21.3, 22.2, 31.5 GHz IR wavelength: μm Constant beamwidth: 4° Constant view direction zenith angle: 50° Originally from Peter and Kämpfer (1992), with many improvements over the years.

5 Additional information from Weather station: surface-air temperature, visibility, precipitation etc. Aqua Satellite (AMSU, AIRS, MODIS): cloud-top temperature

6 Method TROWARA Aqua MODIS, AIRS, AMSU T cloud base T cloud top T Retrieval: T from IR, weather station, τ i from MW Mätzler & Morland IEEE TGRS (2009)

7 Cloud-temperature comparison Differences between measured cloud-top (-base) temperature and estimated mean temperature of 71 liquid cloud episodes (no precipitation)

8 Microwave spectral signatures of atmospheric gases and clouds TROWARA channels

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10 Integrated water vapour (IWV) and integrated liquid water ILW dry atmosphere water vapour liquid water droplets with different size Ice clouds and dry snow are invisible at the TROWARA frequencies

11 ILW cloud IWV T cloud base Results IWV-ILW retrieval

12 Example with fog ( UT) and supercooled clouds later IWV ILW cloud

13 Example with fog and supercooled cloud in the evening Example of a 15-day period without rain

14 Zenith opacities at 21, 22 and 31 GHz versus time on 10 April 2010

15 Opacity at 21 GHz and 22 GHz vs opacity at 31 GHz slow variations due to water vapour fast variations due to changing cloud water 22 GHz 21 GHz Slope = cloud-opacity ratio

16 p = +2 p = -1 i = 21.38, j = 31.5 GHz

17 Same, but i = GHz

18 Measured and modelled cloud-opacity ratio vs cloud T Approximate correction for vapour fluctuations

19 Comparison of dielectric models with laboratory measurements at 9.61 GHz (Bertolini et al. 1982)

20 Conclusions Ratios of fast opacity changes observed over four years at frequency pairs of the TROWARA system (21 to 31 GHz) were related to ratios of cloud absorption coefficients in the Rayleigh Approximation. The 76 cloud episodes studied cover the temperature range from 246 to 279 K. Scatter of the absorption ratios was explained by correlated fluctuations of water vapor. The temperature variation of the measured absorption ratios agrees with the dielectric water model of Stogryn et al. (1995), but for T < 265 K it disagrees with most other models. A discrepancy occurs when our results are compared with the laboratory measurements of Bertolini et al. (1982) at 10 GHz. Clarifications are needed through further studies both in the laboratory and with water clouds. We recommend to extend our method to the frequency range from about 15 up to 100 GHz.

21 References See also Bertolini, D., M. Cassettari, G. Salvetti, The dielectric relaxation time of super-cooled water, J. Chem. Phys. 76, (1982). Ellison W. Freshwater and sea water, Section 5.2 of Mätzler C. (Ed.), P.W. Rosenkranz, A. Battaglia and J.P. Wigneron (Co-Eds.), Thermal Microwave Radiation - Applications for Remote Sensing, IET Electromagnetic Waves Series 52, London, UK (2006). Ellison W., Permittivity of Pure Water at Standard Atmospheric Pressure, over the Frequency Range 0-25 THz and the Temperature Range 0-100C, J. Phys. Chem. Data, 36 (1), 1-17 (2007). Liebe, H.J., G.A. Hufford, T. Manabe, A model for the permittivity of water at frequencies below 1THz, Internat. J. of Infrared and Millimeter Waves 12, (1991). Mätzler C., and J. Morland, Refined Physical Retrieval of Integrated Water Vapor and Cloud Liquid for Microwave Radiometer Data, IEEE Transactions on Geoscience and Remote Sensing, 47 (6), (2009). Mätzler C., P. Rosenkranz and J. Cermak: Microwave absorption of supercooled clouds and implications for the dielectric constant of water, JGR in press, see also IAP-Research Report MW (2010). Peter, R. and N. Kämpfer, Radiometric Determination of Water Vapor and Liquid Water and its Validation With Other Techniques, J. Geophys. Res. Vol. 97 (D16), (1992). Stogryn, A.P., H.T. Bull, K. Rubayi, S. Iravanchy, The microwave permittivity of sea and fresh water, Aerojet Internal Report, Aerojet, Sacramento, California (1995).

22 Comparison of dielectric models of pure liquid water: cloud-opacity ratio γ 21,31 at T=250 K (Col 2), standard deviations (Col 3 to 4) and mean values (Col 5 to 6) of real and imaginary parts of the differences between 363 measurements of dielectric constants (ε' exp and ε" exp ) and the model values, respectively, temperature range -10C to +20C, frequency range 0.9 to 1000 GHz, 90% of which are below 100 GHz. Dielectric data of water compiled by Ellison (2007).

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24 Cloud-free day with peculiar IWV variation IWV ILW

25 Identification of cloud-free (ILW=0) periods from std of ILW cloud free cloudy