Observing the atmospheric diurnal cycle with GPS/COSMIC occultations

Diurnal cycle, driven by solar forcing, plays important roles in the Earth's weather and climate system. Lack of temporal sampling with high vertical resolution measurements in the troposphere hinders our understanding of global diurnal variations in atmospheric thermodynamic variables, such as temperature, water vapor, precipitation and clouds. The six-satellite COSMIC (Constellation Observing System for Meteorology Ionosphere and Climate) radio occultation (RO) provides rather uniform global coverage with high vertical resolution, all-weather and diurnal sampling capability. This paper analyzes the diurnal and semi-diurnal variations of both temperature and refractivity from two-year (2007-2008) COSMIC RO measurements in the troposphere and stratosphere. The RO observations reveal both vertically propagating and trapped structures of diurnal and semi-diurnal variations, including transition regions near the tropopause where high vertical resolution is critically needed. In the stratosphere the upward propagating component of the migrating diurnal tides in the tropics is clearly captured by the monthly GPS RO measurements, which shows downward progress in phase from upper troposphere to the stratopause with a vertical wavelength of ~25 km. Top boundary layer and polar stratosphere show large diurnal variations with strong seasonal variations, of which the cause(s) requires further investigations. The current COSMIC diurnal sampling remains insufficient but has revealed, for the first time, many interesting diurnal features in the troposphere and stratosphere. Diurnal sampling with future GPS RO constellation systems will be discussed.