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 high-resolution 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 17Γ, where Γ 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.