Advanced hodograph-based analysis technique to derive gravity waves parameters from Lidar observations

Abstract

An advanced hodograph-based analysis technique to derive gravity waves (GW) parameters from observations of temperature and winds is developed and presented as a step-by-step recipe with justification of every step in such an analysis. As a most adequate background removal technique the 2D-FFT is suggested. For an unbiased analysis of fluctuation whose amplitude grows with height exponentially we propose to apply a scaling function of the form exp(z/(ςH)), where H is scale height, z is altitude, and the constant ς can be derived by a linear fit to fluctuation profiles and should be in a range 1–10 5 (we derived ς = 2.15 for our data). The most essential part of the proposed analysis technique consist of fitting of cosineswaves to simultaneously measured profiles of zonal and meridional winds and temperature and subsequent hodograph analysis of these fitted waves. The novelty of our approach is that its robustness ultimately allows for automation of the hodograph analysis and resolves many more GWs than it can be inferred by manually applied hodograph technique. This technique is applied to unique lidar measurements of temperature and horizontal winds measured in an altitude range of 30 to 70 km. A 10 case study of continuous lidar observations from January 09 to 12, 2016 with the ALOMAR Rayleigh-Mie-Raman (RMR) Lidar in Northern Norway (69°N) is analyzed. We use linear wave theory to identify 4507 quasi monochromatic waves and apply the hodograph method which allows to estimate several important parameters of the observed GW. This technique allows to unambiguously identify upand downward propagating GW. In the vicinity of the polar night jet ∼ 30 % of the detected waves propagate downwards. The upward propagating GW predominantly propagate against the background wind, whereas 15 downward propagating waves show no preferred direction. The kinetic energy density of upward propagating GW is larger than that of the downward propagating waves, whereas the potential energy is nearly the same for both directions. The mean vertical flux of horizontal momentum in the altitude range of 42 to 70 km for the detected waves is about 0.65 mPa for upward propagating GW and 0.53 mPa for downward propagating GW.