Physical Processes Controlling the Vertical and Longitudinal Distributions of Relative Humidity in the Tropical Tropopause Layer Over the Pacific

Abstract

The vertical distribution of relative humidity with respect to ice (RHI) in the Boreal wintertime Tropical Tropopause Layer (TTL, ≃14-18 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 longitudinal extent of 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-middle TTL (temperatures greater than 195 K); (2) low RHI values in the lower TTL over the central and eastern Pacific; (3) common occurrence of RHI values following a constant mixing ratio in the middle-to-upper TTL (temperatures between about 190 and 200 K), particularly for samples with ozone greater than about 50-100 ppbv indicating mixtures of tropospheric and stratospheric air; (4) RHI values typically near ice saturation in the coldest airmasses sampled (temperatures less than about 190 K); and (5) common occurrence of RHI values near 100% across the TTL temperature range in air parcels with low ozone mixing ratio (O₃ < 50 ppbv) indicative of recent uplift by deep convection. We suggest that the typically saturated air in the lower TTL over the western Pacific is likely driven by a combination of the frequent occurrence of deep convection and the predominance of radiative heating (rising motion) in this region. The low relative humidities in the central/eastern Pacific lower TTL result from the lack of convective influence, the predominance of subsidence, and the relatively warm temperatures in the region. The nearly-constant water vapor mixing ratios in the middle-to-upper TTL likely 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). The numerical simulations generally reproduce the observed RHI distribution features and sensitivity tests further emphasize the strong sensitivities of TTL relative humidity to convective input and vertical motions.

Figure 1

Occurrence frequency of convection extending above potential temperatures of 370 (a), 360 (b), and 350 K (c) during January-February 2013. The cloud-top heights are derived from geostationary-satellite infrared measurements, with TRMM precipitation measurements used to distinguish convective clouds from in situ cirrus. The grey boxes indicate the westPac and centeastPac regions discussed in the text.

Figure 2

ATTREX flight tracks (top panel) and height profiles (bottom panel). The 2013 flight paths are shown with blue lines, and the 2014 flight paths are green (Guam local flights) and red (transits) lines. The subsets of flights in the westPac and centeastPac regions are indicated by the grey boxes.

Figure 3

Frequency distributions of days since most recent convective encounter are plotted based on trajectory analysis from times and locations along the ATTREX flight paths. (a) 2014 western Pacific flights; (b) 2013 central/eastern Pacific flights. Different colors correspond to flight segments in different ranges of potential temperature corresponding to the different temperature regimes in the RHI distributions. The fraction of time along the flight tracks for which back trajectories did not intersect convection within 40 days are indicated in the legend. More recent convective influence in the western Pacific is apparent, particularly in the lower TTL.

Figure 4

The mean ERA-interim temperatures along the flight paths are plotted versus final potential temperature for the ATTREX-2014 (solid dark blue curve) and ATTREX-2013 (dashed blue curve) campaigns. Also shown are the mean minimum temperatures along 40-day back trajectories from the flight tracks (2014: solid dark red; 2013: dashed red curves). The horizontal bars indicate the standard deviations of the final and minimum temperatures.

Figure 5

Same as Figure 5 except that the data has been subsetted into ranges of measured ozone concentration. a: O₃ < 50 ppbv; b: 50 < O₃ < 100 ppbv; c: 100 < O₃ < 300 ppbv. Both clear-sky and in-cloud data are included.

Figure 6

Frequency distributions of relative humidity with respect to ice are plotted in 1-K temperature ranges. The left column shows data from the 2014 western Pacific campaign, and the right column shows data from the 2013 eastern and central Pacific campaign. The top panels include all data, middle panels include only clear-sky data, and bottom panels include only measurements made inside thin cirrus. Clear-sky and in-cloud measurements correspond to FCDP ice concentrations less than 15 L⁻¹ and greater than 50 L⁻¹, correspondingly. In order to avoid excessive influence of stratospheric samples on the statistics, we only include measurements made below 390 K potential temperature. The distributions are normalized by dividing by the total number of measurements within each temperature bin, and the color scale is logarithmic. Relative humidities were calculated using DLH water vapor, MMS temperature, and MMS pressure measurements. The black dotted curve corresponds to a parcel with a constant H₂O mixing ratio of 2 ppmv at 100 hPa undergoing adiabatic ascent/descent through the TTL.

Figure 7

Frequency distributions of relative humidity with respect to ice computed with the 2014 NOAA-H₂O vapor measurements (green curves) and total water measurements (red curves). (a) temperatures less than 190 K (upper TTL); (b) temperatures greater than 200 K (lower TTL).

Figure 8

Same as Figure 5 except that water vapor mixing ratio frequency distributions are shown for the 2014 (top panel) and 2013 (bottom panel) campaigns. The dashed line shows the saturation mixing ratio versus temperature.

Figure 9

Simulated frequency distributions of relative humidity versus temperature along the ATTREX-2014 (western Pacific) and ATTREX-2013 (central and eastern Pacific) flight tracks. The dotted lines indicating 2 ppmv H₂O mixing ratio are included to facilitate comparison with the measured RHI frequency distributions (Figure 5).

Figure 10

Sensitivity tests showing the impact on simulated ATTREX-2014 (WestPac) RHI frequency distribution of excluding convective influence (top panel) and excluding convection as well as using the heating rate profile from the tropics outside the western Pacific (bottom panel).