Rei Ueyama

To What Extent Does High-Latitude Wave Forcing Drive Tropical Upwelling in the Brewer–Dobson Circulation?

Abstract The causes of the annual cycle and nonseasonal variability in the globally averaged, equator-to-pole Brewer–Dobson circulation (BDC; defined here as the equatorially symmetric component of the Lagrangian-mean meridional circulation) are investigated based on zonally averaged, lower-stratospheric temperature data from satellite-borne Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit (AMSU). Time-varying vertical velocities in the BDC are inferred from departures of the meridional temperature profiles from the respective radiative equilibrium temperature profiles. Equatorward of ∼45°N/S, the annual-mean profile of lower-stratospheric temperature and the seasonal and nonseasonal variations about it project almost exclusively onto the equatorially symmetric component. The climatological-mean annual cycle accounts for nearly 90% of the month-to-month variance of the equatorially symmetric component of the temperature field; January/February is colder than July/August equatorward of ∼4...

The NASA Airborne Tropical Tropopause Experiment: High-altitude aircraft measurements in the Tropical Western Pacific

AbstractThe February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX da...

Dehydration in the tropical tropopause layer: A case study for model evaluation using aircraft observations

The dynamical and microphysical processes that influence water vapor concentrations in the tropical tropopause layer (TTL) are investigated in simulations of ice clouds along backward trajectories of air parcels sampled during three flights of the Airborne Tropical Tropopause Experiment over the central to eastern tropical Pacific in boreal fall 2011. ERA-Interim reanalysis temperatures interpolated onto the flight tracks have a negligible (−0.09 K) cold bias compared to aircraft measurements of tropical cold point temperature thus permitting case study simulations of TTL dehydration. When the effects of subgrid-scale waves, cloud microphysical processes, and convection are considered, the simulated water vapor mixing ratios on the final day of 40 day backward trajectories exhibit a mean profile that is within 20–30% of the mean of the aircraft measurements collected during vertical profiling maneuvers between the 350 and 410 K potential temperature levels. Averaged over the three flights, temperature variability driven by subgrid-scale waves dehydrated the 360–390 K layer by approximately −0.5 ppmv, whereas including homogeneous freezing of aqueous aerosols and subsequent sublimation and rehydration of ice crystals increased water vapor below the 380 K level by about +1 ppmv. The predominant impact of convection was to moisten the TTL, resulting in an average enhancement below the 370 K level by +1 to 5 ppmv. Accurate (to within 0.5–1 ppmv) predictions of TTL water vapor using trajectory models require proper representations of waves, in situ ice cloud formation, and convective influence, which together determine the saturation history of air parcels.

The Role of High-Latitude Waves in the Intraseasonal to Seasonal Variability of Tropical Upwelling in the Brewer–Dobson Circulation

The forcing of tropical upwelling in the Brewer‐Dobson circulation (BDC) on intraseasonal to seasonal time scales is investigated in integrations of an idealized general circulation model, ECMWF Interim ReAnalysis, and lower-stratospheric temperature measurements from the (Advanced) Microwave Sounding Unit, with a focus on the extended boreal winter season. Enhanced poleward eddy heat fluxes in the high latitudes (458‐908N) at the 100-hPa level are associated with anomalous tropical cooling and anomalous warming on the poleward side of the polar night jet at the 70-hPa level and above. In both the model and the observations, planetary waves entering the stratosphere at high latitudes propagate equatorward to the subtropics and tropics at levels above 70 hPa over an approximately 10-day period, exerting a force at sufficiently low latitudes to modulate the tropical upwelling in the upper branch of the BDC, even on time scales longer than the radiative relaxation time scale of the lower stratosphere. To the extent that they force the BDCviadownwardasopposedtosidewayscontrol,planetarywavesoriginatinginhighlatitudescontributeto the seasonally varying climatological mean and the interannual variability of tropical upwelling at the 70-hPa level and above. Their influence upon the strength of the tropical upwelling, however, diminishes rapidly with depth below 70 hPa. In particular, tropical upwelling at the cold-point tropopause, near 100 hPa, appears to be modulated by variations in the strength of the lower branch of the BDC.

Dynamical, convective, and microphysical control on wintertime distributions of water vapor and clouds in the tropical tropopause layer

Processes that influence the humidity and cirrus cloud abundance in the Tropical Tropopause Layer (TTL) during boreal winter 2006–2007 are investigated in simulations of clouds along backward trajectories of parcels ending at the 372 K potential temperature (100 hPa) level in the tropics. Trajectories are calculated using offline calculations of seasonal mean tropical radiative heating rates along with reanalysis temperature and wind data with enhanced wave-driven variability in the TTL. The one-dimensional (vertical) time-dependent cloud microphysical model is initialized with water vapor measurements from the Microwave Limb Sounder and the evolution of clouds along each trajectory is simulated using temperature profiles extracted from reanalysis data and convective cloud top heights estimated from 3-hourly geostationary satellite imagery. Averaged over the tropics, waves dehydrate the 100 hPa level by 0.5 ppmv, while convection and cloud microphysical processes moisten by 0.3 and 0.7 ppmv, respectively. The tropical mean cloud occurrence frequencies in the middle to upper TTL agree well with those based on satellite observations (spatial correlation of 0.8). Waves and convection enhance cloud occurrence at the cold point tropopause by 4% and 2%, respectively. Temporal variability of the heating rates as indicated by the ERA-Interim 6-hourly heating rate fields dehydrates the TTL by 0.4 ppmv and decreases the cloud occurrence by 4% because parcels are more likely to encounter the coldest temperatures and dehydrate near the cold point, limiting cloud formation above. The final dehydration locations of parcels, concentrated near the dateline in the tropical Pacific, are insensitive to various model parameters.

