Explore the vast range of titles available.

Tropical tropopause layer (TTL) cirrus clouds play a key role in the Earth climate system. Yet the relative role of the various processes shaping them remains poorly known. Characterizing the temporal evolution of cloudy structures from observations is essential to address this issue but represents a challenge. Indeed, space‐ and airborne platforms move fast and mainly provide instantaneous snapshots. In boreal winter 2021–2022, two balloon‐borne lidars flew over the Equatorial Pacific Ocean, slowly drifting above the clouds. We use those unique nighttime observations to quantify the distribution of TTL cloud lifetime above this homogeneous region. This distribution is strongly asymmetric: half of the clouds live less than 1 hr, but their mean lifetime is about 6 hr. The few long‐lived clouds (>12 ${ >} 12$ hr) dominate the cloud cover. Those results compare reasonably well with TTL cirrus lifetimes in the ERA5 reanalysis, although the modeled TTL cloud cover is largely underestimated.

Aerosol particles play a critical role in the tropical tropopause layer (TTL) through cloud formation and heterogeneous chemistry, influencing the radiative and chemical balance of the stratosphere. However, aerosol measurements in the TTL are sparse, resulting in poor knowledge of aerosol abundance and distribution in this important region. Here, we present in situ aircraft measurements over the western tropical Pacific, revealing a persistent and altitude‐dependent enhancement of aerosol mass in the TTL compared to the convectively influenced troposphere below. Notably, our data demonstrate a striking positive correlation between aerosol mass and ozone. Model simulations suggest that organic materials constitute a substantial fraction of the total aerosol mass within the TTL. We further derived an empirical parameterization of TTL aerosol mass as a function of ozone based on their linear relationship. This framework holds potential for estimating the TTL aerosol abundance but requires further validation and refinement through future measurements. Plain Language Summary We investigated tiny particles called aerosols in a specific atmospheric layer called the tropical tropopause layer (TTL). These particles are crucial because they affect cloud formation and chemical processes in the atmosphere, influencing how energy is distributed. Unfortunately, there hasn't been much research on aerosols in the TTL, leading to gaps in our understanding of their abundance and distribution in this important region. To fill this knowledge gap, we conducted measurements using aircraft over the western tropical Pacific. Our findings revealed that aerosol mass in the TTL is consistently higher compared to the lower troposphere, which is influenced by upward air movement. What's interesting is that we observed a clear connection between the amount of aerosol and ozone. Our model simulations indicated that a significant portion of the aerosol mass in the TTL is made up of organic materials. To make it easier to estimate aerosol levels and their impact on climate, we developed a way to predict TTL aerosol mass based on ozone measurements. Since ozone is relatively straightforward to measure and model, our method could provide a useful framework for understanding aerosol abundance in the TTL and its effects on the climate. Key Points Aircraft measurements reveal persistent enhancement of aerosol mass in the TTL The TTL aerosol enhancement tightly correlates with ozone. An empirical parameterization of TTL aerosol as a function of ozone is derived Modeling suggests that TTL aerosol particles are mainly composed of organics and sulfate

We investigate the impact of tropical tropopause warming on the stratospheric water vapor using the Specified-Dynamics version of the NCAR Whole Atmosphere Community Climate Model. We find that the tropical tropopause warming results in a strengthening of the Brewer–Dobson circulation (BDC). The strengthening of BDC induced by a narrow warming of tropical tropopause within 12° latitude, which is much stronger in boreal winter than that in boreal summer, propagates more dry air from the tropical tropopause into the stratosphere and thus causes a reduction of water vapor in the middle stratosphere. On the contrary, the seasonal difference of the BDC strengthening is weaker in the experiment with a broader tropical tropopause warming within 25° latitude. The drying effect of the BDC is counteracted by the moistening effects of the tropical tropopause warming and methane oxidation. This leads to the moistening in both the lower and upper stratosphere. The results suggest the control of the stratospheric humidity by the tropical tropopause temperature could be significantly offset by the associated BDC changes.

A single-column radiative-convective model (RCM) is a useful tool to investigate the physical processes that determine the tropical tropopause layer (TTL) temperature structures. Previous studies on the TTL using the RCMs, however, omitted the cloud radiative effects. In this study, we examine the impact of cloud radiative effects on the simulated TTL temperatures using an RCM. We derive the cloud radiative effects based on satellite observations, which show heating rates in the troposphere but cooling rates in the stratosphere. We find that the cloud radiative effect warms the TTL by as much as 2 K but cools the lower stratosphere by as much as −1.5 K, resulting in a thicker TTL. With (without) considering cloud radiative effects, we obtain a convection top of ≈167 hPa (≈150 hPa) with a temperature of ≈213 K (≈209 K), and a cold point at ≈87 hPa (≈94 hPa) with a temperature of ≈204 K (≈204 K). Therefore, the cloud radiative effects widen the TTL by both lowering the convection-top height and enhancing the cold-point height. We also examine the impact of TTL cirrus radiative effects on the RCM-simulated temperatures. We find that the TTL cirrus warms the TTL with a maximum temperature increase of ≈1.3 K near 110 hPa.

