11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002
Tropical convection's roles in tropical tropopause cirrus
Matthew T. Boehm, National Research Council and NASA/GSFC, Washington, DC; and D. O. Starr, J. Verlinde, and S. Lee
Poster PDF (366.8 kB)
Remote sensing observations reveal the frequent occurrence of tropopause cirrus, thin cirrus layers located near the tropical cold-point tropopause. Here, we investigate the roles that tropical convection plays in the formation of these clouds. First, we combine high-resolution cloud brightness temperature data from the infrared channels of the GOES satellites with temperature data from the NCEP/NCAR and NASA DAO reanalyses to determine the altitude to which tropical convection penetrates. In particular, we look at the frequency with which convection reaches the cold-point tropopause. This information allows us to estimate the extent to which tropical convection, acting alone, is able to supply the moisture near the cold-point tropopause required for the formation of tropopause cirrus.

Previous studies suggest that tropical convection only infrequently reaches the cold-point tropopause and that the transport of moisture to the cold-point tropopause typically requires large-scale rising motion within the tropical tropopause transition layer (TTL). One mechanism by which this transport may be accomplished involves tropical convection itself. According to this hypothesis, the transport is accomplished by meridional circulations that develop within the TTL in response to Rossby waves generated by tropical convection. Here, we present results of a series of model runs, conducted using a global scale primitive equations model, which were designed to test this hypothesis. In addition, vertical velocity data from the NCEP/NCAR and NASA DAO reanalyses will be examined for evidence of the expected correlation between large-scale rising motion in the tropical upper troposphere and tropical convection.

Once moisture is present near the cold-point tropopause, large-scale cooling is required to initiate tropopause cirrus formation. One source of this cooling is stratospheric tropical waves induced by tropical convection, as we will show in a time series of radiosonde temperature data superimposed with data on cloud occurrence from the DOE ARM Nauru99 field experiment.

Tropical convection therefore plays roles in all aspects of tropopause cirrus formation. First, it directly transports large quantities of moisture from the surface to the base of the tropical TTL. Next, it induces Rossby waves that help produce the large-scale rising motion within the TTL required to explain the transport of moisture the remainder of the distance to the cold-point tropopause. Finally, it generates stratospheric tropical waves that are one source of the large-scale cooling required for tropopause cirrus formation.

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