Physical Processes Controlling the Distribution of Relative Humidity in the Tropical Tropopause Layer over the Pacific

The distribution of relative humidity with respect to ice (RHI) in the Boreal wintertime Tropical Tropopause Layer (about 14-19 km) over the Pacific is examined with the extensive dataset of measurements from the NASA Airborne Tropical TRopopause EXperiment (ATTREX).  Multiple deployments of the Global Hawk during ATTREX provided hundreds of vertical profiles spanning the Pacific with accurate measurements of temperature, pressure, water vapor concentration, ozone concentration, and cloud properties.  We also compare the measured RHI distributions with results from a transport and microphysical model driven by meteorological analysis fields.  Notable features in the distribution of RHI versus temperature and longitude include (1) the common occurrence of RHI values near ice saturation over the western Pacific in the lower TTL (temperatures greater than 200 K) and in airmasses with low ozone concentrations indicating recent detrainment from deep convection; (2) low RHI values in the lower TTL over the eastern Pacific where deep convection is infrequent; (3) RHI values following a constant H2O mixing ratio in the upper TTL (temperatures below about 195 K), particularly for samples with ozone mixing ratios greater than about 50-100 ppbv indicating mixtures of tropospheric and stratospheric air, and (4) RHI values typically near ice saturation in the coldest airmasses sampled (temperatures less than about 190 K). We find that the typically saturated air in the lower TTL over the western Pacific is largely driven by the frequent occurrence of deep convection in this region. The nearly-constant water vapor mixing ratios in the upper TTL result from the combination of slow ascent (resulting in long residence times) and wave-driven temperature variability on a range of time scales (resulting in most air parcels having experienced low temperature and dehydration).