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Ming Cai
,
Xiaoming Hu
,
Jie Sun
,
Feng Ding
, and
Jing Feng

Abstract

This paper introduces a climate feedback kernel, referred to as the “energy gain kernel” (EGK). EGK allows for separating the net longwave radiative energy perturbations given by a Planck feedback matrix explicitly into thermal energy emission perturbations of individual layers, and thermal radiative energy flux convergence perturbations at individual layers resulting from the coupled atmosphere-surface temperature changes in response to the unit forcing in individual layers. The former is represented by the diagonal matrix of a Planck feedback matrix and the latter by EGK. Elements of EGK are all positive, representing amplified energy perturbations at a layer where forcing is imposed and energy gained at other layers, both of which are achieved through radiative thermal coupling within an atmosphere-surface column.

Applying EGK to input energy perturbations, whether external or internal due to responses of non-temperature feedback processes to external energy perturbations, such as water vapor and albedo feedbacks, yields their total energy perturbations amplified through radiative thermal coupling within an atmosphere-surface column.

As the strength of EGK depends exclusively on climate mean states, it offers a solution for effectively and objectively separating control climate state information from climate perturbations for climate feedback studies. Given that an EGK comprises critical climate mean state information on mean temperature, water vapor, clouds, and surface pressure, we envision that the diversity of EGK across different climate models could provide insight into the inquiry of why, under the same anthropogenic greenhouse gas increase scenario, different models yield varying degrees of global mean surface warming.

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Yevgenii Rastigejev
and
Sergey A. Suslov

Abstract

This study focuses on the influence of the sea spray polydispersity on the vertical transport of momentum in a turbulent marine atmospheric boundary layer in high-wind conditions of a hurricane. The Eulerian multifluid model treating air and spray droplets of different sizes as interacting inter-penetrating continua is developed and its numerical solutions are analyzed. Several droplet size distribution spectra and correlation laws relating wind speed and spray production intensity are considered. Polydisperse model solutions have confirmed the difference between the roles small and large spray droplets play in modifying the turbulent momentum transport that have been previously identified by monodisperse spray models. The obtained results have also provided a physical explanation for the previously unreported phenomenon of the formation of thin low-eddy-viscosity “sliding” layers in strongly turbulent boundary layer flows laden with predominantly fine spray.

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Danyang Wang
and
Daniel R. Chavas

Abstract

Tropical cyclones are known to expand to an equilibrium size on the f-plane, but the expansion process is not understood. In this study, an analytical model for tropical cyclone outer-size expansion on the f-plane is proposed. Conceptually, the storm expands because the imbalance between latent heating and radiative cooling drives a lateral inflow that imports absolute vorticity. Volume-integrated latent heating increases more slowly with size than radiative cooling, and hence the storm expands towards an equilibrium size. The predicted expansion rate is given by the ratio of the difference in size from its equilibrium value (rt,eq ) to an environmentally-determined time scale τ rt of 10 ∼ 15 days. The model is fully predictive if given a constant rt,eq , which can also be estimated environmentally. The model successfully captures the first-order size evolution across a range of numerical simulation experiments in which the potential intensity and f are varied. The model predictions of the dependencies of lateral inflow velocity and expansion rate on latent heating rate also compare well with numerical simulations. This model provides a useful foundation for understanding storm size dynamics in nature.

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Hugh Morrison
,
Kamal Kant Chandrakar
,
Shin-Ichiro Shima
,
Piotr Dziekan
, and
Wojciech W. Grabowski

Abstract

Various coalescence methods for Lagrangian microphysics schemes are tested in box and large-eddy simulation (LES) models, including the stochastic all-or-nothing super-droplet method (SDM) and a version of SDM (dSDM) that applies a fractional approach similar to the average impact method. In LES, variabilities driven by microphysics and by flow realizations are separated using the “piggybacking” technique. Rain initiation averaged over many realizations of the box model is delayed and rain variability increases as the number of super-drops per collision volume (NSD ) is decreased using SDM. In contrast, rain initiation time using SDM in LES is insensitive to NSD for 32 ≤ NSD ≤ 512. This is explained through the interaction between LES grid boxes, each acting as a separate collision volume. Variability across the ensemble of LES collision volumes using SDM results in rain quickly initiating in some of the LES grid cells at low NSD and leading to a similar overall timing of rain initiation from the cloud compared to simulations with high NSD . There is a ∼20% decrease in the total rain mass and mean rain flux as NSD is increased from 32 to 256, with little additional change as NSD is increased from 256 to 512. The fractional coalescence approach in dSDM leads to reduced microphysical variability and a 15-18 min delay in rain initiation compared to SDM. An additional LES ensemble with microphysical variability feeding back to the dynamics shows that flow variability dominates the impact of microphysical variability on rain properties. Thus, flow variability must be constrained to isolate impacts of microphysical variability.

