F. Guichard, D. B. Parsons and E. Miller
National Center for Atmospheric Research, Boulder, Colorado, USA
Journal of Climate, 2000, vol. 13, pp. 3611-3624.
Abstract: Accurate measurements of atmospheric water vapor are
crucial to many aspects of climate research and atmospheric science. This
paper discusses some of the meteorological implications of a bias discovered
in the measurement of water vapor in widely deployed radiosonde systems.
This problem apparently arose in the early 1990s and a correction scheme
has been recently developed that intends to remove the bias. The correction
scheme also includes improvements in the humidity measurements in the upper
troposphere and near the surface. This scheme has been applied to data
taken during the Tropical Ocean and Global Atmosphere Coupled OceanAtmosphere
Response Experiment (TOGA COARE). The impact of the bias on the general
stability of the tropical atmosphere to deep convection, as measured by
the convective available potential energy (CAPE) and the convective inhibition
(CIN), is quite large. On the basis of the uncorrected data set, one might
erroneously conclude that it is difficult to trigger deep convection over
the region. When the correction is taken into account, the atmosphere over
the tropical western Pacific becomes typically unstable to deep convection,
with convective instability similar to that measured from aircraft in the
vicinity of active convective sytems. Radiative fluxes are also significantly
modified. For clear sky conditions, it is found that, on average, the net
surface radiative flux increases by 4 W m-2 due to the humidity
correction. Under more realistic cloudy conditions, the differences are
weaker, but still significant. Changes in radiative fluxes are explained
at first order by the the precipitable water increase.