A new method of estimating Tropical Tropopause Layer Cirrus (TTLC) formation mechanism (object method) is compared to interpretations of formation from previous studies using back trajectory calculations matched to convection identified from satellites and statistical relationships of TTLC with temperature and water vapor. The object method groups neighboring Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) TTLC cloud profiles into cloud objects and classifies them as convective (35% of TTLC) if they are directly attached to a convective cloud along the CALIPSO track. The percentage of back trajectory calculations that intersect convection (39–95% of TTLC within 5 days) depends strongly on the spatial and temporal resolution of the convection data set, and the manner in which deep convection is identified. Using minimum brightness temperature in 3 hourly, 1° resolution grid boxes to define convection, 46% of non‐convective TTLC (37% of all TTLC) did not trace back to convection within 24 h. Back trajectory calculations of thin cirrus identified by the High Resolution Dynamics Limb Sounder (HIRDLS) gave similar results. Temperature is not a good proxy for formation mechanism as convective and non‐convective TTLC frequencies both increase monotonically with decreasing temperature at approximately the same rate. Non‐convective TTLC frequencies have a stronger relationship with relative humidity than convective TTLC frequencies, though are not sufficiently different to distinguish object method categories. A decrease in TTL cirrus frequency found at low temperatures in previous studies is caused by insufficient variability in reanalysis temperature data and is not indicative of TTLC formation mechanism. Key Points TTL cirrus (TTLC) formed both by detrainment and in situ ice nucleation Back trajectory definitions of TTLC formation sensitive to convective definition Temperature not sufficient to distinguish TTLC formation mechanisms

We describe Global Atmosphere 7.0 and Global Land 7.0 (GA7.0/GL7.0), the latest science configurations of the Met Office Unified Model (UM) and the Joint UK Land Environment Simulator (JULES) land surface model developed for use across weather and climate timescales. GA7.0 and GL7.0 include incremental developments and targeted improvements that, between them, address four critical errors identified in previous configurations: excessive precipitation biases over India, warm and moist biases in the tropical tropopause layer (TTL), a source of energy non-conservation in the advection scheme and excessive surface radiation biases over the Southern Ocean. They also include two new parametrisations, namely the UK Chemistry and Aerosol (UKCA) GLOMAP-mode (Global Model of Aerosol Processes) aerosol scheme and the JULES multi-layer snow scheme, which improve the fidelity of the simulation and were required for inclusion in the Global Atmosphere/Global Land configurations ahead of the 6th Coupled Model Intercomparison Project (CMIP6).In addition, we describe the GA7.1 branch configuration, which reduces an overly negative anthropogenic aerosol effective radiative forcing (ERF) in GA7.0 whilst maintaining the quality of simulations of the present-day climate. GA7.1/GL7.0 will form the physical atmosphere/land component in the HadGEM3–GC3.1 and UKESM1 climate model submissions to the CMIP6.

The relationship between relative humidity and cirrus clouds in the tropical tropopause layer (TTL) is investigated using balloon‐borne cryogenic frostpoint hygrometers (CFH) and quasi‐collocated measurements of space‐borne Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) over Biak (1.17°S, 136.06°E) and Kototabang (0.20°S, 100.32°E) both in Indonesia in Januaries 2007 and 2008. At Kototabang, thin layers of high supersaturation, up to ∼160% in relative humidity with respect to ice (RHi), are often observed co‐existing with cirrus clouds at altitudes of ∼15–18 km. At Biak, RHi inside cirrus is around 100% or less without large supersaturation layers, and most clouds are limited to altitudes below 16 km. We found that the presence and the degree of supersaturation may strongly depend on the phases of large‐scale disturbances such as the MJO rather than geographical difference. Key Points Many cases of high supersaturations inside cirrus in the TTL were found The sensors for water vapor and cirrus were the most reliable instruments Dependence of supersaturation on phase of large‐scale disturbance was suggested

