Dry Layers in the Tropical West Pacific as Observed with GPS Radio Occultation and CONTRAST Aircraft Measurements

Abstract:

The generally moist region of the tropical West Pacific Ocean hosts
lower- and mid-tropospheric intrusions of dry air. Through
their thermal structure, dry layers inhibit vertical cloud development
and deep convection. Subtropical dry layers play an important role in
the troposphere through radiative cooling.

Case studies of tropical dry air intrusions have been conducted with
data from in-situ and remotely sensed observations as well as model
data. In this work we show that observations from GPS Radio
Occultation (RO) are capable of detecting dry layers.

GPS RO is a limb sounding technique that provides accurate and precise
measurements of atmospheric refractivity, which is related to density,
with high vertical resolution and global coverage. Vertical profiles
of water vapor pressure can be derived through a One-Dimensional
Variation retrieval (1DVAR) using model data as the first guess.

In this work, we compare RO data from the COSMIC mission to aircraft
measurements from the CONTRAST campaign, conducted from Guam in the
tropical West Pacific during January and February 2014. We derived the
relative humidity from both data sets to investigate dry layers in the
lower and mid troposphere.

It is shown that GPS RO observations have the ability to robustly
detect tropospheric dry layers with a relative humidity of only a few
percent. Together with the high vertical resolution, global
availability, and 14 years of GPS RO data record, this opens the
opportunity to study the spatio-temporal pattern of dry layers
globally and their dependency on climatological phenomena such as El
Niño–Southern Oscillation.