Although the tropopause is a well‐established concept, its definition and physical properties remain an active research topic. In the tropics, both the World Meteorological Organization established ...lapse rate tropopause definition and the minimum in the temperature profile (the cold point) are used to determine the tropopause height. We examine the differences produced by these two definitions using high‐resolution airborne in situ measurements of temperature, water vapor, and ozone in the tropical tropopause layer from a recent experiment over the western Pacific using the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aircraft system. When the two definitions do not produce the same tropopause height, which is in about half of the cases, the combined temperature and trace gas analysis shows that the lapse rate definition better identifies the transition from the troposphere to the stratosphere.
Plain Language Summary
Discovered more than a century ago, the tropopause is known to mark the boundary of two dynamically and chemically distinct layers of atmosphere, the stratosphere, and the troposphere. In the tropics, the location, temperature, and physical/chemical gradients of the tropopause are important as part of the fundamental knowledge of the atmosphere and for regulating the amount of water vapor entering the stratosphere, which has a significant contribution to climate forcing. The tropopause over the tropical western Pacific, in particular, is known as the decisive region for determining the amount of stratospheric water vapor. High‐resolution measurements for this region are rare because the region is remote and tropopause altitudes are difficult to access. An airborne experiment targeting this decisive region was conducted in 2014, using the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aircraft system. These high‐resolution temperature and trace gas data provided an unprecedented opportunity to examine the physical meaning of the two tropical tropopause definitions, known as the lapse‐rate tropopause and the cold‐point tropopause. In this work, we demonstrate how the relationship of two chemical tracers, ozone and water vapor, can unambiguously identify the transition from troposphere to stratosphere and therefore serve to diagnose the effectiveness of the different tropopause definitions.
Key Points
The tropical tropopause definitions are examined using airborne in situ measurements over the tropical western Pacific
O3 and H2O relationship is used to identify the air mass change from the troposphere to the stratosphere
The lapse rate definition is shown to more consistently identify the tropopause based on the tracer diagnostic
The NASA Airborne Tropical Tropopause Experiment (ATTREX) deployment in January–March 2014 yielded more than 34 h of cirrus cloud sampling in the tropical tropopause layer (TTL) over the western ...Pacific. Cirrus were encountered throughout the TTL, at temperatures between 185 and 207 K, with ice water content (IWC) ranging from >10 mg m−3 to below the instrumental detection limit of 1 μg m−3. Geometric optical extinction (σ) values determined from cloud particle probe measurements ranged from 10−3 m−1 to <10−7 m−1. The median effective diameter (Deff) for cirrus sampled at T > 192 K was 40–50 μm and exhibited a weak temperature dependence, while colder than 192 K, Deff decreased more strongly with decreasing T. From the ATTREX data, a new parameterization of the IWC‐σ relationship for TTL cirrus is derived that will improve the estimation of IWC from lidar and optical probe observations of these clouds.
Plain Language Summary
Cold, high‐altitude tropical cirrus clouds are an important component of the climate system but are significantly underconstrained in climate models. Lidar measurements, especially from satellites, have the spatial and temporal coverage to produce statistically meaningful observations for model comparison and validation but do not directly measure quantities such as cloud ice water content that are important predicted quantities in the models. We have used an extensive data set of cloud ice water content and microphysical properties collected during a 2014 aircraft campaign in the western Pacific to derive a new parameterization that will improve the estimation of ice water content from lidar remote sensing measurements of tropical cirrus. With this parameterization, lidar observations can be used to derive a more accurate ice water content for tropical cirrus, in order to improve its treatment in models.
Key Points
The 2014 deployment of the ATTREX mission yielded an extensive data set of TTL cirrus IWC and microphysical properties
The effective diameter for TTL cirrus exhibited a weak temperature dependence above 192 K but decreased strongly between 192 and 186 K
New parameterization of the IWC‐extinction relationship will improve lidar‐derived IWC for climatically important TTL cirrus clouds
Dispersion of backward air parcel trajectories that are initially tightly grouped near the tropical tropopause is examined using three ensemble approaches: “RANWIND,” in which different ensemble ...members use identical resolved wind fluctuations but different realizations of stochastic, multifractal simulations of unresolved winds; “PERTLOC,” in which members use identical resolved wind fields but initial locations are perturbed 2° in latitude and longitude; and a multimodel ensemble (“MULTIMODEL”) that uses identical initial conditions but different resolved wind fields and/or trajectory formulations. Comparisons among the approaches distinguish, to some degree, physical dispersion from that due to data uncertainty and the impacts of unresolved wind fluctuations from those of resolved variability. Dispersion rates are robust properties of trajectories near the tropical tropopause. Horizontal dispersion rates are typically ~3°/d, which is large enough to spread parcels throughout the tropics within typical tropical tropopause layer transport times (30–60 days) and underscores the importance of averaging large collections of trajectories to obtain reliable parcel source and pathway distributions. Vertical dispersion rates away from convection are ~2–3 hPa/d. Dispersion is primarily carried out by the resolved flow, and the RANWIND approach provides a plausible representation of actual trajectory dispersion rates, while PERTLOC provides a reasonable and inexpensive alternative to RANWIND. In contrast, dispersion from the MULTIMODEL calculations is important because it reflects systematic differences in resolved wind fields from different reanalysis data sets.
