We have used temperature
profiles derived from Global Positioning System (GPS) radio occultation measurements
to study thermal variability of the tropical tropopause region. GPS/MET
was a demonstration system that obtained data during April 1995 - February
1997. The data are characterized by very high vertical resolution (~0.2
km), and global sampling (especially advantageous in the tropics, where the
radiosonde network is sparse). The high vertical resolution is important
for analyses of the cold point tropopause, as shown in comparisons with meteorological analyses. Our
work used the GPS/MET data to characterize the mean structure and variability
of the tropopause during these two years. The key results are:
1) The mean tropopause
structure is similar to that obtained from radiosonde data (eg. Seidel et
al., 2001) and global analyses, but the GPS data offer higher spatial sampling.
Here's an example of the cold point tropopause during NH
winter (DJF). We have developed a climatology of cold point temperature
and height from GPS data, which are available to anyone interested.
2) Much of the variability in tropical cold point temperature and height in GPS data appears related to wave-like fluctuations (such as inertia-gravity waves or Kelvin waves). Here are some examples of extrema in tropopause temperatures in individual profiles (sampled over 10 N- 10 S during DJF 1996-97). Note that the profiles are very 'wavy' above ~15 km (see the analysis of gravity waves in GPS/MET data in Tsuda et al., 2000).
3) Significant correlations are found between the GPS/MET temperatures and daily, gridded outgoing longwave radiation (OLR) data (obtained from the Climate Diagnostics Center web site). OLR is a proxy for convection in the tropics, and these calculations quantify temperature variability associated with fluctuations in tropical convection. These significant correlations are an important result, providing independent confirmation of the GPS/MET temperature fluctuations. The plot below shows correlations between GPS/MET temperature profiles and the gridded OLR data during February 1997, plotted to show the temperature response to transient convection as a function of height and longitude. These patterns show upward and eastward tilting wave structures over ~12-18 km (above convection), similar to equatorially-trapped Kelvin waves. Note that the patterns are coherent over an entire hemisphere, showing that localized convection influences temperatures over 10, 000 km away!
4) The GPS/MET data also show clear evidence of the stratospheric QBO in temperature over altitudes 16-40 km. Here's a plot of deseasonalized temperature anomalies over the equator, showing downward propagating patterns with an approximate 2-year periodicity. The heavy line near 17 km is the tropopause.
Seidel, D.J., R.J. Ross, J.K. Angell and G.C. Reid, 2001: Climatological characteristics of the tropical tropopause as revealed by radiosondes. J. Geophys. Res., 106, 7875-7878.
Tsuda, T., M. Nishida,
C. Rocken and R.H. Ware, 2000: A global morphology of gravity wave activity
in the stratosphere revealed by the GPS occultation data (GPS/MET). J. Geophys.
Res., 105, 7257-7273.
For lots more details,
see our paper (January
2003 JGR, doi10.1029/2002JD002595)