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

Abstract

The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was an aircraft‐based field campaign conducted from Guam (14°N, 145°E) during January–February 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 (~3–9 km). 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 ~50–80% 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 ~320–340 K, which extend into the extratropical upper troposphere‐lower stratosphere (UTLS). 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 quasi‐isentropic transport from the extratropical UTLS and suggest a ubiquitous UTLS influence on the subtropical middle troposphere.

Tropospheric dry layers in the tropical western Pacific: comparisons of GPS radio occultation with multiple data sets

“…Overall, dry layers occur throughout the year in some regions of the SH, but there is also a seasonal cycle with higher occurrence in the winter hemisphere. This behavior is consistent with the dry layer climatology derived from ERA (results not shown) and from GFS data in Randel et al (2016).…”

“…RO q values are biased positively compared to CONTRAST for low q values and biased negatively for high q values. Comparison on q between CONTRAST and GFS in Randel et al (2016) shows a similar bias. Because of the highly accurate aircraft water vapor and temperature measurements and the very small scale of the observation (essentially a point observation), the CONTRAST measurements are capable of detecting extremes of dry and moist air more frequently than RO observations or model estimates, whose data represent averages over larger scales.…”

“…Finally, Randel et al (2016) performed a detailed comparison between aircraft measurements from the CONvective TRansport of Active Species in the Tropics (CONTRAST) experiment and GFS meteorological analyses, demonstrating that the analyses accurately capture the behavior of subtropical dry layers. A global climatology from GFS data shows that the dry layers are a ubiquitous feature of the subtropics, with maximum occurrence frequency in the winter hemisphere (linked to the strongest subtropical jets).…”

Evaluating tropospheric humidity from GPS radio occultation, radiosonde, and AIRS from high-resolution time series

“…This is supported by individual profiles (e.g. Randel et al (2016), Fig. 4), as well as our comparison of ERA with RS ( Fig.…”

“…The changing humidity pattern 10 above 800 hPa results from the alternation of the tropical conditions (high humidity, strong convection) and dry air intrusions from the subtropical UTLS in December to June (Randel et al, 2016). These dry intrusions (relative humidity as low as a few percent) are very stable and suppress convection.…”