1. Introduction
The study of hurricanes and their dynamics has long been a topic of great importance, in particular, due to significant property damage and loss of life they can cause. To minimize them, it is important to accurately forecast hurricane track and intensity. There is observational and theoretical evidence (Fairall et al. 1994; Mestayer et al. 1996; Van Eijk et al. 2001; Donelan et al. 2004; Makin 2005; Wu et al. 2015; Druzhinin et al. 2017; Tang et al. 2017; Garg et al. 2018; He et al. 2018; Rastigejev and Suslov 2019; Xu et al. 2021; Huang et al. 2022; Rastigejev and Suslov 2022; Richter and Wainwright 2023; Troitskaya et al. 2023; Xu et al. 2023) that ocean spray plays an important role in modifying the vertical enthalpy and momentum fluxes in the marine atmospheric boundary layer (MABL) of a hurricane, thereby affecting its dynamics both thermodynamically and mechanically. However, the present understanding of the role of ocean spray in the air–sea turbulent exchange under high-wind conditions of hurricanes remains limited. This knowledge gap has hindered progress in the accurate prediction of hurricane intensity motivating further in-depth studies of sea spray that are also relevant to many other contexts such as understanding the influence of spray on ice accumulation on vessels at high latitudes (Panov 1978; Line et al. 2022), gas exchange between ocean and atmosphere (Staniec et al. 2021; Gutiérrez-Loza et al. 2022), formation of aerosols (Burrows et al. 2022; Su et al. 2022), and climate dynamics (Song et al. 2022; Bruch et al. 2023).
There are two distinct ways in which sea spray can affect the MABL: thermodynamic and mechanical (Rastigejev and Suslov 2016). The thermodynamic effect is due to moisture and heat introduced or removed by spray droplets. It leads to variations in the temperature, humidity, and the vertical fluxes of sensible and latent heat within the hurricane MABL (Van Eijk et al. 2001; Rastigejev and Suslov 2019). The mechanical effect of ocean spray on the MABL is related to its capacity to modify the mechanical characteristics of the airflow via air–droplet momentum transfer by several competing physical mechanisms: weakening of the wind due to spray inertia and its acceleration due to the spray-induced suppression of turbulence (Barenblatt et al. 2005; Rastigejev et al. 2011; Rastigejev and Suslov 2022). The spray-induced inertial air deceleration is caused by the air momentum transfer to the accelerating sea spray ripped off wave crests and entering an airflow with initial velocity that is much smaller than that of the wind and increasing the density of the air–spray mixture near the ocean surface. The competing wind acceleration effect is due to two different mechanisms responsible for reducing the turbulence intensity of the air: the turbulence attenuation due to air–droplet slip (DS) (Kulick et al. 1994; Li et al. 2016) and the gravity lubrication (GL) (Barenblatt and Golitsyn 1974; Bertsch et al. 2015). The DS turbulence attenuation is caused by turbulent energy dissipation due to air–droplet friction, while GL results from the reduction in the turbulent kinetic energy (TKE) due to the vertical transport of spray against gravity. The TKE suppression by both DS and GL mechanisms reduces the effective vertical turbulent transport coefficients such as eddy viscosity. Subsequently, to maintain a constant momentum flux from the MABL to the sea, the wind velocity profile develops a steeper gradient, leading to an increase in the airflow velocity above wave crests (Barenblatt et al. 2005; Kudryavtsev 2006; Rastigejev et al. 2011; Rastigejev and Suslov 2014, 2022).
Our previous studies have demonstrated that the mechanical suppression of turbulence and the subsequent reduction of turbulent vertical exchange strongly affect heat fluxes in the MABL (Rastigejev and Suslov 2016, 2019) and can induce variations in the sea surface temperature, ocean mixed layer depth, and currents and other phenomena (Feng et al. 2021; Z. Sun et al. 2021). At the same time, the thermodynamic influence on mechanical transport in the MABL is found to be much weaker, thereby justifying decoupling their studies as is done in the current work.