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Yu Du
,
Richard Rotunno
,
Zijian Chen
, and
Hongpei Yang

Abstract

This study presents a simple 2D linear analytical model aimed at investigating gravity waves forced by temporally periodic convection near a coastline. This investigation encompasses two distinct convective heating scenarios: deep convective heating and stratiform heating/cooling. Our model explores the intricate behavior of gravity waves in proximity to a time-dependent convective source and examines their propagation characteristics across diverse atmospheric conditions. Close to the convective source, gravity waves demonstrate nearly horizontal propagation with vertically aligned phase lines. The velocity of their propagation primarily depends on the vertical scale of the convective heating. The presence of a tropopause further extends their horizontal reach through partial wave ducting between the surface and the tropopause. However, the horizontal scale of the convective heating also plays a crucial role in determining the horizontal wavelength, and consequently, affecting the horizontal propagation speed of the gravity waves. If the heating horizontal scale is small compared to the horizontal scale of free waves at the forcing frequency, the heating vertical scale determines the vertical wavelength and thus the horizontal wavelength. However, if the heating horizontal scale is large, the horizontal wavelength determined by the heating vertical scale has little energy, so that the horizontal wavelength is mainly determined by the heating horizontal scale. Moreover, longer periods of convective heating and stronger background winds contribute to an increased downstream propagation distance of the gravity waves away from the source. Additionally, inertia-gravity waves generated by diurnal convection can propagate horizontally over greater distances at a higher latitude but become confined or trapped at latitudes exceeding 30°.

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Sylvain Dupont
,
Mark R. Irvine
, and
Caroline Bidot

Abstract

Turbulence in canopy plays a crucial role in biosphere-atmosphere exchanges. Traditionally, canopy turbulence has been analyzed under stationary conditions based on atmospheric thermal stability, disregarding the time of the day and the atmospheric boundary layer (ABL) depth, although recent studies have suggested that daytime canopy turbulence might be influenced by ABL-scale motions. The morning transition offers an intriguing period when the ABL grows and when an increasing influence of large-scale motions on canopy turbulence might be anticipated. Using large-eddy simulations resolving both canopy and ABL turbulence, we investigate here how the turbulence and exchanges at canopy top change along the morning transition according to the wind intensity. Under significant wind, simulations show that canopy turbulence and exchanges are dominated by mixing-layer-type motions whose characteristics remain constant during the morning transition, even though ABL-scale motions imprint on the canopy’s instantaneous velocity fields as the ABL grows. Under low wind, the canopy turbulence is dominated by plumes, whose horizontal sizes extend with the ABL, while their vertical sizes reach a limit before the morning transition ends. In the early morning, canopy-top exchanges are influenced by sources from both the canopy top and the ABL entrainment zone, explaining some of the dissimilarity in turbulent transport between scalars, apart from the differences in the location of canopy scalar sources. When reaching the residual layer, the ABL grows quicker, with intense water vapour and carbon dioxide exchanges, dominated by large-scale motions penetrating deep within the canopy, releasing into the atmosphere the nocturnal accumulated carbon dioxide.

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Daniel J. Kirshbaum
,
Hugh Morrison
, and
John M. Peters

Abstract

In recent years, direct calculations of simulated cumulus entrainment and detrainment have facilitated new physical insights into these highly elusive but critically important processes. However, these calculations require substantial computational resources that may limit their widespread usage. To facilitate such calculations, two simplified approximations of direct cumulus entrainment and detrainment are examined herein. The first approximation, termed the “semi-direct” method, follows a standard bulk approach but makes more realistic assumptions about the sources of entrained and detrained air near the cloud edges. In contrast, the second approximation (the “projection” method) uses the governing equations of motion to project whether grid points near the cloud edge will entrain or detrain as the mean cloud ascends by one grid point. Verification exercises using large-eddy simulations reveal that both methods generally agree better with corresponding direct entrainment/detrainment estimates than the traditional bulk formulation, with the projection method outperforming the semi-direct method. The two methods can be used in a synergistic fashion, with the semi-direct method helping to optimize the projection method, to suit a wide range of applications. Because the latter incorporates the essential dynamics of entrainment and detrainment at the local scale, it can be used to gain physical insight into the causal mechanisms regulating these complex processes.

