Dry, high ozone layers in the tropical troposphere observed during CONTRAST and global behavior from GFS analyses

Dry regions in the subtropical troposphere are a climatological feature that have a strong impact on global radiative balances, as they influence the ability of the climate system to radiate heat to space [Pierrehumbert, 1995]. Satellite observations show that dry regions (areas with relative humidity (RH) less than 20%) occur as a climatological feature in the lower-middle troposphere, centered near 2030°N and 2030°S [e.g., Gettelman et al., 2006; Ruzmaikin et al., 2014]. The mechanisms that maintain these dry regions are complex and are linked to some combination of quasi-isentropic transport from the extratropical upper troposphere-lower stratosphere (UTLS) and downward circulation in the Hadley cells.   The Convective Transport of Active Species in the Tropics (CONTRAST) experiment provided a unique set of in situ observations to investigate the mechanisms creating and maintaining the dryness in the subtropics.

The CONTRAST experiment was an aircraft-based field campaign conducted from Guam (14°N, 145°E) during JanuaryFebruary 2014. Aircraft measurements included over 80 vertical profiles from the boundary layer to the upper troposphere (~15 km).  A large fraction of these profiles revealed layered structures with very low water vapor (relative humidity <20%) and enhanced ozone, primarily in the lower-middle troposphere (~39 km). A two-dimensional distribution of ozone vs. relative humidity (Fig. 1) from the CONTRAST measurements illustrates the frequent occurrence and close connection between enhanced ozone and dry air, highlighting a ‘bi-modal’ distribution (first described in Pan et al, 2016).

2-d distribution of ozone vs relative humidity for measurements obtained during CONTRAST
Figure 1:
Two-dimensional distribution of ozone versus relative humidity for aircraft
measurements obtained during CONTRAST, derived from over 80 vertical profiles
near Guam during January-February 2014.

Comparing CONTRAST water vapor measurements with co-located profiles from National Centers for Environmental Prediction Global Forecast System (GFS) analyses, we find good agreement for dry layers, including profile-by-profile comparisons and statistical behavior. We then utilize GFS data to evaluate the frequency of occurrence and 3-D structure of dry layers for the CONTRAST period to provide perspective to the campaign measurements and evaluate the global climatological behavior based on a longer record. GFS data show that dry layers occur ~5080% of the time in the subtropical troposphere, maximizing on the equatorward side of the subtropical jets in the winter hemisphere. Subtropical dry layers occur most frequently over isentropic levels ~320340 K, which extend into the extratropical upper troposphere-lower stratosphere (UTLS). The figure provides an example of in situ and GFS agreement and the extended dry layer occurrence for the CONTRAST campaign period.

Similar statistical behavior of dry, ozone-rich layers is found in long-term balloon measurements from Reunion Island (21°S, 56°E). The climatologically frequent occurrence of dry, ozone-rich layers, plus their vertical and spatial structures linked to the subtropical jets, all suggest that dry layers are linked to quasiisentropic transport from the extratropical UTLS and suggest a ubiquitous UTLS influence on the subtropical middle troposphere.

vertical profiles of water vapor mixing ration from CONTRAST

Figure 2: (Left) Vertical profiles of water vapor mixing ratio from CONTRAST (blue lines) and co-located GFS
analyses (orange), for measurements from CONTRAST research flight 12 on February 16, 2014. (right) Spatial
distribution of the occurrence frequency of dry air (RH <20%) at the 330 K isentropic level for statistics during
JanuaryFebruary 2014 (during CONTRAST), derived from GFS analyses. The pink lines show the corresponding
zonal winds (m/s). The location of Guam is noted.


Publication information:

Pan, L.L., S. B. Honomichl, W. J. Randel, E. C. Apel, E. L. Atlas, S. P. Beaton, J. F. Bresch, R. Hornbrook, D. E. Kinnison, J-F Lamarque, A. Saiz-Lopez, R. J. Salawitch, and A. J. Weinheimer, 2015: Bimodal distribution of free tropospheric ozone over the tropical western Pacific revealed by airborne observations, Geophys. Res. Lett., 42, 7844–7851, doi: 10.1002/2015GL065562

Randel, W.J., L. Rivoire, L. L. Pan and S. B. Honomichl, 2016: Dry layers in the tropical troposphere observed during CONTRAST and global behavior from GFS analyses, J. Geophys. Res. Atmos., 121, 14,142–14,158, doi:10.1002/2016JD025841.





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