The conventional relationships for the ocean surface roughness length Charnock (1955), Large and Pond (1981) state that the drag coefficient increases monotonically with the wind speed. However, the data collected in recent field observations and laboratory experiments (Powell et al. 2003; Donelan et al. 2004; Jarosz et al. 2007; Black et al. 2007; French et al. 2007; Haus et al. 2010; Takagaki et al. 2012; Potter et al. 2015; Zhao and Li 2019; Zhang et al. 2023) demonstrate that the drag coefficient increases with wind speed only when it is sufficiently small, but it reaches maximum at
Our previous papers (Rastigejev et al. 2011; Rastigejev and Suslov 2014, 2016, 2019, 2022) also contained a comprehensive investigation of the influence of a monodisperse spray on the vertical fluxes within the MABL. In agreement with studies undertaken by other authors (e.g., Peng and Richter 2020), they have shown that both thermodynamic and mechanical influences of ocean spray on hurricane dynamics sensitively depend on the spray droplet size. However, realistic sprays are polydisperse mixtures of droplets of different sizes. Therefore, in the current study, we aim to determine how the variation in droplet size spectra affects various mechanical characteristics of the MABL. We also discuss the influence of the ocean surface roughness length on the airflow characteristics in the presence of polydisperse ocean spray.
We focus on the mechanical effect of a polydisperse spray. We employ the modern theory of turbulent multiphase flows (Drew and Passman 2006; Brennen 2005) and adopt the multifluid approach (Reeks 1992; Massot 2007) that treats air and droplets of different sizes as separate interacting and interpenetrating turbulent continua. This approach, which has recently gained popularity in numerical simulations of multiphase flows in various fields (Andreini et al. 2016; Kartushinsky et al. 2016; Senapati and Dash 2020; Li et al. 2021; Lian et al. 2022), is also known as the Eulerian–Eulerian method. In our analysis, each continuum represents a distinct entity with its own velocity and TKE and obeys mass and momentum conservation laws. A multifluid E–ϵ epsilon (also known as k–ϵ) model (Jakobsen 2008) is used as a turbulence closure. This choice is motivated by the fact that presently, it is one of the most extensively validated and widely used turbulence models for multiphase flow simulations across diverse applications (Torno et al. 2020; Pouraria et al. 2021; Song et al. 2021; Che et al. 2022). It has also found widespread application in simulating the atmospheric boundary layer (Chalikov and Rainchik 2011; van der Laan et al. 2017; Walsh et al. 2017).
Spray droplet size distributions characterized by two spectral shape functions (SSFs) derived from volumetric spray generation functions (VSGFs) proposed by Andreas (1998), hereafter A98, and Ortiz-Suslow et al. (2016), referred to as OS16 below, are considered. The key distinction between these spray distributions important in the context of our study is that OS16 SSF contains a higher proportion of larger droplets compared with the A98 counterpart (see Figs. 1c and 1d). Additionally, we examine estimations for the aerodynamic roughness length: the classical expression suggested by Charnock (1955) and the empirical formulation proposed by Large and Pond (1981) that is frequently used by atmospheric research and meteorological communities.
The SSFs for VSGFs given in (a) A98 and (b) OS16. (c),(d) The corresponding number density SSFs. Distributions (b) and (d) contain larger droplets.
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
The paper is organized as follows. Section 2 introduces the main governing equations for polydisperse spray. The results of numerical simulation and their discussion are presented in section 4. It includes a comparative analysis of the spray polydispersity effects for A98 and OS16 SSFs for two different choices of aerodynamic roughness length. Section 5 presents a qualitative explanation of the mechanism behind the formation of a sliding layer and the associated turbulence crisis. Section 6 provides a concise overview of the main results of the paper. Details of spray distributions and spectral shape functions are given in the appendixes.
