Tropospheric Dry Layers in the Tropical Western Pacific: Comparisons of GPS Radio Occultation with Multiple Data Sets

The generally moist regions of the tropical Western Pacific Ocean also contain large-scale lower- and mid-tropospheric intrusions of extremely dry air. These dry regions have a strong radiative impact on the climate system through their ability to radiate heat to space, preventing a “runaway greenhouse effect”. Furthermore, they suppress deep convection and affect the planetary boundary layer height.

Studies of the structure of these dry air intrusions have been conducted with data from in-situ and remotely sensed observations as well as model data. However, all current humidity observation techniques have limitations for analyzing global water vapor fields (such as low vertical or horizontal resolution, being influenced by weather/clouds, or sparse or non-existent measurements over open oceans).

In this paper we examine whether GPS Radio Occultation (RO) observations are capable of detecting these intrusions of extremely low (less than 10 %) Relative Humidity (RH). RO is a limb sounding technique that provides accurate and precise measurements of atmospheric refractivity with high vertical resolution and global coverage. Vertical profiles of temperature and water vapor pressure can be derived from RO observed refractivities through a One-Dimensional Variation retrieval (1DVAR) using model or other observational data as the first guess.

We compare several different types of observations over the Tropical Western Pacific during the six-week period of the CONTRAST (CONvective TRansport of Active Species in the Tropics) experiment (January and February 2014). We used data from the NSF/NCAR Gulfstream V research aircraft as a reference for RO profiles. Furthermore, we used radiosonde observations from Guam and AIRS (Atmospheric Infrared Sounder) profiles, as well as data from the GFS (U.S. National Weather Service Global Forecast System) model and the ERA-Interim Reanalysis.

A comparison of these diverse data sets is challenging, considering their large differences in sampling characteristics and technologies. Furthermore, RH can undergo very strong changes in the horizontal and the vertical, and does not follow a “typical” vertical profile, such as temperature or specific humidity do (which generally decrease significantly with increasing height). Despite all of their differences, our results show that these diverse data sets are in very good agreement.

We show that RO is capable of detecting layers with very low RH in the lower and mid troposphere. Individual profile comparisons of RO to other types of observations show that dry layers are captured quite accurately regarding both shape and intensity. Both the GFS and ERA analyses show the overall correct structure, but they show less or no small-scale variations and no sharp vertical gradients due to a much lower vertical resolution.

Statistics on all profile pairs revealed a dry bias of CONTRAST for low humidity values and a moist bias for high humidity values (compared to RO, ERA and GFS).

Extremely strong horizontal moisture gradients (more than 50 % RH change within 100 km) can yield profile pairs that strongly disagree, even though they are close in space and time, which was confirmed by the ERA-Interim field. The reanalysis field also showed the high correlation between low humidity and high ozone concentration in air masses, which suggests stratospheric origin of dry intrusions.

Finally, we compared a global climatology of dry layers throughout the year using RO data with the climatologies of the ERA and GFS models. The RO data confirm both the ERA and GFS climatologies, which show a very similar seasonal occurrence of dry layers.