surface cooling
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2022 ◽  
pp. 1-54

Abstract State-of-the-art climate models exhibit significant spread in the climatological value of atmospheric shortwave absorption (SWA). This study investigates both the possible causes and climatic impacts of this SWA inter-model spread. The inter-model spread of global-mean SWA largely originates from the inter-model difference in water vapor shortwave absorptivity. Hence, we alter the water vapor shortwave absorptivity in the Community Earth System Model, version 1, with Atmosphere Model, version 4 (CESM1-CAM4). Increasing the water vapor shortwave absorptivity leads to a reduction in global-mean precipitation and a La Niña-like cooling over the tropical Pacific. The global-mean atmospheric energy budget suggests that the precipitation is suppressed as a way to compensate for the increased SWA. The precipitation reduction is driven by the weakened surface winds, stabilized planetary boundary layer, and surface cooling. The La Niña-like cooling over the tropical Pacific is attributed to the zonal asymmetry of climatological evaporative damping efficiency and the low cloud enhancement over the eastern basin. Complementary fixed SSTs simulations suggest that the latter is more fundamental and that it primarily arises from atmospheric processes. Consistent with our experiments, the CMIP5/6 models with a higher global-mean SWA tend to exhibit the tropical Pacific toward a more La Niña-like mean state, highlighting the possible role of water vapor shortwave absorptivity for shaping the mean-state climate patterns.


2021 ◽  
Author(s):  
Luciano Ponzi Pezzi ◽  
Mario F. L. Quadro ◽  
João Lorenzzetti ◽  
Arthur J Miller ◽  
Eliana B Rosa ◽  
...  

Abstract The South Atlantic Convergence Zone (SACZ) is an atmospheric system occurring in austral summer on the South America continent and sometimes extending over the adjacent South Atlantic. It is characterized by a persistent and very large, northwest-southeast-oriented, cloud band. Its presence over the ocean causes sea surface cooling that some past studies indicated as being produced by a decrease of incoming solar heat flux induced by the extensive cloud cover. Here we investigate ocean-atmosphere interaction processes in the Southwestern Atlantic Ocean (SWA) during SACZ oceanic episodes, as well as the resulting modulations occurring in the oceanic mixed layer and their possible feedbacks on the marine atmospheric boundary layer. Our main interests and novel results are on verifying how the oceanic SACZ acts on dynamic and thermodynamic mechanisms and contributes to the sea surface thermal balance in that region. In our oceanic SACZ episodes simulations we confirm an ocean surface cooling. Model results indicate that surface atmospheric circulation and the presence of an extensive cloud cover band over the SWA promote sea surface cooling via a combined effect of dynamic and thermodynamic mechanisms, which are of the same order of magnitude. The sea surface temperature (SST) decreases in regions underneath oceanic SACZ positions, near Southeast Brazilian coast, in the South Brazil Bight (SBB) and offshore. This cooling is the result of a complex combination of factors caused by the decrease of solar shortwave radiation reaching the sea surface and the reduction of horizontal heat advection in the Brazil Current (BC) region. The weakened southward BC and adjacent offshore region heat advection seems to be associated with the surface atmospheric circulation caused by oceanic SACZ episodes, which rotate the surface wind and strengthen cyclonic oceanic mesoscale eddy. Another singular feature found in this study is the presence of an atmospheric cyclonic vortex Southwest of the SACZ (CVSS), both at the surface and aloft at 850 hPa near 24°S and 45°W. The CVSS induces an SST decrease southwestward from the SACZ position by inducing divergent Ekman transport and consequent offshore upwelling. This shows that the dynamical effects of atmospheric surface circulation associated with the oceanic SACZ are not restricted only to the region underneath the cloud band, but that they extend southwestward where the CVSS presence supports the oceanic SACZ convective activity and concomitantly modifies the ocean dynamics. Therefore, the changes produced in the oceanic dynamics by these SACZ events may be important to many areas of scientific and applied climate research. For example, episodes of oceanic SACZ may influence the pathways of pollutants as well as fish larvae dispersion in the region.


Coatings ◽  
2021 ◽  
Vol 12 (1) ◽  
pp. 25
Author(s):  
Li Shi ◽  
Yuanfeng Lu ◽  
Hanze Huang

Hot streaks and rotor–stator interaction have a great influence on the aerothermal performance of turbine blades. Previous investigations have conducted limited study of the film-cooled blade. To investigate the combined effects of a hot streak and rotor–stator interaction on the coated blade, an unsteady numerical simulation has been conducted with an efficient unsteady Navier–Stokes solver in this paper. The numerical results at four different relative stator–rotor locations (t = 0/4 T, 1/4 T, 2/4 T, and 3/4 T) have been investigated in one stator period. Compared with the stator, rotor–stator interaction exerts a significant impact on the cooling performance of the rotor blade under hot streak inlet conditions. The overall cooling effectiveness distribution of the coated rotor blade is similar to that of the uncoated blades in one stator period. Relatively lower overall cooling performance of the rotor blade can be observed in the 1/4 stator period. Then, the cooling performance begins to increase and relatively larger cooling effectiveness can be observed in the 3/4 stator period. The addition of a TBC is generally beneficial to the improvement of blade surface cooling performance, especially for the areas with low overall cooling performance. However, a negative cooling effectiveness increment can be observed at the trailing edge. It shows that for an area with poor cooling performance, the addition of thermal barrier coating will have the opposite effect. Therefore, it is necessary to enhance the design of cooling arrangements at the trailing edge to maximize the insulation performance of TBCs for the coated rotor blade.


