Proceedings with Extended Abstracts (single PDF file) - Radio ...

RETRIEVAL OF ATMOSPHERIC STATIC STABILITY FROM MSTRADAR RETURN SIGNALDavid A Hooper 1,2 , Johan Arvelius 1 and Kerstin Stebel 1,31 Swedish Institute of Space Physics, Box 812, 981 28, Kiruna, SWEDEN2 now at Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK3 now at Norwegian Institute for Air Research, NO-9296, Tromsø, NORWAY1. IntroductionThe vertical gradient of potential temperature gives a measure of the atmosphere's staticstability, i.e. of its resistance to vertical motions. It is conveniently quantified in terms of thesquare of the Brunt-Väisälä frequency, ω B 2 (rad 2 s -2 ):g ∂θ⎛ ∂ lnθ⎞(1)2ωB= = g⎜⎟θ ∂z⎝ ∂z⎠where θ (K) is potential temperature [= T(1000/p) 2/7 ], T (K) is absolute temperature, p (hPa)is pressure, g (m s -2 ) is gravitational acceleration and z (m) is altitude. Perturbations of ω B2values are commonly seen in association with a variety of atmospheric phenomena includingfronts, gravity waves and turbulence. Moreover, the variations of ω B 2 as a function of altitudehave an effect on both gravity wave propagation and turbulence generation. Routinelyderived profiles of this parameter will therefore have a number of uses.The purpose of this paper is to describe a method of retrieving profiles of ω B 2 based on theMST radar return signal power, P, for a vertically directed beam. This method has much incommon with that described by Gage and Green (1982). Data are considered fromobservations made by the ESRAD MST radar (Chilson et al., 1999), which is located atEsrange, the Swedish Space Corporation's rocket range in northern Sweden (67.9°N, 21.1°E).Comparisons are made between values of ω B 2 derived from the retrieval method and thosederived from measurements made by 221 radiosondes launched from the same site during thewinters of 1996/1997 to 1999/2000. In deriving the radiosonde values of ω B 2 using Equation1, gradients of potential temperature are evaluated over a vertical interval of 300 m andsubsequently transferred to a vertical grid, between 1.0 and 15.7 km altitude at 300 mintervals, so as to correspond to the radar measurements.2. MethodIt has long been appreciated that perturbations of P and ω B 2 are closely related. The theory ofFresnel scatter (Gage et al., 1981) gives a direct relationship between the two throughconsideration of the mean vertical gradient of generalised potential refractive index, M(Ottersten, 1969). The latter is often conveniently approximated by the dry term, M D :M D= −77.6× 10−6 p ∂ lnθ(2)T ∂zalthough the full term additionally depends on the both the specific humidity, q (g kg -1 ), andits vertical gradient:42