Dynamics of Atmospheres and Oceans最新文献

Based on the data from 245 observation stations in Northeast China and the atmospheric reanalysis dataset, we investigate the impact and causes of the Northeast China cold vortex (NCCV) and its different circulation configurations on high-impact agroclimatic events in the past 60 years. The results show that the NCCV, in coordination with large-scale circulations at different geopotential heights, results in anomalous high-impact agroclimatic events. In April, the upper-level jet stream at 30°N over Northeast Asia is more robust, the stronger mid-level NCCV controls Northeast China, and the low-level cold air moves eastward and southward along the rear of the NCCV. This circulation pattern results in the delay in the date for spring temperature to exceed critical temperature stably. Consequently, the sowing, growth and maturation periods for warm-loving crops are postponed. Besides, in April, the upper-level westerly jet at 45°N over Northeast Asia is more intense, the rear portion of the strong mid-level NCCV is located over eastern Northeast China, and the low-level water vapor is transported to Northeast China from the Indian Ocean and the South China Sea. This circulation pattern can lead to an earlier onset of spring soaking rain, thereby increasing soil moisture during the spring ploughing and improving the emergence rate of crops. In June, the upper-level westerly jet at 45°N is more intense, the mid-level Ural blocking high is stronger, and a relatively vigorous NCCV controls eastern Northeast China. Additionally, the low-level water vapor is transported southwestward to Northeast China from the Bay of Bengal and the South China Sea, as well as is transported eastward and northwestward from the Northwest Pacific, leading to a relatively greater number of heavy rainfall days in Northeast China during June, This situation protects crops from the effects of droughts but may pose a risk of reduced yields due to potential flooding.

Sea spray generation is vital for air-sea interactions, however, it has been showing large differences of four orders of magnitude under the same wind speed. It is well known that wind speed can affect sea spray generation, and other physical processes should also be further considered. Surface wave states are widely believed to have significant impacts on sea spray generation, however, they have not yet been specifically and quantitatively verified. This study investigated the effects of surface waves on sea spray generation through a series of controlled laboratory experiments in a large wind-wave tank. Background mechanical waves with different wave heights and periods at a constant high wind speed (U₁₀ = 22 m/s) were synchronically introduced to generate sprays, and sprays with radii ranging from 1.5 to 25 μm were recorded by an optical particle counter. The experimental results indicated that different wave steepness has dramatic effects on sea spray generation function (SSGF) under the same wind speed, which can be close to two orders of magnitude. Then, a power law as a function of wind and wave steepness $u_{*}^{1/3}S{C}_D^{-1/2}$was proposed to describe the sea spray production rate (F).

Plain Language Summary

Sea sprays generated by wave breaking exist at the air-sea interface and have significant impacts on the air-sea mass, momentum and heat exchanges, especially under extreme ocean conditions. In the past 30 years, sea spray generation function (SSGF) controlled by wind speed has been widely adopted, although surface wave effects on sea spray generation have received high attention. The impacts of surface waves have rarely been considered due to sea spray observation difficulties at high sea states, and previous SSGFs have shown large differences of four orders of magnitude. Focusing on this tremendous difference, we conducted a series of wind-wave tank experiments under different wave conditions at a high wind speed to investigate the impact of surface waves on the SSGF. Our results showed that wave steepness is an important factor modulating the SSGF, which provides quantitative evidence for better understandings on sea spray generation and hence their influences on air-sea momentum and heat fluxes.

The Tropical Indian Ocean (TIO) is one of the most vulnerable regions to climate change due to its unique ocean-atmosphere interactions. The region is characterized by warm temperatures, high evaporation rates, and strong convection, making it particularly vulnerable to greenhouse gases. This susceptibility leads to an overall trend of warming across the Earth's surface and atmosphere, intensifying sea surface temperatures (SST) and increasing evaporation rates, consequently influencing Surface Latent Heat Flux (SLHF). The strong influence of both SST and SLHF on atmospheric circulation and precipitation patterns makes them critical factors in determining the region's climate, particularly in monsoon-dominated regions. Analysis reveals a consistent upward trend in both SST and SLHF in the central region of TIO. To model this, standard Logistic Curve Model (LCM) has been applied to these parameters averaged over the central region of the TIO. The model is run for a period of 20 years from 2001 to 2020, grouped into four lustrums. The LCM-derived SST and SLHF are in good consistent with observed datasets during the above periods with, correlation coefficients ranging from 0.92 to 0.96 for SST and 0.86–0.90 for SLHF. Extending the model spatially across the entire TIO region explains the ability to project fluctuations in SST and SLHF values across different seasons. These findings highlight the model's relevance for capturing short-term, long-term, and seasonal variability of the parameters, providing important insights into regional climate dynamics.