Investigation of the transport processes controlling the geographic distribution of carbon monoxide at the tropical tropopause

Convectively influenced trajectory calculations are used to investigate the impact of different Tropical Tropopause Layer (TTL) transport pathways for establishing the distribution of carbon monoxide (CO) at 100 hPa as observed by the Microwave Limb Sounder (MLS) on board the Aura satellite. Carbon monoxide is a useful tracer for investigating TTL transport and convective influence because the CO lifetime (≃1–2 months) is comparable to the time required for slow ascent through the TTL. MERRA horizontal winds are used for the diabatic trajectories, and off-line calculations of TTL radiative heating are used to determine the vertical motion field. The locations and times of convective influence events along the trajectories are determined from 3-hourly, geostationary satellite measurements of convective clouds. The trajectory model reproduces most of the prominent features in the 100 hPa CO geographic distribution indicated by the MLS observations for the winter and summer 2007 periods simulated. CO concentrations and tendencies simulated with the Whole Atmosphere Climate Chemistry Model (WACCM) are used to specify boundary-layer concentrations for convective influence and CO loss rates resulting from reaction with OH. The broad maximum in CO concentration over the Pacific during Boreal winter is primarily a result of the strong radiative heating (corresponding to upward vertical motion) associated with the abundant TTL cirrus in this region. Convection over the Pacific brings clean maritime air to the tropopause region and actually decreases the 100 hPa CO. The relative abundance of CO over the continental convective regions during wintertime is sensitive to small variations in convective cloud-top height. Both the simulated and the observed summertime 100 hPa CO distributions are dominated by the maximum co-located with the upper level anticyclone forced by the Asian monsoon convection. Sensitivity tests indicate that the summertime Asian monsoon anticyclone 100 hPa CO maximum is dominated by extreme convective systems with detrainment of polluted air above about 360–365 K potential temperature. This result stems directly from the fact that the heating rates are negative (downward motion) below 360–365 K during summertime through most of the tropics; therefore, air detrained from convection at lower levels will generally just sink back down into the middle troposphere. We find that most of the CO feeding into the Asian monsoon anticyclone comes from convection over the Tibetan Plateau and India, with relatively minor contributions from southeast Asia and eastern China.

On the Susceptibility of Cold Tropical Cirrus to Ice Nuclei Abundance

AbstractNumerical simulations of cirrus formation in the tropical tropopause layer (TTL) during boreal wintertime are used to evaluate the impact of heterogeneous ice nuclei (IN) abundance on cold cloud microphysical properties and occurrence frequencies. The cirrus model includes homogeneous and heterogeneous ice nucleation, deposition growth/sublimation, and sedimentation. Reanalysis temperature and wind fields with high-frequency waves superimposed are used to force the simulations. The model results are constrained by comparison with in situ and satellite observations of TTL cirrus and relative humidity. Temperature variability driven by high-frequency waves has a dominant influence on TTL cirrus microphysical properties and occurrence frequencies, and inclusion of these waves is required to produce agreement between the simulated and observed abundance of TTL cirrus. With homogeneous freezing only and small-scale gravity waves included in the temperature curtains, the model produces excessive ice con...

High-frequency gravity waves and homogeneous ice nucleation in tropical tropopause layer cirrus

The impact of high-frequency gravity waves on homogeneous-freezing ice nucleation in cold cirrus clouds is examined using parcel model simulations driven by superpressure balloon measurements of temperature variability experienced by air parcels in the tropical tropopause region. We find that the primary influence of high-frequency waves is to generate rapid cooling events that drive production of numerous ice crystals. Quenching of ice nucleation events by temperature tendency reversal in the highest-frequency waves does occasionally produce low ice concentrations, but the overall impact of high-frequency waves is to increase the occurrence of high ice concentrations. The simulated ice concentrations are considerably higher than indicated by in situ measurements of cirrus in the tropical tropopause region. One-dimensional simulations suggest that although sedimentation reduces mean ice concentrations, a discrepancy of about a factor of 3 with observed ice concentrations remains. Reconciliation of numerical simulations with the observed ice concentrations will require inclusion of physical processes such as heterogeneous nucleation and entrainment.

Physical processes controlling the spatial distributions of relative humidity in the tropical tropopause layer over the Pacific

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.