Geosphere ◽  
2021 ◽  
Author(s):  
Xiaowen Liu ◽  
Claire A. Currie ◽  
Lara S. Wagner

Most flat-slab subduction regions are marked by an absence of arc volcanism, which is consistent with closure of the hot mantle wedge as the subducting plate flattens below the continent. Farther inland, low surface heat flow is observed, which is generally attributed to cooling of the continent by the underlying flat slab. However, modern flat slabs have only been in place for <20 Ma, and it is unclear whether there has been sufficient time for cooling to occur. We use numerical models to assess temporal variations in continental thermal structure during flat-slab subduction. Our models show that the flat slab leads to continental cooling on timescales of tens of millions of years. Cool slab temperatures must diffuse through the continental lithosphere, resulting in a delay between slab emplacement and surface cooling. Therefore, the timescales primarily depend on the flat-slab depth with shallower slabs resulting in shorter timescales. The magnitude of cooling increases for a shallow or long-lived flat slab, old subducting plate, and fast convergence rates. For regions with flat slabs at 45–70 km depth (e.g., Mexico and Peru), shallow continental cooling initiates 5–10 Ma after slab emplacement, and low surface heat flow in these regions is largely explained by the presence of the flat slab. However, for the Pampean region in Chile, with an ~100-km-deep slab, our models predict that conductive cooling has not yet affected the surface heat flow. The low heat flow observed requires additional processes such as advective cooling from the infiltration of fluids released through dehydration of the flat slab.


Abstract The interaction between upper-ocean submesoscale fronts evolving with coherent features, such as vortex filaments and eddies, and finescale convective turbulence generated by surface cooling of varying magnitude is investigated. While convection is energized by gravitational instability, predominantly at the finescale (FS), which feeds off the potential energy that is input through cooling, the submesoscale (SMS) is energized at larger scales by the release of available potential energy stored in the front. Here, we decompose the flow into FS and SMS fields explicitly to investigate the energy pathways and the strong interaction between them. Overall, the SMS is energized due to surface cooling. The frontogenetic tendency at the submesoscale increases, which counters the enhanced horizontal diffusion by convection-induced turbulence. Downwelling/upwelling strengthens, and the peak SMS vertical buoyancy flux increases as surface cooling is increased. Furthermore, the production of FS energy by SMS velocity gradients is significant, up to half of the production by convection. Examination of potential vorticity reveals that surface cooling promotes higher levels of secondary symmetric instability, which coexists with the persistent baroclinic instability.


2021 ◽  
Author(s):  
Claire Martinot ◽  
Clara T Bolton ◽  
Anta-Clarisse Sarr ◽  
Yannick Donnadieu ◽  
Marta Garcia ◽  
...  

2021 ◽  
Author(s):  
Claire Martinot ◽  
Clara T Bolton ◽  
Anta-Clarisse Sarr ◽  
Yannick Donnadieu ◽  
Marta Garcia ◽  
...  

Abstract Understanding the connections between latent heating from precipitation and cloud radiative effects is essential for accurately parameterizing cross-scale links between cloud microphysics and global energy and water cycles in climate models. While commonly examined separately, this study adopts two cloud impact parameters (CIPs), the surface radiative cooling efficiency, Rc, and atmospheric radiative heating efficiency, Rh, that explicitly couple cloud radiative effects and precipitation to characterize how efficiently precipitating cloud systems influence the energy budget and water cycle using A-Train observations and two reanalyses. These CIPs exhibit distinct global distributions that suggest cloud energy and water cycle coupling are highly dependent on cloud regime. The dynamic regime (ω500) controls the sign of Rh, while column water vapor (CWV) appears to be the larger control on the magnitude. The magnitude of Rc is highly coupled to the dynamic regime. Observations show that clouds cool the surface very efficiently per unit rainfall at both low and high sea surface temperature (SST) and CWV, but reanalyses only capture the former. Reanalyses fail to simulate strong Rh and moderate Rc in deep convection environments but produce stronger Rc and Rh than observations in shallow, warm rain systems in marine stratocumulus regions. While reanalyses generate fairly similar climatologies in the frequency of environmental states, the response of Rc and Rh to SST and CWV results in systematic differences in zonal and meridional gradients of cloud atmospheric heating and surface cooling relative to A-Train observations that may have significant implications for global circulations and cloud feedbacks.


2021 ◽  
pp. 1-65

Abstract One of the most puzzling observed features of recent climate has been a multidecadal surface cooling trend over the subpolar Southern Ocean (SO). In this study we use large ensembles of simulations with multiple climate models to study the role of the SO meridional overturning circulation (MOC) in these sea surface temperature (SST) trends. We find that multiple competing processes play prominent roles, consistent with multiple mechanisms proposed in the literature for the observed cooling. Early in the simulations (20th century and early 21st century) internal variability of the MOC can have a large impact, in part due to substantial simulated multidecadal variability of the MOC. Ensemble members with initially strong convection (and related surface warming due to convective mixing of subsurface warmth to the surface) tend to subsequently cool at the surface as convection associated with internal variability weakens. A second process occurs in the late 20th and 21st centuries, as weakening of oceanic convection associated with global warming and high latitude freshening can contribute to the surface cooling trend by suppressing convection and associated vertical mixing of subsurface heat. As the simulations progress, the multidecadal SO variability is suppressed due to forced changes in the mean state and increased oceanic stratification. As a third process, the shallower mixed layers can then rapidly warm due to increasing forcing from greenhouse gas warming. Also, during this period the ensemble spread of SO SST trend partly arises from the spread of the wind-driven Deacon cell strength. Thus, different processes could conceivably have led to the observed cooling trend, consistent with the range of possibilities presented in the literature. To better understand the causes of the observed trend it is important to better understand the characteristics of internal low-frequency variability in the SO and the response of that variability to global warming.


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