The selection of meteorological inputs in storm surge is crucial, with wind and pressure fields playing a significant role in energy transfer and the study area's bathymetry. While using observed track information for wind and pressure drop has been the standard approach for the past decade, recent studies have highlighted the need for atmospheric inputs from models like the Weather Research and Forecast model (WRF) for storm surge forecast. This study aims to compare the performance of a DELFT-3D FM storm surge model forced with inputs from IMD (India Meteorological Department) observed the best track and pressure drop (EXP-1) and wind and pressure fields from WRF (EXP-2) for Tropical Cyclone Vayu, which occurred in the southern Arabian Sea basin in June 2019. The study compares the simulated storm water levels and highlights the benefits of using time and space-varying wind and pressure input for improved surge representation. Results show that the WRF-DELFT setup outperforms the IMD-DELFT setup, particularly for tide gauge locations close to the storm eye. The simulated storm surge captures the intensified stage of Vayu and provides a more realistic representation than the model forced with IMD track data. However, biases and limitations, such as inadequate representation of land surface parameterization, are identified. The study suggests further exploring wave-induced effects on total water level and multiple cyclone scenarios to enhance wind speed and track displacement prediction accuracy and improved land-sea parameterization can help address these limitations.

The Earth’s atmospheric interface is highly vulnerable to anthropogenic aerosol pollution and changes caused by numerous industrial and allied sectors. The boundary layer aerosol emissions interact directly or indirectly with the dynamics and microphysical processes, impact cloud properties, precipitation accumulations, and subsequently affect socio-economic growth. The primary objective of this study is to synthesize the aerosol-convection-precipitation interactions concerning atmospheric microphysics and identify the modern methods to explore such dynamics. The secondary objective of this study is to understand and analyze the scientific literature with a bibliometric analysis to find the significant theme of influence from the scientific literature. The results highlighted the main and critical trends in aerosol research and reveal that the research interest seemingly improved in the past 5 years with an annual scientific growth rate of approx. 6%. It is evident from a plethora of relevant research findings that aerosol loading in the atmosphere up to a certain effective droplet concentration may increase the precipitation, however, further increment in aerosol concentration will decrease the precipitation efficiency. Regional feedback mechanisms (boundary layer, radiative etc.) play a pivotal role in governing aerosol and convective precipitation dynamics. The combination of satellite observations from space and ground-based (in-situ) measurements and climate models offers a practical possibility for resolving the complexity in cloud micro-phenomenology.

A one-dimensional Atmospheric Boundary Layer (ABL1D) model is coupled with the NEMO ocean model and implemented over the Iberian–Biscay–Ireland (IBI) area at 1/36° resolution to investigate the damping effect of the current and the thermal feedback on the kinetic energy (KE) at the mesoscale. This type of coupling between an ocean model and an ABL1D is a newly proposed approach as an alternative of intermediate complexity between bulk forcing and full coupling with an atmosphere model. In ABL1D, the prognostic tracers are nudged toward large-scale variables and the wind is guided by a low-frequency geostrophic wind provided from the ERA-Interim reanalyses. First, the ABL1D is successfully validated against satellite observations regarding the wind, and the dynamic coupling coefficient (linking the near surface wind and wind-stress to the of the surface currents) are consistent with the literature, over the period 2016–2017. Our results show that the thermal feedback has a negligible impact on kinetic energy (KE) and does not influence the strength of the current feedback in the region. Given the ABL1D physics, this further indicates that the changes in the vertical wind structure caused by CFB are primarily governed by local mechanical mechanisms associated with surface wind-stress condition, rather than by thermodynamic or non-local processes within the planetary boundary layer. The induced KE reduction by the current feedback amounts to 14% at the surface and propagates down to 2000 m, indicating that it can modify the vertical distribution of KE throughout the water column. KE reductions in the surface boundary layer (0 – 300 m) and in the interior (300 – 2000 m) are attributed to a reduction of the surface wind work by 4%, and of the pressure work by 7%, respectively. The Ekman pumping anomalies induced by the current feedback tend to attenuate eddy activity and horizontal pressure gradients at depth, illustrating the potential of the current feedback to induce a geostrophic adjustment on the water column. These results illustrate the relevance of the proposed ABL1D coupling approach for reproducing the wind-current coupling (a.k.a. current feedback effect) which cannot be taken into account straightforwardly with simple bulk forcing.

