More Like this

1. Sensitivity of large eddy simulations of tropical cyclone to sub-grid scale mixing parameterization.

   https://doi.org/10.1016/j.atmosres.2021.105922  

   Li, Yubin; Zhu, Ping; Gao, ZhiQiu; Cheunge, Kevin (January 2022, Atmospheric research)

   The surface wind structure and vertical turbulent transport processes in the eyewall of hurricane Isabel (2003) are investigated using six large-eddy simulations (LESs) with different horizontal grid spacing and three-dimensional (3D) sub-grid scale (SGS) turbulent mixing models and a convection permitting simulation that uses a coarser grid spacing and one-dimensional vertical turbulent mixing scheme. The mean radius-height distribution of storm tangential wind and radial flow, vertical velocity structure, and turbulent kinetic energy and momentum fluxes in the boundary layer generated by LESs are consistent with those derived from historical dropsonde composites, Doppler radar, and aircraft measurements. Unlike the convection permitting simulation that produces storm wind fields lacking small-scale disturbances, all LESs are able to produce sub-kilometer and kilometer scale eddy circulations in the eyewall. The inter-LES differences generally reduce with the decrease of model grid spacing. At 100-m horizontal grid spacing, the vertical momentum fluxes induced by the model-resolved eddies and the associated eddy exchange coefficients in the eyewall simulated by the LESs with different 3D SGS mixing schemes are fairly consistent. Although with uncertainties, the decomposition in terms of eddy scales suggests that sub-kilometer eddies are mainly responsible for the vertical turbulent transport within the boundary layer (~1 km depth following the conventional definition) whereas eddies greater than 1 km become the dominant contributors to the vertical momentum transport above the boundary layer in the eyewall. The strong dependence of vertical turbulent transport on eddy scales suggests that the vertical turbulent mixing parameterization in mesoscale simulations of tropical cyclones is ultimately a scale-sensitive problem. 

   more » « less
2. Submesoscale Mixing Across the Mixed Layer in the Gulf of Mexico

   https://doi.org/10.3389/fmars.2021.615066  

   Liu, Guangpeng; Bracco, Annalisa; Sitar, Alexandra (March 2021, Frontiers in Marine Science)

   null (Ed.)

   Submesoscale circulations influence momentum, buoyancy and transport of biological tracers and pollutants within the upper turbulent layer. How much and how far into the water column this influence extends remain open questions in most of the global ocean. This work evaluates the behavior of neutrally buoyant particles advected in simulations of the northern Gulf of Mexico by analyzing the trajectories of Lagrangian particles released multiple times at the ocean surface and below the mixed layer. The relative role of meso- and submesoscale dynamics is quantified by comparing results in submesoscale permitting and mesoscale resolving simulations. Submesoscale circulations are responsible for greater vertical transport across fixed depth ranges and also across the mixed layer, both into it and away from it, in all seasons. The significance of the submesoscale-induced transport, however, is far greater in winter. In this season, a kernel density estimation and a detailed vertical mixing analysis are performed. It is found that in the large mesoscale Loop Current eddy, upwelling into the mixed layer is the major contributor to the vertical fluxes, despite its clockwise circulation. This is opposite to the behavior simulated in the mesoscale resolving case. In the “submesoscale soup,” away from the large mesoscale structures such as the Loop Current and its detached eddies, upwelling into the mixed layer is distributed more uniformly than downwelling motions from the surface across the base of the mixed layer. Maps of vertical diffusivity indicate that there is an order of magnitude difference among simulations. In the submesoscale permitting case values are distributed around 10 –3 m 2 s –1 in the upper water column in winter, in agreement with recent indirect estimates off the Chilean coast. Diffusivities are greater in the eastern portion of the Gulf, where the submesoscale circulations are more intense due to sustained density gradients supplied by the warmer and saltier Loop Current. 

   more » « less
3. Computational Domain Size Effects on Large-Eddy Simulations of Precipitating Shallow Cumulus Convection

   https://doi.org/10.3390/atmos14071186  

   Lamaakel, Oumaima; Venters, Ravon; Teixeira, Joao; Matheou, Georgios (July 2023, Atmosphere)

   Idealized large-eddy simulations of shallow convection often utilize horizontally periodic computational domains. The development of precipitation in shallow cumulus convection changes the spatial structure of convection and creates large-scale organization. However, the limited periodic domain constrains the horizontal variability of the atmospheric boundary layer. Small computational domains cannot capture the mesoscale boundary layer organization and artificially constrain the horizontal convection structure. The effects of the horizontal domain size on large-eddy simulations of shallow precipitating cumulus convection are investigated using four computational domains, ranging from 40×40km2 to 320×320km2 and fine grid resolution (40 m). The horizontal variability of the boundary layer is captured in computational domains of 160×160km2. Small LES domains (≤40 km) cannot reproduce the mesoscale flow features, which are about 100km long, but the boundary layer mean profiles are similar to those of the larger domains. Turbulent fluxes, temperature and moisture variances, and horizontal length scales are converged with respect to domain size for domains equal to or larger than 160×160km2. Vertical velocity flow statistics, such as variance and spectra, are essentially identical in all domains and show minor dependence on domain size. Characteristic horizontal length scales (i.e., those relating to the mesoscale organization) of horizontal wind components, temperature and moisture reach an equilibrium after about hour 30. 