2. Problem formulation and mathematical model
The developed mathematical model of the spray-laden MABL is designed to capture the complexity of air–spray interaction and consistently quantify the main mechanical effect of spray on the airflow. To achieve this in a tractable way, we assume that turbulent incompressible airflow is statistically steady and horizontally homogeneous. The air is laden with polydisperse sea spray generated at the average wave crest level. Therefore, all variables depend only on the elevation above the mean sea level. The spray is modeled as thermodynamically inert media, which implies no evaporation or heat exchange with the surrounding air. The droplets are idealized as spheres of various radii subject to drag and gravity forces; droplet breakup and coalescence are neglected. The forces acting on spray droplets due to the perturbations they cause to the undisturbed flow, the so-called Faxen correction, are estimated to be at least three orders of magnitude smaller than Stokes’ drag (Maxey and Riley 1983) and so is the Basset memory force (Armenio and Fiorotto 2001; Daitche 2015; Liang and Michaelides 1992). Therefore, they are also neglected so that coupling between the turbulent airflow and droplets is assumed to have two components: airflow carrying the slipping droplets and droplets influencing the drag force experienced by the airflow.
The spray droplet radius spectrum is partitioned into N bins (classes). Because it falls off rapidly at large sizes, to maintain a better representation it could be beneficial to consider bins of nonequal width, but for algebraic simplicity in the current model, we opted for 10 equally spaced bins. Comparison of the so-obtained numerical results for several representative regimes with their counterparts for 5 and 15 bins did not reveal any qualitative discrepancies.
The proposed mathematical model adopts a multifluid approach treating droplets of various sizes and the air as distinct interacting turbulent media. As a result, the air and droplets from different bins are governed by their respective sets of coupled equations for mass, momentum, and turbulent kinetic energy. Each droplet class is characterized by its own velocity, turbulent kinetic energy, and transport coefficients. The model flow uses a 1.5-order E–ϵ multiphase turbulence closure. The Boussinesq approximation is employed to represent turbulent transport in both air and water phases.
To assess the sensitivity of model predictions to the choice of spray generation correlations, we employ two SSFs
The variable parameters entering model (1)–(13) and the discretized SSFs pi in (12) depend on four physical quantities: the friction velocity
3. Mechanical effect of sea spray
4. Results of numerical simulation
The governing equations introduced in section 2 were solved numerically using a standard MATLAB (MATLAB 2021) routine bvp5c using at least 2000 spatial discretization points distributed over the interval between z = 1 and z = 100 (from the wave crest level up to the altitude of 500 m). This ensures that all reported numerical results are accurate up to at least three decimal places.
a. Suppression of TKE by polydisperse spray and subsequent wind acceleration
Typical vertical distributions of characteristics of the spray-free reference boundary layer and of that laden with polydisperse spray are presented in Fig. 2. Despite that the shown quantities have been computed for specific values of the main independent parameters
Vertical distributions of (a) airspeed, (b) air TKE, (c) dissipation rate of TKE, (d) turbulent viscosity in the airflow, (e) total spray concentration, and (f) total TKE of spray in the MABL for
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
For comparison, in Fig. 2, we also present the computational results obtained for a monodisperse spray with the same concentration s0 = 10−4 and the droplet radius req corresponding to the terminal droplet velocity in quiescent air given by (A6) (shown by thin curves). Physically, this choice ensures the equivalence of the total volumetric spray injection fluxes in the cases of poly- and monodisperse sprays with identical values of the total spray volumetric fraction s0 in a quiescent air. We found that for A98 and OS16 SSFs, (a0, req) ≈ (0.86 m s−1, 122 μm) and (a0, req) ≈ (2.56 m s−1, 304 μm). The main observations are that the monodisperse spray assumption leads to a quantitative underprediction of the wind velocity and of the total spray concentration at higher altitudes (see Figs. 2a and 2c) and overprediction of the eddy viscosity (see Fig. 2d) that are stronger pronounced for the spectrum containing larger droplets (OS16, dashed lines). The distribution of the TKE dissipation is almost insensitive to whether poly- or monodisperse spray is considered. Importantly, the difference between the air TKE distributions computed using poly- and monodisperse models depends on the adopted SSF: The monodisperse model predicts larger (smaller) air TKE values for spectra favoring larger (smaller) droplets. Such nonmonotonicity of variations between results obtained adopting mono- and polydisperse assumptions demonstrates that the model explicitly and consistently accounting for the natural spray polydispersity is a nontrivial extension of the previously suggested models (Rastigejev and Suslov 2014, 2022) effectively considering a single-sized spray.
