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

The generally moist region of the tropical West Pacific Ocean hostslower- and mid-tropospheric intrusions of dry air. Throughtheir thermal structure, dry layers inhibit vertical cloud developmentand deep convection. Subtropical dry layers play an important role inthe troposphere through radiative cooling.Case studies of tropical dry air intrusions have been conducted withdata from in-situ and remotely sensed observations as well as modeldata. In this work we show that observations from GPS RadioOccultation (RO) are capable of detecting dry layers.GPS RO is a limb sounding technique that provides accurate and precisemeasurements of atmospheric refractivity, which is related to density,with high vertical resolution and global coverage. Vertical profilesof water vapor pressure can be derived through a One-DimensionalVariation retrieval (1DVAR) using model data as the first guess.In this work, we compare RO data from the COSMIC mission to aircraftmeasurements from the CONTRAST campaign, conducted from Guam in thetropical West Pacific during January and February 2014. We derived therelative humidity from both data sets to investigate dry layers in thelower and mid troposphere.It is shown that GPS RO observations have the ability to robustlydetect tropospheric dry layers with a relative humidity of only a fewpercent. Together with the high vertical resolution, globalavailability, and 14 years of GPS RO data record, this opens theopportunity to study the spatio-temporal pattern of dry layersglobally and their dependency on climatological phenomena such as ElNiño-Southern Oscillation.