A Preliminary Meteorological Interpretation of Correlated Huygens DWE and HASI Data

1. A Preliminary Meteorological Interpretation of Correlated Huygens DWE and HASI Data M. Allison1, F. Ferri2, M.K. Bird3, M. Fulchignoni4,5, S.W. Asmar6, D.H. Atkinson7, G. Colombatti2, G.L. Tyler8 and the DWE and HASI Experiment Teams 1 NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025; 2 CISAS G. Colombo, Università di Padova, Via Venezia 15, 35131 Padova, Italy; 3 Radioastronomisches Institut, Universität Bonn, 53121 Bonn, Germany; 4 LESIA, Observatoire de Paris, 5 Place Janssen, 92195 Meudon, France 5 Université Denis Diderot – Paris 7, UFR de Physique, 2 Place Jussieu, 75006 Paris, France; 6 Jet Propulsion Laboratory, Calif. Inst. Technology, 4800 Oak Grove Dr., Pasadena, CA 91109; 7 Department of Electrical & Computer Engineering., University of Idaho, Moscow, ID 83844; 8 Center for Radar Astronomy, Stanford University, Stanford, CA 94305 4. Abstract. The highly resolved vertical profiles of wind and temperature in Titan's atmosphere afforded by the Huygens Atmospheric Structure Instrument (HASI) and Doppler Wind Experiment (DWE) have been analyzed as profiles of wind shear and buoyant static stability, using a variety of smoothing methods. Above the boundary layer, the regions of strongest vertical wind shear, as smoothed over half-scale height intervals, occur within the most statically stable regions of Titan's lower stratosphere. We have computed vertical profiles of the Richardson number (Ri), representing the squared ratio of the Brunt-Väisälä frequency to the vertical wind shear. As evaluated over small vertical difference intervals, Ri appears to approach the limiting value of 1/4 (for Kelvin-Helmholtz instability) within localized regions both near the surface and above 100 km altitude, possibly indicative of wave breaking there. As evaluated for smoothed fits to the shear, Ri is as small as ~2-5 over roughly half-scale height layers between 70 and 90 km altitude, but is generally large at lower levels. These results have important implications for the setting of the latitudinal distribution of angular momentum and velocity by efficient eddy mixing of potential vorticity. 1. Fig. 4. A (south) latitude-height wind section derived from the Zero Potential Vorticity (ZPV) hypothesis of Allison et al. (1994), assuming the DWE vertical profile for the near-equatorial zonal wind and the combined HASI-DWE analysis of the vertical profile of the Richardson number. 5. Fig. 1. Point-by-point plot of Huygens Doppler wind (left) and HASI temperature data (right), along with smooth functional fits shown as solid colored curves. The blue curve provides an interpolation through the Doppler tracking data gap in the lower troposphere, and at the ~110km level for the exchange of staged parachutes. The functional representation of temperature (red curve) is within 5K of all measured stratospheric temperatures below 130 km altitude, and within 1K in the troposphere. 2. Fig. 5. The vertical profile of the potential temperature (left), the DWE zonal wind (right), and implied Richardson number (Ri) within the lowest 3000m of Titan’s atmosphere. The red solid curves represent smooth functional fits to the data points, as used for the schematic estimate of the Ri profile marked in blue. • Preliminary Conclusions: • An initial correlated analysis of Huygens DWE and HASI data reveals a significant correspondence of wind-shear and buoyant stability structures, both in Titan’s stratosphere and the apparent planetary boundary layer region of its lower troposphere. • (b) The unanticipated strong vertical wind shear region between 60 and 100 km altitude is correlated with the most buoyantly stratified region of Titan’s lower stratosphere, but corresponds to a roughly one scale-height layer where the averaged Richardson number is as small as ~2 – 5. The “potential vorticity mixing” theory for planetary circulation suggests that this (15 – 30 hPa) region may be characterized by relatively flat isotachs, as may be further illuminated by thermal wind studies with Cassini CIRS. • (c) The HASI and DWE data in Titan’s lower atmosphere suggest a boundary layer characterized by well-mixed potential temperature, strong vertical shears, and a thin region of near-unitary Richardson number over the lowest three kilometers. Fig. 2. Profiles of vertical wind shear per scale-height (left) and buoyant static stability NH (right), both given in units of m/s. The smooth thick curves correspond to the functional fits shown in Fig.1. Thin lines representing the fine-scale structure have been computed as the derivatives of polynomial interpolations to a 5-point smoothing of the individual data points. 3. References: Allison, M., Del Genio, A.D. and Zhou, W. (1994). Zero potential vorticity envelopes for the zonal-mean velocity of the Venus/Titan atmospheres. J. Atmos. Sci. 51, 694-702. Cushman-Roisin, B. (1994). Introduction to Geophysical Fluid Dynamics. Prentice Hall, New Jersey. Bird, M.K. et al. (2005) The vertical profile of winds on Titan. Nature, in press. Ferri, F. et al. (2005) Huygens ASI measurements at Titan: A new insight of Titan's atmosphere. Eos Trans. AGU 86 (18), Joint Assem. Suppl., Abstract P34A-05. Flasar, F.M. et al. (2005). Titan's atmospheric winds, temperatures, and composition. Science 308, 975-978. Fulchignoni, M. (2005). Titan's physical characteristics measured by the Huygens Atmospheric Structue Instrument (HASI). Nature, in press. Lindal, G.F. et al. (1983). The atmosphere of Titan: An analysis of the Voyager 1 radio-occultation measurements. Icarus 53, 348-363. Lindzen, R.S. (1970). Internal gravity waves in atmospheres with realistic dissipation and temperature Part I. Mathematical development and propagation of waves into the thermosphere. Geophys. Fluid Dyn. 1, 303-355. Tennekes, H. (1973) A model for the dynamics of the inversion above a convective boundary layer. J. Atmos. Sci. 30, 558-567. Turner, J.S. (1973) Buoyancy Effects in Fluids. Cambridge University Press, London. Contact: mallison@giss.nasa.gov Fig. 3. Vertical profiles of the Richardson number (Ri) measure of the combined buoyant and wind-shear layering of Titan’s atmosphere (shown in the left-side panel), as derived from the combined Huygens HASI and DWE data. (The vertical profile of the inverse of this generally large number is shown in the panel to the right.) The red solid curves have been derived from the smooth profiles shown in Fig.2, while the thin (cerulean blue) lines correspond to the fine-scaled structure displayed in the same plot.