Least-squares harmonic estimation of the tropopause parameters using GPS radio occultation measurements

In order to investigate temporal variations of the tropopause parameters, Least-Squares Harmonic Estimation (LS-HE) is applied to the time series of the tropopause temperatures and heights derived from Global Positioning System Radio Occultation (GPS RO) atmospheric profiles of CHAMP, GRACE and COSMIC missions from January 2006 until May 2010 in different regions of Iran. By applying the univariate LS-HE to the completely unevenly spaced time series of the tropopause temperatures and heights, annual and diurnal components are detected together with their higher harmonics. The multivariate LS-HE estimates the main periodic signals, particularly diurnal and semidiurnal cycles, more clearly than the univariate LS-HE. Mixing in the values of the tropopause height and temperature is seen to occur in winter at lower latitudes (around 30°) as a result of subtropical jet, and in summer at higher latitudes (36°–42°) as an effect of subtropical high. A bimodal pattern is observed in the frequency histograms of the tropopause heights, in which the primary modes for the southern and northern parts of Iran correspond to subtropical and extratropical heights, respectively.