b. Dependence of the air–sea drag coefficient on a spray production rate
In view of large uncertainty currently existing in the literature regarding the spray concentration data at the wave crest level (A98) that is required as the input to the model developed in section 2, here, we vary it over a wide interval producing a comprehensive parametric maps that could also be viewed as sensitivity maps of the model outputs.
The distributions of spray volumetric density
Distributions of (a) the volumetric spray density
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
As in Fig. 3, but for the OS16 SSF.
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
As in Fig. 3, but for the aerodynamic roughness length given by (15) and (16).
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
The figures show that the air–sea drag coefficient Cd decreases with the spray source intensity s0 and grows with
The comparison of Figs. 3 and 5 reveals that the values of Cd are lower when they are computed for the roughness length z0,LP rather than z0,Ch for the same SSF. This is expected since z0,LP < z0,Ch for the considered wind speed range so that the predicted wind speed is higher when the air moves over a smoother surface.
The thick solid, dashed, and dash–dotted lines in Figs. 3a,c, 4a,c, and 5a,c map the wind speed/spray concentration correlations (21) and (22) for n = 8 and n = 5.37 and δ = 0.3, respectively, to the
The mechanical effect of ocean spray cannot be consistently characterized by the behavior of the drag coefficient alone even though its reduction can be explained by the spray-induced decrease in the turbulent intensity. It is more appropriate to evaluate the mechanical influence of spray by assessing the extent to which the spray attenuates the TKE in the MABL. This is because the influence of the spray extends beyond a thin layer above the ocean surface, where it is produced, and can be substantial up to several hundred meters above it. Furthermore, the spray indirectly affects the MABL dynamics by mechanically suppressing turbulence and subsequently reducing turbulent vertical exchange, which strongly affects heat fluxes in the MABL. Nonetheless, we can compare our results with those presented in the literature by observing the drag coefficient behavior in a spray-laden MABL.
c. Weak influence of the spray injection velocity
As we established previously (Rastigejev and Suslov 2022), the outputs of the models based on E–ϵ turbulence closure are almost insensitive to the speed with which spray droplets enter the airflow. Since in hurricane conditions, most of spray originates from spume that is torn off the wave crests by a wind shear, it is expected that the initial speed of such droplets would be approximately equal to the ocean wave propagation speed. Even for large ocean waves created by hurricane winds, it does not exceed around 60 km h−1. Thus, below we consider two droplet injection speed values: typical 6 m s−1 (U = 2) and near-maximum 15 m s−1 (U = 5).
As seen from Fig. 6a, the nonzero droplet injection speed leads to a very minor change in the overall wind velocity (well under 0.1%). There are several physical reasons for that. First is that even the maximum expected droplet injection speed is at least 3–5 times slower than that of the hurricane winds at the wave crest level. The second is that in a statistically steady regime, the spray droplets that have just been injected with a nonzero speed constitute a relatively small proportion of the total droplet population. The majority of droplets found at the wave crest level at any moment in time are those that have been ejected previously, then carried across the MABL by turbulent diffusion, and then brought back by the gravity pull. Since such droplets have been suspended in the air for a sufficiently long time, their mean speed relaxes to the local flow conditions with their initial injection momentum well dissipated. This scenario is illustrated in Figs. 7a and 7c, where the vertical distributions of ratios of the droplet velocities (uiU) injected with speed U ≠ 0 to those (ui0) of droplet injected with U = 0 (thick curves corresponding to the positive values). The figure demonstrates that while larger droplets retain the information about their initial speed over a longer distance due to their inertia, regardless of the injection speed in the physically expected range, even they “forget it” by the time they reach the altitude of at most 1.5 wave crest heights (i.e., by traveling less than 2.5 m up from the 5-m wave). After that, it takes about another half-wave height for droplets injected with any initial speed to assume the local wind speed (the thin lines corresponding to the negative values). Therefore, the influence of the injection speed dispersion is limited to a thin [exponential as shown in Rastigejev and Suslov (2022)] region just above the waves.