   more » « less
4. Evaluation of Eddy Diffusion Adjustments on Improving Hurricane Simulations in Weather Forecasting Models

   Momen, Mostafa; Matak, Leo (January 2024, 104th AMS Annual Meeting)

   Rotation in hurricane flows can highly affect the dynamics and structure of hurricane boundary layers (HBLs). Recent studies (Momen et al. 2021) showed that there is a significant distinction between turbulence characteristics in hurricane and regular atmospheric boundary layers due to the strong rotational effects of hurricane flows. Despite these unique features of HBLs, the current planetary boundary layer (PBL) and turbulence schemes in numerical weather prediction (NWP) models are neither specifically designed nor comprehensively tested for major hurricane flows. In this talk, we will address this knowledge gap by characterizing the role of horizontal and vertical eddy diffusion under different PBL schemes in simulated hurricane intensity, size, and track. To this end, the results of multiple simulated hurricane cases will be presented using the Weather Research and Forecasting (WRF) model. The impacts of changing the grid resolution, horizontal turbulence, PBL scheme, vertical eddy diffusivity, and PBL height on hurricane dynamics and accuracy will be characterized. The results indicate that the current turbulence and PBL schemes in WRF are overly diffusive for simulating major hurricanes (Romdhani et al. 2022; Li et al. 2023) primarily since they do not account for turbulence suppression effects in rotating hurricane flows. We will also show new suites of simulations in which the default horizontal and vertical diffusion in WRF are modulated to determine the impacts of eddy diffusion changes on hurricane dynamics. The results indicate that reducing the default vertical diffusion depth and magnitude led to ~38% and ~24% improvements, on average, in hurricane intensity forecasts compared to the default models in the considered cases (Matak and Momen 2023). Moreover, by decreasing the default horizontal mixing length, we managed to decrease the intensity errors on average between ~8-23% in the WRF’s default models for both low and high resolutions. Figure A displays an example of the simulations in which our new adjustment of the vertical diffusion (reduced diffusion, blue line) agrees better with the observed data (black line) compared to the default WRF results (gray line). The figure also depicts wind speed contours that how this change in vertical diffusion can remarkably influence the structure, size, and intensity of hurricane simulations. The results of this study provide notable insights into the role of turbulent fluxes in simulated hurricanes that can be useful to advance the turbulence and PBL parameterizations of NWP models for accurate tropical cyclone forecasts. References: Li M, Zhang JA, Matak L, Momen M (2023) The impacts of adjusting momentum roughness length on strong and weak hurricanes forecasts: a comprehensive analysis of weather simulations and observations. Mon Weather Rev. https://doi.org/10.1175/MWR-D-22-0191.1 Matak L, Momen M (2023) The role of vertical diffusion parameterizations in the dynamics and accuracy of simulated intensifying hurricanes . Boundary Layer Meteorology. https://doi.org/10.1007/s10546-023-00818-w Momen M, Parlange MB, Giometto MG (2021) Scrambling and reorientation of classical boundary layer turbulence in hurricane winds. Geophys Res Lett 48:.https://doi.org/10.1029/2020GL091695 Romdhani O, Zhang JA, Momen M (2022) Characterizing the impacts of turbulence closures on real hurricane forecasts: A comprehensive joint assessment of grid resolution, horizontal turbulence models, and horizontal mixing length. J Adv Model Earth Syst. https://doi.org/10.1029/2021MS002796 

   more » « less
5. Maintenance of mid-latitude oceanic fronts by mesoscale eddies

   https://doi.org/10.1126/sciadv.aba7880  

   Jing, Zhao; Wang, Shengpeng; Wu, Lixin; Chang, Ping; Zhang, Qiuying; Sun, Bingrong; Ma, Xiaohui; Qiu, Bo; Small, Justing; Jin, Fei-Fei; et al (July 2020, Science Advances)

   null (Ed.)

   Oceanic fronts associated with strong western boundary current extensions vent a vast amount of heat into the atmosphere, anchoring mid-latitude storm tracks and facilitating ocean carbon sequestration. However, it remains unclear how the surface heat reservoir is replenished by ocean processes to sustain the atmospheric heat uptake. Using high-resolution climate simulations, we find that the vertical heat transport by ocean mesoscale eddies acts as an important heat supplier to the surface ocean in frontal regions. This vertical eddy heat transport is not accounted for by the prevailing inviscid and adiabatic ocean dynamical theories such as baroclinic instability and frontogenesis but is tightly related to the atmospheric forcing. Strong surface cooling associated with intense winds in winter promotes turbulent mixing in the mixed layer, destructing the vertical shear of mesoscale eddies. The restoring of vertical shear induces an ageostrophic secondary circulation transporting heat from the subsurface to surface ocean. 

   more » « less