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Yunshuai Zhang
,
Cunbo Han
,
Yaoming Ma
,
Shizuo Fu
,
Hongchao Zuo
, and
Qian Huang

Abstract

Applying 1-D surface heterogeneity and observed atmospheric vertical profiles as initial conditions, two sets of large-eddy simulation experiments provided insight into the influence of lake size and soil moisture on the development of lake breezes and moist convection over land beside the lake. When the lake diameter increased from 20 km to 50 km and 70 km, the maximum precipitation increased by 71.4% and 1.29 times, respectively. There are two reasons for larger precipitation over land in large-lake simulations: 1. Stronger and broader updrafts were found near the lake breeze front (LBF); 2. The air at 2–4 km was moister, probably because more water vapor below 2 km was advected by the lake breezes and transported upward through turbulent exchange. Moreover, When the lake diameter increased from 20 km to more than 50 km, the deep moist convection (DMC) occurred 20 minutes earlier. This may be related to broader shallow convective cloud and larger vertical velocity of cloud-initiating parcels in large-lake simulations. Shallow moist convection transitioned to DMC earlier with broader clouds under moderate and high soil moisture conditions. Notably, stronger and broader updrafts near the LBFs, along with the advection of moisture induced by the lake breezes, caused the shallow moist convection to reach its peak 1 hour earlier in the driest soil moisture case. However, smaller evapotranspiration could not provide sufficient moisture for the development of DMC. Our simulation results show that lake breeze circulations are essential for the development of moist convections in lake region.

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ZiJian Chen
and
Yu Du

Abstract

A significant diurnal offshore propagation of rainfall is observed extending from the eastern coast of India to the central Bay of Bengal. This study focuses on understanding the influence of topography over the Indian subcontinent on this rainfall propagation through a series of semi-idealized mesoscale numerical simulations. These simulations with varying topography highlight the crucial role of inertia-gravity waves, driven by diurnal mountain-land-sea thermal contrast between India and the Bay of Bengal, in initiating and promoting the offshore propagation of convective systems in the Bay. These waves’ phase speed of around 14.8 m s−1 aligns well with the speed of diurnal rainfall propagation. Even after eliminating the impact of Indian topography, the offshore propagating signal persists, suggesting a secondary rather than dominant effect of terrain on offshore rainfall propagation. Furthermore, the topography affects the depth of diurnal heating within the land’s boundary layer, which thus influences the amplitude, phase, and speed of the inertia-gravity waves. Specifically, the presence of higher mountains along the coastal area drives faster waves by increasing heating depth, resulting in faster rainfall propagation.

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Tsung-Yung Lee
and
Allison A. Wing

Abstract

Recent modeling studies have suggested a potentially important role of cloud-radiative interactions in accelerating tropical cyclone (TC) development, but there has been only limited investigation of this in observations. Here, we investigate this by performing radiative transfer calculations based on cloud property retrievals from the CloudSat Tropical Cyclone (CSTC) dataset. We examine the radius-height structure of radiative heating anomalies, compute the resulting radiatively-driven circulations, and use the moist static energy variance budget to compute radiative feedbacks. We find that inner-core mid-level ice water content and anomalous specific humidity increase with TC intensification rate, resulting in enhanced inner-core deep-layer longwave warming anomalies and shortwave cooling anomalies in rapidly-intensifying TCs. This leads to a stronger radiatively-driven deep in-up-and-out overturning circulation and inner-core radiative feedback in rapidly-intensifying TCs. The longwave-driven circulation provides radially inward momentum fluxes and upward moisture fluxes which benefit TC development, while the shortwave-driven circulation suppresses TC development. The longwave anomalies, which dominate the inner-core positive radiative feedback, are mainly generated from cloud-radiative interactions, with ice particles dominating the deep-layer circulation and liquid droplets and water vapor contributing to the shallow circulation. Moreover, the variability in ice water content, as opposed to variability in liquid water content and the effective radii of ice particles and liquid droplets, dominates the uncertainty in TC-radiative interaction. These results provide observational evidence for the importance of cloud-radiative interactions in TC development and suggest that the amount and spatial structure of ice water content is critical for determining the strength of this interaction.

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