Teleconnections between the Quasi Biennial Oscillation (QBO) and the Northern Hemisphere zonally averaged zonal winds, mean sea level pressure (mslp) and tropical precipitation are explored. The standard approach that defines the QBO using the equatorial zonal winds at a single pressure level is compared with the empirical orthogonal function approach that characterizes the vertical profile of the equatorial winds. Results are interpreted in terms of three potential routes of influence, referred to as the tropical, subtropical and polar routes. A novel technique is introduced to separate responses via the polar route that are associated with the stratospheric polar vortex, from the other two routes. A previously reported mslp response in January, with a pattern that resembles the positive phase of the North Atlantic Oscillation under QBO westerly conditions, is confirmed and found to be primarily associated with a QBO modulation of the stratospheric polar vortex. This mid-winter response is relatively insensitive to the exact height of the maximum QBO westerlies and a maximum positive response occurs with westerlies over a relatively deep range between 10 and 70 hPa. Two additional mslp responses are reported, in early winter (December) and late winter (February/March). In contrast to the January response the early and late winter responses show maximum sensitivity to the QBO winds at ∼ 20 and ∼ 70 hPa respectively, but are relatively insensitive to the QBO winds in between (∼ 50 hPa). The late winter response is centred over the North Pacific and is associated with QBO influence from the lowermost stratosphere at tropical/subtropical latitudes in the Pacific sector. The early winter response consists of anomalies over both the North Pacific and Europe, but the mechanism for this response is unclear. Increased precipitation occurs over the tropical western Pacific under westerly QBO conditions, particularly during boreal summer, with maximum sensitivity to the QBO winds at 70 hPa. The band of precipitation across the Pacific associated with the Inter-tropical Convergence Zone (ITCZ) shifts southward under QBO westerly conditions. The empirical orthogonal function (EOF)-based analysis suggests that this ITCZ precipitation response may be particularly sensitive to the vertical wind shear in the vicinity of 70 hPa and hence the tropical tropopause temperatures.

The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere with air masses showing chemical and dynamical properties of both regions. The representation of the TTL in meteorological reanalysis data sets is important for studying the complex interactions of circulation, convection, trace gases, clouds, and radiation. In this paper, we present the evaluation of climatological and long-term TTL temperature and tropopause characteristics in the reanalysis data sets ERA-Interim, ERA5, JRA-25, JRA-55, MERRA, MERRA-2, NCEP-NCAR (R1), and CFSR. The evaluation has been performed as part of the SPARC (Stratosphere–troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The most recent atmospheric reanalysis data sets (ERA-Interim, ERA5, JRA-55, MERRA-2, and CFSR) all provide realistic representations of the major characteristics of the temperature structure within the TTL. There is good agreement between reanalysis estimates of tropical mean temperatures and radio occultation data, with relatively small cold biases for most data sets. Temperatures at the cold point and lapse rate tropopause levels, on the other hand, show warm biases in reanalyses when compared to observations. This tropopause-level warm bias is related to the vertical resolution of the reanalysis data, with the smallest bias found for data sets with the highest vertical resolution around the tropopause. Differences in the cold point temperature maximize over equatorial Africa, related to Kelvin wave activity and associated disturbances in TTL temperatures. Interannual variability in reanalysis temperatures is best constrained in the upper TTL, with larger differences at levels below the cold point. The reanalyses reproduce the temperature responses to major dynamical and radiative signals such as volcanic eruptions and the quasi-biennial oscillation (QBO). Long-term reanalysis trends in temperature in the upper TTL show good agreement with trends derived from adjusted radiosonde data sets indicating significant stratospheric cooling of around −0.5 to −1 K per decade. At 100 hPa and the cold point, most of the reanalyses suggest small but significant cooling trends of −0.3 to −0.6 K per decade that are statistically consistent with trends based on the adjusted radiosonde data sets. Advances of the reanalysis and observational systems over the last decades have led to a clear improvement in the TTL reanalysis products over time. Biases of the temperature profiles and differences in interannual variability clearly decreased in 2006, when densely sampled radio occultation data started being assimilated by the reanalyses. While there is an overall good agreement, different reanalyses offer different advantages in the TTL such as realistic profile and cold point temperature, continuous time series, or a realistic representation of signals of interannual variability. Their use in model simulations and in comparisons with climate model output should be tailored to their specific strengths and weaknesses.

Ozone is a key radiative species near the tropical tropopause, which acts as a gateway to the stratosphere for ascending air. Ozone concentrations at fixed heights in this region fluctuate seasonally and interannually as the strength of stratospheric upwelling varies, influencing local temperatures and stratospheric composition. Models ranging in complexity suggest that an accelerated stratospheric circulation, along with tropospheric expansion, could reduce tropical lower stratospheric ozone following surface warming. These modes of variability are often equated with variability at the tropical tropopause; however, tropopause height varies seasonally and interannually, and it is expected to rise as Earth's surface warms. Here, we explore how tropical tropopause ozone varies when considering changes to tropopause pressure. We first examine 15 years of MERRA-2 reanalysis data to distinguish variability at the tropical tropopause from nearby fixed pressure levels on annual-to-interannual timescales. We show that changes to tropopause pressure drive ozone's annual cycle at the tropical tropopause to be substantially smaller and out of phase from those at 95 or 105 hPa. We then investigate how tropical tropopause ozone responds to surface warming under a range of forcing scenarios using output from the Chemistry-Climate Modeling Initiative (CCMI). We find that pressure-dependent ozone production coupled with tropospheric expansion leads tropical tropopause ozone variability to remain distinct from fixed pressure levels following surface warming, with divergent trends in the strongest forcing scenario. Finally, we show that increases to tropical tropopause ozone correspond with local warming in CCMI projections, while tropospheric expansion increases lower stratospheric ozone.