Relative variation in the airflow characteristics with droplet injection speed U: (a) horizontal airspeed ua and (b) turbulent eddy viscosity ka for
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
Relative variation in spray characteristics with droplet injection speed U: (a),(c) horizontal droplet speed ui and (b),(d) spray concentration si for various droplet radii in a polydisperse spray with the A98 spectrum and other parameters as in Fig. 6.
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
Another noteworthy observation following from Fig. 6a is that while spray injection with positive speed accelerates the local airflow in a thin region near the wave crest level, it leads to the overall wind reduction, which may appear counterintuitive at the first glance. The physical explanation for such a phenomenon becomes evident from Fig. 6b. The steep air velocity gradient induced by the spray injection leads to a more intense turbulence production [pau term in (4) and (7)], which in turn leads to the enhancement of turbulent viscosity ka illustrated in Fig. 6b and the related reduction of the wind speed. Such an increase in the turbulent transport coefficient also intensifies spray flux across the MABL, which results in a decrease in the spray concentration compared with the zero-injection-speed case that is more pronounced for large droplets (see Figs. 7a,b).
Having observed these trends, in principle, we note that quantitatively, they are very weak, and thus, we do not pursue this aspect of the model further.
5. Turbulence crisis in a thin near-surface layer
Our numerical simulations reported in section 4b revealed that when the spray production intensity s0 exceeds the critical value, the computed drag coefficients decrease quickly (see the top right corners of Figs. 3c and 5c). Typical flow quantity distributions presented in Fig. 8 indicated by solid lines (corresponding to A98 SSF) for such regimes shed light on the physical reasons for such a phenomenon. They demonstrate that a thin layer with a small turbulent eddy viscosity (see Fig. 8c) is formed a few meters above wave crests, under which a large portion of the injected spray is confined (see Fig. 8e). We refer to this layer as “a sliding layer” in the subsequent discussion. The presence of a large amount of spray drastically increases the local rate of turbulence destruction due to the friction between droplets slipping relative to the surrounding air and due to the vertical transport of spray against gravity. In statistically steady regimes, such TKE destruction must be compensated by the increasing rate of shear production, which implies the steepening of the vertical gradient of wind speed (see Fig. 8a). The only way this can be achieved is through a drastic local reduction in the turbulent eddy viscosity
As in Fig. 2, but for
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
To clarify the influence that the spectral composition of spray mixture has on the processes discussed above, in Fig. 8, we also show computational results obtained for the same physical parameters but the OS16 SSF (see the dashed lines). Such a spray with spectrum shifted toward larger droplets does not induce the formation of a thin layer with strong turbulence suppression. This is because the TKE balance in this case can be achieved without its formation. Therefore, no evidence of the drag coefficient crisis is found in Fig. 4 computed for this spectrum.
The TKE crisis has a profound influence on the distribution of spray in the upper part of the MABL. When the crisis develops, the amount of spray reaching higher altitudes drastically decreases. Indeed, as seen from Fig. 2e, in the absence of the crisis, the total spray concentration for OS16 SSF favoring larger droplets is significantly lower than that for the lighter AS98 spray. However, when the latter gets affected by the crisis, its concentration distribution above the sliding layer becomes very close to that of the heavier OS16 spray that remains unaffected by the crisis (see Fig. 8e). This leads to another noteworthy effect: the degree of the air TKE recovery in higher atmosphere is larger when the drag crisis develops preventing spray from traveling to high altitudes. Since the crisis is more likely to develop for a fine rather than large droplet spray, their dominant roles in the suppression of turbulence swap at high altitudes: According to relationships (C8), at equal concentrations, heavy droplets suppress turbulence more efficiently (see Figs. 2b and 8b).