This work evaluates the influence of cloud microphysical schemes on prediction of a warm-sector heavy rainfall with an operational modeling system of CMA-GD in Guangzhou Regional Meteorological Center (GRMC). The heavy rainfall is produced by a MCS occurred over Pearl River Delta in South China on May 21, 2020. Four cloud microphysical schemes (including WSM6, WDM6, THOMP and LIUMA) are investigated to understand their impacts on structure and evolution of rainfall system. Results show that the WSM6 over-predicts the 24-hour accumulated rainfall, while the other three schemes underestimate the rainfall. In general, these deviations of total rainfall are mainly caused by generated rainfall during mature stage of MCS. Four schemes all underestimate precipitation during this period, but the deviation is the least in WSM6 and WDM6 schemes. As far as both WSM6 and WDM6 schemes, quantitative verification shows that the threat score (TS) and the false alarm ratio (FAR) as well as the proportion of convective/stratiform precipitation in the WSM6 single-moment scheme are superior to those in the WDM6 double-moment scheme. Investigation of vertical distributions of precipitation particles and the associated thermodynamic response in the environment shows that compared with THOMP and LIUMA, WSM6 and WDM6 simulate more ice and snow in the upper level and more rain and cloud water in the low troposphere as MCS evolves into mature state. With the top-heavier heating and the strongest upward motion lasting longer time, WSM6 may lead to stronger dynamical feedback to large-scale environment compared with THOMP. All these reveal that WSM6 is the most accurate scheme simulating this warm-sector rainstorm and the importance of more accurate simulation on the evolution and structure of precipitation.

Numerous studies have suggested that the North Atlantic subpolar gyre (SPG), Atlantic Meridional Overturning Circulation (AMOC), and Arctic sea ice impact the polar and global climate. Here, we use a fully linked atmosphere-ocean-sea ice Earth system model to investigate the North Atlantic subpolar ocean dynamics over the last 21 thousand years before the present (ka). We found that the SPG strength, net ocean surface heat flux, and mixed layer depth in the North Atlantic deep convection sites declined during the Heinrich 1 (H1; ∼19–17 ka) and Younger Dryas (YD; ∼12.9–11.3 ka) cold events. Consequently, the deep convection and AMOC strength declined, reducing the northward meridional heat transport and causing the expansion of Atlantic sea ice coverage. We also found that the North Atlantic subpolar net ocean surface heat flux varied coherently with AMOC strength throughout the past 21 ka. Subsequently, we observed a sea ice-capping mechanism wherein an increase (decrease) in Atlantic sea ice coverage during H1/YD (Bølling-Allerød (BA; ∼17–14.35 ka)) reduces (increases) net ocean surface heat flux and deep convection, thereby influencing the AMOC strength. Meanwhile, the SPG and AMOC strengths have been in-phase throughout the past 21 ka, except during the abrupt termination and input of freshwater flux during the BA and Meltwater Pulse 1 A (∼14.4–13.9 ka) events, respectively. In conclusion, our study suggests that a sudden shift in freshwater discharge into the subpolar North Atlantic may disturb the polar ocean dynamics.

A CTD/ADCP/surface-drifter survey in fall 2004 reveals the behaviour of a mesoscale unidirectional flow coming from the Cretan sea in the south with depths ∼1000 m and entering a channel-like area of the Cycladic shelf in the north, that forms a contraction which leads to a bottom elevation (sill depth ∼100 m), and finally returning into the Cretan Sea in the lee-side of the sill. The flow decelerates/accelerates upstream/downstream of the sill. The along-stream density contours near the sill bottom are raised prior to reaching the sill, while they deepen in the lee side of it indicating supercriticality. The long-wavelength internal wave speeds with realistic stratification and no-rotation are higher than the section averaged flow speeds and indicate subcriticality. A key element in this apparent paradox is the large height of the sill that potentially increases the body (drag) force exerted on the flow by the sill while flow blocking is also observed upstream of the sill.

This work examines ocean Ekman current dynamics in fractal dimensions based on the concept of product-like fractal measure introduced recently by Li and Ostoja-Starzewski in their formulation of anisotropic continuum media. We show that fractal dimensions of the fluid affects the amplitude and the shape of the velocity profile. It was observed that fast-moving current occurs for fractal dimensions much less than unity whereas slow-moving current arises for fractal dimensions close to unity. A large flow velocity leads to a decrease in the shear rate and an increase in the viscosity, a fact that has been observed in physical oceanography.