Key Points
Dispersion rates are robust properties of trajectories near the tropical tropopause
Dispersion is primarily carried out by the resolved flow
Physical dispersion can be distinguished from trajectory error
From 2016 to 2018 a DC-8 aircraft operated by the US National
Aeronautics and Space Administration (NASA) made four series of flights,
profiling the atmosphere from 180 m to ∼12 km above sea level ...(km a.s.l.)
from the Arctic to the Antarctic over both the Pacific and Atlantic oceans.
This program, the Atmospheric Tomography Mission (ATom), sought to sample
the troposphere in a representative manner, making measurements of
atmospheric composition in each season. This paper describes the
aerosol microphysical measurements and derived quantities obtained during
this mission. Dry size distributions from 2.7 nm to 4.8 µm in
diameter were measured in situ at 1 Hz using a battery of instruments: 10
condensation particle counters with different nucleation diameters, two
ultra-high-sensitivity aerosol size spectrometers (UHSASs), one of which
measured particles surviving heating to 300 ∘C, and a laser
aerosol spectrometer (LAS). The dry aerosol measurements were complemented
by size distribution measurements from 0.5 to 930 µm diameter at
near-ambient conditions using a cloud, aerosol, and precipitation
spectrometer (CAPS) mounted under the wing of the DC-8. Dry aerosol number,
surface area, and volume, and optical scattering and asymmetry parameters at
several wavelengths from the near-UV to the near-IR ranges were calculated from the
measured dry size distributions (2.7 nm to 4.8 µm). Dry aerosol mass
was estimated by combining the size distribution data with particle density
estimated from independent measurements of aerosol composition with a
high-resolution aerosol mass spectrometer and a single-particle soot
photometer. We describe the instrumentation and fully document the aircraft
inlet and flow distribution system, the derivation of uncertainties, and the
calculation of data products from combined size distributions. Comparisons
between the instruments and direct measurements of some aerosol properties
confirm that in-flight performance was consistent with calibrations and
within stated uncertainties for the two deployments analyzed. The unique
ATom dataset contains accurate, precise, high-resolution in situ
measurements of dry aerosol size distributions, and integral parameters, and
estimates and measurements of optical properties, for particles < 4.8 µm in diameter that can be used to evaluate aerosol abundance and
processes in global models.
In situ measurements of aerosol microphysical, chemical, and optical properties were made during global-scale flights from 2016-2018 as part of the Atmospheric Tomography Mission (ATom). The NASA ...DC-8 aircraft flew from â¼ 84.sup." N to â¼ 86.sup." S latitude over the Pacific, Atlantic, Arctic, and Southern oceans while profiling nearly continuously between altitudes of â¼ 160 m and â¼ 12 km. These global circuits were made once each season. Particle size distributions measured in the aircraft cabin at dry conditions and with an underwing probe at ambient conditions were combined with bulk and single-particle composition observations and measurements of water vapor, pressure, and temperature to estimate aerosol hygroscopicity and hygroscopic growth factors and calculate size distributions at ambient relative humidity. These reconstructed, composition-resolved ambient size distributions were used to estimate intensive and extensive aerosol properties, including single-scatter albedo, the asymmetry parameter, extinction, absorption, Ãngström exponents, and aerosol optical depth (AOD) at several wavelengths, as well as cloud condensation nuclei (CCN) concentrations at fixed supersaturations and lognormal fits to four modes. Dry extinction and absorption were compared with direct in situ measurements, and AOD derived from the extinction profiles was compared with remotely sensed AOD measurements from the ground-based Aerosol Robotic Network (AERONET); this comparison showed no substantial bias.
Abstract
Numerical 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 concentrations compared with in situ observations. Inclusion of relatively numerous heterogeneous ice nuclei (NIN ≥ 100 L−1) in the simulations improves the agreement with observed ice concentrations. However, when IN contribute significantly to TTL cirrus ice nucleation, the occurrence frequency of large supersaturations with respect to ice is less than indicated by in situ measurements. The model results suggest that the sensitivity of TTL cirrus extinction and ice water content statistics to heterogeneous ice nuclei abundance is relatively weak. The simulated occurrence frequencies of TTL cirrus are quite insensitive to ice nuclei abundance, both in terms of cloud frequency height distribution and regional distribution throughout the tropics.