Overall, Fig. 8 demonstrates that the spectral composition of spray has a highly nonlinear influence on the practically important MABL characteristics such as the wind speed and the air–sea drag coefficient. The monodisperse spray model results, which are shown for comparison by thin lines in this figure, demonstrate that while they also exhibit the tendency toward forming a thin sliding layer when a sufficiently large amount of fine spray is injected in the air, it is much weaker in a uniform spray. This is the main reason why this process has not been reported previously. As a consequence of that, a monodisperse model results may noticeably underpredict the wind acceleration in a spray-laden atmosphere. Note that the appearance of sliding is controlled by the TKE balance, and in the present model, it is considered in the average sense. Ocean waves can vary this balance and thus affect the characteristics of such layers. To quantify such an influence, an accurate description of the average action of ocean waves has to be included in the model, which is left for future studies given the current lack of relevant field data.
6. Conclusions
This study has been concerned with the Eulerian modeling of a turbulent statistically steady MABL laden with polydisperse ocean spray, where droplets of different sizes have been treated as interacting and interpenetrating continua with individual transport properties. Two spray size spectra, A98 with the large number of fine droplets and OS16 favoring big droplets, have been considered. Just above the wave crest level, spray causes a slight wind speed decrease due to its inertia. However, several meters higher, the spray-induced turbulence suppression overcomes the effect of inertia. This leads to a noticeable increase in the wind speed and the corresponding decrease in the drag coefficient for both spectra and roughness lengths compared to the reference spray-free atmosphere. The turbulence suppression and the wind acceleration and the air–sea drag reduction that it causes are stronger for the A98 droplet size distribution than that for OS16. This is due to the larger proportion of fine spray in A98, which has a superior ability to propagate upward in turbulent flows, leading to a higher volumetric spray density above wave crests.
We have tested several correlation laws that relate the wind speed and spray production intensity and observed that the drag coefficient gradually deviates from its reference value in the spray-free atmosphere reaching its maximum at a spray volumetric fraction smax ∼ 10−4 as the wind speed increases. The specific value of smax depends sensitively on the chosen SSF but only weakly on the correlation law or the aerodynamic roughness length. However, for a fixed correlation law, the value of smax is approximately twice as high for OS16 SSF than for A98. In contrast, the wind speed at which the drag coefficient reaches its maximum is primarily determined by the correlation law and is almost insensitive to the choice of SSF and the roughness length.
Our numerical calculations have also shown the possibility of formation of a thin sliding layer, which is characterized by a low turbulent eddy viscosity, a few meters above wave crests, where the presence of large amount of spray strongly enhances turbulence destruction by the air–droplet friction. Numerical experiments have demonstrated that such layers occur in an MABL laden with A98 SSF spray with volumetric densities ∼2 × 10−4. However, sliding layers have not been observed for OS16 SSF containing a larger proportion of heavy droplets at the same volumetric spray production rate. The intense TKE destruction in a sliding layer is balanced by the increased shear, leading to a steepening of the vertical gradient of the wind speed profile, which implies a local reduction in the turbulent eddy viscosity. It is shown that this reduction can be achieved only via a substantial increase in the TKE dissipation. This also implies a significant reduction in the local turbulence length scale. The sliding layer with a strongly suppressed vertical turbulent transport caused by the low turbulent eddy viscosity effectively isolates the bulk of the boundary layer from a rough sea surface, ultimately leading to the crisis of the drag coefficient.
