Temperatures of Anvil Clouds and Radiative Tropopause in a Wide Array of Cloud-Resolving Simulations

We present ninety-nine cloud-resolving simulations to study how temperatures of anvil clouds and radiative tropopause change with surface warming. Our simulation results show that the radiative tropopause warms at approximately the same rate as anvil clouds. This relationship persists across a variety of modeling choices, including surface temperature, greenhouse gas concentration, and the representation of radiative transfer. We further show that the shifting ozone profile associated with climate warming may give rise to a fixed tropopause temperature as well as a fixed anvil temperature. This result points to the importance of faithful treatment of ozone in simulating clouds and climate change; the robust anvil-tropopause relationship may also provide alternative ways to understand what controls anvil temperature.

Diurnal Variability of Lower and Middle Atmospheric Water Vapour Over The Asian Summer Monsoon Region: First Results From COSMIC-1 and TIMED-SABER Measurements

First observations on the vertical structure of diurnal variability of tropospheric water vapour in the lower and middle atmosphere using 13 years of COSMIC and 18 years of SABER observations are presented in this paper. The most significant and new observation is that the middle stratospheric water vapour (SWV) enhancement is observed between 9-18 LT, whereas it is between 6-15 LT near tropopause in all the seasons. The diurnal amplitude of water vapour near tropopause is between 0.3-0.4 ppmv. Bimodal peaks are found in the diurnal amplitude of SWV, maximizing between 25-30 km (~0.4 ppmv) and 45-50 km (~0.6 ppmv). The analysis reveals that the diurnal variability in the lower SWV is controlled by the tropical tropopause temperature, whereas the middle and upper SWV is controlled by methane oxidation. The results are presented and discussed in the light of present understanding.

Geometric estimation of volcanic eruption column height from GOES-R near-limb imagery – Part 2: Case studies

In a companion paper (Horváth et al., 2021), we introduced a new technique to estimate volcanic eruption column height from extremely oblique near-limb geostationary views. The current paper demonstrates and validates the technique in a number of recent eruptions, ranging from ones with weak columnar plumes to subplinian events with massive umbrella clouds and overshooting tops that penetrate the stratosphere. Due to its purely geometric nature, the new method is shown to be unaffected by the limitations of the traditional brightness temperature method, such as height underestimation in subpixel and semitransparent plumes, ambiguous solutions near the tropopause temperature inversion, or the lack of solutions in undercooled plumes. The side view height estimates were in good agreement with plume heights derived from ground-based video and satellite stereo observations, suggesting they can be a useful complement to established techniques.

Geometric estimation of volcanic eruption column height from GOES-R near-limb imagery – Part 2: Case studies

In a companion paper (Horváth et al., 2021), we introduced a new technique to estimate volcanic eruption column height from extremely oblique near-limb geostationary views. The current paper demonstrates and validates the technique in a number of recent eruptions, ranging from ones with weak columnar plumes to subplinian events with massive umbrella clouds and overshooting tops that penetrate the stratosphere. Due to its purely geometric nature, the new method is shown to be unaffected by the limitations of the traditional brightness temperature method, such as height underestimation in subpixel and semitransparent plumes, ambiguous solutions near the tropopause temperature inversion, or the lack of solutions in undercooled plumes. The side view height estimates were in good agreement with plume heights derived from ground-based video and satellite stereo observations, suggesting they can be a useful complementary to established techniques.

Impacts of Ozone Changes in the Tropopause Layer on Stratospheric Water Vapor

Stratospheric water vapor (SWV) changes play an important role in regulating global climate change, and its variations are controlled by tropopause temperature. This study estimates the impacts of tropopause layer ozone changes on tropopause temperature by radiative process and further influences on lower stratospheric water vapor (LSWV) using the Whole Atmosphere Community Climate Model (WACCM4). It is found that a 10% depletion in global (mid-low and polar latitudes) tropopause layer ozone causes a significant cooling of the tropical cold-point tropopause with a maximum cooling of 0.3 K, and a corresponding reduction in LSWV with a maximum value of 0.06 ppmv. The depletion of tropopause layer ozone at mid-low latitudes results in cooling of the tropical cold-point tropopause by radiative processes and a corresponding LSWV reduction. However, the effect of polar tropopause layer ozone depletion on tropical cold-point tropopause temperature and LSWV is opposite to and weaker than the effect of tropopause layer ozone depletion at mid-low latitudes. Finally, the joint effect of tropopause layer ozone depletion (at mid-low and polar latitudes) causes a negative cold-point tropopause temperature and a decreased tropical LSWV. Conversely, the impact of a 10% increase in global tropopause layer ozone on LSWV is exactly the opposite of the impact of ozone depletion. After 2000, tropopause layer ozone decreased at mid-low latitudes and increased at high latitudes. These tropopause layer ozone changes at different latitudes cause joint cooling in the tropical cold-point tropopause and a reduction in LSWV. Clarifying the impacts of tropopause layer ozone changes on LSWV clearly is important for understanding and predicting SWV changes in the context of future global ozone recovery.

Axisymmetric Hadley Cell Theory with a Fixed Tropopause Temperature Rather than Height

Axisymmetric Hadley cell theory has traditionally assumed that the tropopause height (Ht) is uniform and unchanged from its radiative–convective equilibrium (RCE) value by the cells’ emergence. Recent studies suggest that the tropopause temperature (Tt), not height, is nearly invariant in RCE, which would require appreciable meridional variations in Ht. Here, we derive modified expressions of axisymmetric theory by assuming a fixed Tt and compare the results to their fixed-Ht counterparts. If Tt and the depth-averaged lapse rate are meridionally uniform, then at each latitude Ht varies linearly with the local surface temperature, altering the diagnosed gradient-balanced zonal wind at the tropopause appreciably (up to tens of meters per second) but the minimal Hadley cell extent predicted by Hide’s theorem only weakly (≲1°) under standard annual-mean and solsticial forcings. A uniform Tt alters the thermal field required to generate an angular-momentum-conserving Hadley circulation, but these changes and the resulting changes to the equal-area model solutions for the cell edges again are modest (<10%). In numerical simulations of latitude-by-latitude RCE under annual-mean forcing using a single-column model, assuming a uniform Tt is reasonably accurate up to the midlatitudes, and the Hide’s theorem metrics are again qualitatively insensitive to the tropopause definition. However imperfectly axisymmetric theory portrays the Hadley cells in Earth’s macroturbulent atmosphere, evidently its treatment of the tropopause is not an important error source.

Variations in The Frequency of Sudden Stratospheric Warmings in CMIP5 and CMIP6

<p>The frequency of sudden stratospheric warming events (SSWs) is an essential characteristic of the coupled stratosphere-troposphere system. This study is motivated by the fact that many of the CMIP5 and CMIP6 climate models considerably over- or underestimate the observed SSW frequency. The goal is to understand the causes for the large intermodel spread in the number of SSWs and relate it to specific model configurations. To this end, various dynamical quantities associated with the simulation of SSWs are investigated. It is found that variations in the SSW frequency are closely related to the strength of the polar vortex and the stratospheric wave activity. While it is difficult to explain the variations in the strength of the polar vortex, the stratospheric wave activity is strongly influenced by the background state (i.e., zonal wind and index of refraction) of the lower stratosphere. An important regulator for the background is the extratropical tropopause temperature, which in turn is associated with the vertical model resolution. Low-resolution models tend to have large biases in simulating the location and temperature of the extratropical tropopause. The results indicate that the simulated SSW frequency is a useful metric for model performance, as the frequency is highly sensitive to a number of stratospheric and tropospheric factors.</p>