On 13 July 2002 a widespread, subvisible tropopause cirrus layer occurred over the Florida region. This cloud was observed in great detail with the NASA Cirrus Regional Study of Tropical Anvils and ...Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE) instrumentation, including in situ measurements with the WB‐57 aircraft. In this paper, we use the 13 July cloud as a case study to evaluate the physical processes controlling the formation and evolution of tropopause cirrus layers. Microphysics measurements indicate that ice crystal diameters in the cloud layer ranged from about 7 to 50 μm, and the peak number mode was about 10–25 μm. In situ water vapor and temperature measurements in the cloud indicated supersaturation with respect to ice throughout, with ice saturation ratios as large as 1.8. Even when the ice surface area density was as high as about 500 μm2 cm−3, ice supersaturations of 20–30% were observed. Trajectory analysis shows that the air sampled near the tropopause on this day generally came from the north and cooled considerably during the previous few days. Examination of infrared satellite imagery along air parcel back trajectories from the WB‐57 flight track indicates that the tropopause cloud layer formation was, in general, not simply left over ice from recently generated anvil cirrus. Simulations of cloud formation using time‐height curtains of temperature along the trajectory paths show that the cloud could have formed in situ near the tropopause as the air was advected into the south Florida region and cooled to unusually low temperatures. If we assume a high threshold for ice nucleation via homogeneous freezing of aqueous sulfate aerosols, the model reproduces the observed cloud structure, ice crystal size distributions, and ice supersaturation statistics. Inclusion of observed gravity wave temperature perturbations in the simulations is essential to reproduce the observed cloud properties. Without waves, crystal number densities are too low, crystal sizes are too large, and the crystals fall out too fast, leaving very little cloud persisting at the end of the simulations. In the cloud simulations, coincidence of high supersaturations and high surface areas can be produced by either recent nucleation or sedimentation of crystals into supersaturated layers. The agreement between model results and observed supersaturations is improved somewhat if we assume that the steady state relative humidity within cirrus at T < 200 K is enhanced by about 30%. The WB‐57 measurements and the model results suggest that the cloud layer irreversibly dehydrated air near the tropopause.
Abstract
The contribution of heterogeneous ice nucleation to the formation of cirrus cloud ice crystals is still not well quantified. This results in large uncertainties when predicting cirrus ...radiative effects and their role in Earth’s climate system. The goal of this case study is to simulate the composition, and thus activation conditions, of ice nucleating particles (INPs) to evaluate their contribution to heterogeneous cirrus ice formation in relation to homogeneous ice nucleation. For this, the regional model COSMO—Aerosols and Reactive Trace Gases (COSMO-ART) was used to simulate a synoptic cirrus cloud over Texas on 13 April 2011. The simulated INP composition was then compared to measured ice residual particle (IRP) composition from the actual event obtained during the NASA Midlatitude Airborne Cirrus Properties Experiment (MACPEX) aircraft campaign. These IRP measurements indicated that the dominance of heterogeneous ice nucleation was mainly driven by mineral dust with contributions from a variety of other particle types. Applying realistic activation thresholds and concentrations of airborne transported mineral dust and biomass-burning particles, the model implementing the heterogeneous ice nucleation parameterization scheme of Ullrich et al. is able to reproduce the overall dominating ice formation mechanism in contrast to the model simulation with the scheme of Phillips et al. However, the model showed flaws in reproducing the IRP composition.
Abstract The 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field experiment obtained a diverse set of in situ and remotely sensed measurements before and during a ...pyrocumulonimbus (pyroCb) event over the Williams Flats fire in Washington State. This unique dataset confirms that pyroCb activity is an efficient vertical smoke transport pathway into the upper troposphere and lower stratosphere (UTLS). The magnitude of smoke plumes observed in the UTLS has increased significantly in recent years, following unprecedented wildfire and pyroCb activity observed worldwide. The FIREX-AQ pyroCb dataset is therefore extremely relevant to a broad community, providing the first measurements of fresh smoke exhaust in the upper troposphere, including from within active pyroCb cloud tops. High-resolution remote sensing reveals that three plume cores linked to localized fire fronts, burning primarily in dense forest fuels, contributed to four total pyroCb “pulses.” Rapid changes in fire geometry and spatial extent dramatically influenced the magnitude, behavior, and duration of pyroCb activity. Cloud probe measurements and weather radar identify the presence of large ice particles within the pyroCb and hydrometers below cloud base, indicating precipitation development. The resulting feedbacks suggest that vertical smoke transport efficiency was reduced slightly when compared with intense pyroCb events reaching the lower stratosphere. Physical and optical aerosol property measurements in pyroCb exhaust are compared with previous assumptions. A large suite of aerosol and gas-phase chemistry measurements sets a foundation for future studies aimed at understanding the composition of smoke plumes lifted by pyroconvection into the UTLS and their role in the climate system.
THE NASA AIRBORNE TROPICAL TROPOPAUSE EXPERIMENT Jensen, Eric J.; Pfister, Leonhard; Jordan, David E. ...
Bulletin of the American Meteorological Society,
01/2017, Letnik:
98, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The 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 data are publicly available online (at https://espoarchive.nasa.gov/).
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