The comparison of the current numerical results with those obtained using the previously suggested monodisperse spray models has shown that the latter may noticeably underestimate the spray-induced wind acceleration. It is also concluded that due to the strong nonlinearity of the considered phenomena, the quantitative and qualitative differences between mono- and polydisperse model predictions of the air TKE depend on the specifics of the actual spray spectra. This means that more realistic polydisperse spray results cannot be generally expected to be qualitatively similar to those obtained using a simpler monodisperse spray approach. Therefore, the model explicitly taking into account the spray droplet size variation is a nontrivial extension of the single-size models reported in the literature previously.
Finally, we note that the k–ϵ multifluid turbulence model was chosen here and in our previous studies because of its strong track record in both fluid-particle flows and atmospheric boundary layer simulations. Even though it has its limitations, this well-validated model has been widely used for numerical modeling of diverse industrial and environmental multiphase flows. Notably, it enables one to simulate the energy exchange between continuous and disperse phases. However, the model may not fully capture air–droplet energy exchange across the spectrum of turbulent eddy sizes. Addressing this issue presents an opportunity for a future model refinement. One promising avenue lies in employing multiple time-scale (MTS) models (Hanjalic et al. 1980). It is expected that the MTS approach would account for the air–droplet energy exchange within diverse eddy scales through separate consideration of different spectral parts more consistently than a standard k–ϵ model. The incorporation of the MTS framework would be a natural extension of the current mathematical model.
Acknowledgments.
The authors acknowledge the support from the U.S. National Science Foundation via the Grant Awards 1832089 and 2302221.
Data availability statement.
The MATLAB code used for numerical simulations of the mathematical model will be made available at https://github.com.
APPENDIX A
Spray Generation Function
APPENDIX B
Discrete Spray Generation Function
APPENDIX C
Asymptotic Expressions for Turbulence Destruction Rate
a. Rate of turbulence destruction at z ∼ 1
b. Rate of turbulence destruction at z ≫ 1
APPENDIX D
Mathematical Analysis of Physical Mechanisms Causing the Appearance of Sliding Layers
Figure D1 shows that the vertical distributions of the nondimensional air TKE flux fe = −kadea/dz has a minimum at some location z = zmin in the lower part of MABL. We will demonstrate that the TKE budget balance at this location requires a large TKE dissipation rate ϵa ≫ 1 whenever spray concentration is high. Therefore, the turbulent eddy viscosity ka = (αea)2/ϵa achieves its minimum and, subsequently, sliding layers occur near the minimum of the TKE flux.
The vertical distributions of the TKE flux fe = −kadea/dz for the same parameters and A98 SSF as in Fig. 8. The vertical dashed, solid, and dash–dotted lines correspond to locations of the minimum of ea (and, thus, fe = 0), the minimum of ka, and the maximum of ϵa, respectively, all clustered in the close vicinity of the minimum of fe.
Citation: Journal of the Atmospheric Sciences 81, 7; 10.1175/JAS-D-23-0195.1
The sliding layer starts developing when the TKE dissipation rate becomes larger than its value ϵa = 1/(kpz) in the reference spray-free atmosphere. This happens when s0 exceeds a certain critical value s0,c. We show that s0,c increases with the average terminal speed of spray a0 but decreases with the friction velocity
Furthermore, we employ (D13) to estimate ϵa(zmin) for A98 and OS16 SSFs for which the average terminal speeds are a0 = 0.86 m s−1 and a0 = 2.56 m s−1, respectively, for the same parameter values as in Fig. 8. By substituting ea = 1.83 and sw = 0.9 for A98 SSF and ea = 2.06 and sw = 0.57 for OS16 SSF obtained for typical values τd = 0.2 and c0 = 0.9, we obtain ϵa ≈ 4.6 and ϵa ≈ 0.74 for A98 and OS16, respectively. These values of the TKE dissipation rate are close to ϵa ≈ 5.5 and 0.78 obtained numerically. Therefore, to achieve the TKE balance, ϵa(zmin) has to be large for A98 when both
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