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.
A ubiquitous cold signal near the tropopause, here called “tropopause layer cooling” (TLC), has been documented in deep convective regions such as tropical cyclones (TCs). Temperature retrievals from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) reveal cooling of order 0.1–1 K day−1 on spatial scales of order 1000 km above TCs. Data from the Cloud Profiling Radar (onboard CloudSat) and from the Cloud–Aerosol Lidar with Orthogonal Polarization [onboard the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)] are used to analyze cloud distributions associated with TCs. Evidence is found that convective clouds within TCs reach the upper part of the tropical tropopause layer (TTL) more frequently than do convective clouds outside TCs, raising the possibility that convective clouds within TCs and associated cirrus clouds modulate TLC. The contribution of clouds to radiative heating rates is then quantified using the CloudSat and CALIPSO datasets: in the lower TTL (below the tropopause), clouds produce longwave cooling of order 0.1–1 K day−1 inside the TC main convective region, and longwave warming of order 0.01–0.1 K day−1 outside; in the upper TTL (near and above the tropopause), clouds produce longwave cooling of the same order as TLC inside the TC main convective region, and one order of magnitude smaller outside. Considering that clouds also produce shortwave warming, cloud radiative effects are suggested to explain only modest amounts of TLC while other processes must provide the remaining cooling.
Tropical cyclones (TCs) are associated with tropopause‐level cooling above tropospheric warming. We collect temperature retrievals from 2007 to 2014 near worldwide hurricane‐strength TCs using three remote sensing platforms: the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), the Advanced Microwave Sounding Unit‐A (AMSU‐A), and geostationary infrared (IR) imagery. These retrievals are composited about the lifetime maximum intensity (LMI) to examine the evolution of the fine‐scale temperature structure within TCs. The convective structure evolves highly asymmetrically about LMI, while intensity evolution shows a much weaker degree of asymmetry. Relative to the far‐field structure, tropopause‐level cooling occurs before a tropospheric warm core is established. We speculate that the associated convective destabilization exerts a positive feedback on TC development by increasing the depth of existing convection. Tropopause‐level cold anomalies move away from the storm after LMI, potentially increasing the near‐surface horizontal pressure gradient toward the storm center and increasing the maximum winds.
<p>Tropopause folds are documented to be frequent occurrences in the vicinity of the polar and subtropical jets. The rapidly changing nature of the folds and their complex fine scale structure make quantifying the associated cross-tropopause transport a significant challenge. To date, observational data sets do not provide sufficient coverage or resolution to easily overcome this challenge. In addition, ground-based observations are only representative of local processes or extreme events and do not directly inform global behavior. As a result, cross-tropopause transport estimates have relied on global models and reanalyses. However, observational evidence suggests that such models are prone to errors in both the occurrence frequency of tropopause folds and the amount of transport they generate individually. These limitations serve as the basis for our work, and we focus on a new framework to quantify the occurrence frequency of tropopause folds.</p><p>&#160;</p><p>Existing literature provides various methods to quantify the occurrence frequency of tropopause folds, with some using Lagrangian parcel trajectories and others using tracer-like quantities and dynamical proxies for transport. Results vary greatly in distribution and in amplitude. Overall, because tropopause folds are associated with jet streams, a central problem lies in tracking said jet streams. Existing jet tracking algorithms tend to be complex, computationally expensive, and rely on a variety of ad hoc parameters and thresholds that are based on current climatologies (such as a minimum wind speed threshold). Consequently, these algorithms produce outputs that are sensitive to arbitrary choices and that are not well suited for climate studies.</p><p>&#160;</p><p>We develop a jet tracking algorithm with two central improvements:</p><p>1) it includes temporal information about the evolution of features of interest, by using a time-integrated variable that provides information about parcel transport;</p><p>2) it minimizes the use of ad hoc parameters by defining jet features qualitatively, i.e., as spatially and temporally coherent local maxima in parcel transport;</p><p>By including temporal information, we are able to track dynamically relevant features, which is a substantial improvement over existing algorithms that use instantaneous meteorological fields. We present a comparison of the jet stream features identified by our algorithm versus existing ones. We also use the output of our algorithm as a jet-relative coordinate system, which allows us to identify tropopause folding events in global data sets, and to quantify their occurrence frequency.</p>
<p>Given the couplings between the circulation of the stratosphere and its composition, tracking the evolution of both is crucial. At present however, much remains to be learned about long term trends in the composition of the stratosphere, and there is still little to no agreement between the modeled trends in the Brewer Dobson Circulation and their observational counterparts; while models indicate that the BDC is accelerating at a pace of 2-3 %/decade, observational estimates suggest that the BDC is slowing down. These shortcomings are attributable in part to the relatively short length of the historical record and in part to difficulty characterizing the BDC using observations.</p> <p>To alleviate these shortcomings, we propose to re-visit historical and projected BDC trends using the metric time of emergence (ToE), defined as the length of record needed to separate long-term trends from internal variability with a chosen degree of statistical confidence. We use ToE as it enables the evaluation of current observational capabilities for the detection and validation of BDC trends predicted by models. ToE also provides tangible motivation for the continued monitoring of the composition of the stratosphere by space borne platforms, a topic recently brought to light by the planned decommissioning of the Aura satellite in the absence of a follow-up flight mission.</p> <p>ToE is calculated using two methods, for which results are compared: a) classic bootstrapping based on a reference CMIP6 run (a pre-industrial run, or a run with fixed contemporary greenhouse gas concentrations), and b) an analytical method published by Li et al. (2017) that does not require a very long reference run. We focus the analysis on a comparison of ToE for trends in a) the diabatic circulation, taken as reference for the &#8220;true&#8221; BDC, and b) the BDC metric based on age of air developed by Linz et al. (2016), used as a proxy for observational trend estimates. The results shed light on how internal variability shapes our understanding of long term trends, and provide minimum requirements for the robust detection of trends in the BDC using observations.&#160;</p> <p>Li, J., Thompson, D.W., Barnes, E.A. and Solomon, S., 2017. Quantifying the lead time required for a linear trend to emerge from natural climate variability. <em>J. of Climate</em>.</p> <p>Linz, M., Plumb, R.A., Gerber, E.P. and Sheshadri, A., 2016. The relationship between age of air and the diabatic circulation of the stratosphere. <em>JAS</em>.</p>
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