First In Situ Evidence for Coexisting Submeter Temperature and Humidity Sheets in the Lower Free Troposphere

In recent years, temperature ‘‘sheets’’ with thicknesses on the order of 1 m have been observed with highresolution radiosondes throughout the stably stratified atmosphere, suggesting that they give rise to a major part of the well-known near-zenith aspect sensitivity of VHF radar echo intensities. It has been presumed but as yet not directly observed that these temperature sheets are accompanied by humidity sheets. In this paper, observations using the new helicopter-borne in situ turbulence measurement system HELIPOD are presented and discussed. For the first time it has been possible to observe in situ coexisting atmospheric temperature and humidity sheets with thicknesses down to a few decimeters and with temperature gradients of up to 17G, where G denotes the adiabatic lapse rate. Moreover, the first directly observed tropospheric temperature, humidity, and wind velocity profiles of a turbulent layer with a thickness of less than 10 m confined between two submeter sheets are presented. Simple theoretical reasoning leads to a lower limit for the sheet thicknesses: regardless of whether they are the remnants of Kelvin‐Helmholtz instability or attributed to viscosity/thermal-conduction waves, it should amount (apart from a numerical factor) to the square root of the product of molecular kinematic viscosity and a timescale that characterizes the age of a laminar sheet, the lifetime of a Kelvin‐Helmholtz billow, or the period of a primary gravity wave.

[1]  Hubert Luce,et al.  Direct comparison between in situ and VHF oblique radar measurements of refractive index spectra: A new successful attempt , 1996 .

[2]  M. L. V. Pitteway,et al.  The viscous damping of atmospheric gravity waves , 2013 .

[3]  K. Browning,et al.  Richardson number limited shear zones in the free atmosphere , 1970 .

[4]  Jean Vernin,et al.  Direct Evidence of “Sheets” in the Atmospheric Temperature Field , 1994 .

[5]  J. Woods Wave-induced shear instability in the summer thermocline , 1968, Journal of Fluid Mechanics.

[6]  J. Röttger,et al.  A study of signal statistics of VHF radar echoes from clear air , 1985 .

[7]  K. Gage,et al.  Fresnel scattering model for the specular echoes observed by VHF radar , 1981 .

[8]  K. S. Gage,et al.  Evidence for specular reflection from monostatic VHF radar observations of the stratosphere , 1978 .

[9]  Frank D. Eaton,et al.  A new frequency‐modulated continuous wave radar for studying planetary boundary layer morphology , 1995 .

[10]  Y. Desaubies,et al.  Reversible and Irreversible Finestructure , 1981 .

[11]  J. J. Hicks,et al.  Radar observations of breaking gravitational waves in the visually clear atmosphere. , 1968 .

[12]  P. Chilson,et al.  First observations of Kelvin‐Helmholtz billows in an upper level jet stream using VHF frequency domain interferometry , 1997 .

[13]  C. Wode,et al.  The helicopter-borne sensor package helipod-features and capabilities of a new turbulence measurement system for meteorological research , 1996 .

[14]  D. Zrnic,et al.  Reflection and scatter formula for anisotropically turbulent air , 1984 .

[15]  J. Klostermeyer,et al.  Radar observation and model computation of a jet stream‐generated Kelvin‐Helmholtz instability , 1980 .

[16]  E. Posmentier The Generation of Salinity Finestructure by Vertical Diffusion , 1977 .

[17]  Earl E. Gossard,et al.  Finestructure of Elevated Stable Layers Observed by Sounder and In Situ Tower Sensors , 1985 .

[18]  Kenneth S. Gage,et al.  Radar Observations of the Free Atmosphere: Structure and Dynamics , 1990 .

[19]  D. Fritts,et al.  Convective and dynamical instabilities due to gravity wave motions in the lower and middle atmosphere: Theory and observations , 1985 .

[20]  K. S. Gage,et al.  Vertical profiles of refractivity turbulence structure constant: Comparison of observations by the Sunset Radar with a new theoretical model , 1978 .

[21]  J. Richter,et al.  The Shape of Internal Waves of Finite Amplitude from High-Resolution Radar Sounding of the Lower Atmosphere , 1970 .

[22]  J. Röttger Reflection and scattering of VHF radar signals from atmospheric refractivity structures , 1980 .

[23]  J. Richter,et al.  Effect of wind shear on atmospheric wave instabilities revealed by FM/CW radar observations , 1973 .

[24]  W. Plant,et al.  The Naval Research Laboratory's Air-Sea Interaction Blimp Experiment , 1989 .

[25]  T. Tsuda,et al.  Viscosity waves and thermal‐conduction waves as a cause of “specular” reflectors in radar studies of the atmosphere , 1991 .

[26]  Earl E. Gossard Radar Research on the Atmospheric Boundary Layer , 1990 .

[27]  C. D. Watkins,et al.  Observations of Clear Air Turbulence by High Power Radar , 1970, Nature.

[28]  D. R. Jensen,et al.  An Analytical Study of Tropospheric Structure as Seen by High-Resolution Radar , 1971 .

[29]  W. Heisenberg,et al.  Zur statistischen Theorie der Turbulenz , 1948 .

[30]  Ch. Jacobi,et al.  On wind shear at fronts and inversions , 1996 .

[31]  David Atlas,et al.  Microscale ordered motions and atmospheric structure associated with thin echo layers in stably stratified zones , 1973 .

[32]  H. Luce,et al.  Interpretation of VHF ST radar vertical echoes from in situ temperature sheet observations , 1995 .

[33]  Ronald F. Woodman,et al.  ASPECT SENSITIVITY MEASUREMENTS OF VHF BACKSCATTER MADE WITH THE CHUNG-LI RADAR - PLAUSIBLE MECHANISMS , 1989 .

[34]  F. Takens,et al.  On the nature of turbulence , 1971 .

[35]  POSSIBLE KEY TO THE DILEMMA OF METEOROLOGICAL “ANGEL” ECHOES , 1960 .

[36]  Jürgen Röttger,et al.  Partial reflection and scattering of VHF radar signals from the clear atmosphere , 1978 .

[37]  J. H. Richter,et al.  The Birth of “CAT” and Microscale Turbulence , 1970 .

[38]  S. Thorpe Experiments on the Stability of Stratified Shear Flows , 1969 .

[39]  W. Hocking,et al.  Studies of seasonal behaviour of the shape of mesospheric scatterers using a 1.98 MHz radar , 1992 .