skip to main content


Title: Sea Spray and Its Feedback Effects: Assessing Bulk Algorithms of Air–Sea Heat Fluxes via Direct Numerical Simulations

Sea spray exchanging momentum, heat, and moisture is one of the major uncertainties in modeling air–sea surface heat fluxes under high wind speeds. As a result of several untested assumptions in existing models and low fidelity in the measurements, questions regarding the appropriate method for modeling the effects of spray on air–sea fluxes still exist. In this study, we implement idealized direct numerical simulations (DNS) via an Eulerian–Lagrangian model to simulate spray droplets in turbulent flows. Then, we verify the bulk spray models of Fairall et al. and Andreas et al. with the detailed physics from DNS. We find that the quality of the underlying assumptions of bulk models is sensitive to the time scales governing spray microphysics and lifetime. While both models assume that spray experiences a uniform and steady ambient condition, our results show that this assumption only works well for droplets with long thermodynamic time scales and relatively short lifetime. When the thermodynamic time scales are short, the models fail to predict the correct temperature and radius change of spray (e.g., condensation), thus spray-mediated heat fluxes, which in turn overestimates the total heat fluxes. Moreover, using our two-way coupled simulations, we find a negative feedback induced by the spray evaporation that may be missing in the bulk models, which could lead to further overestimates of the total heat flux when the spray-mediated flux is treated as an add-on to the corresponding interfacial flux. We further illustrate that the feedback effects are consistent under different flow Reynolds numbers, which suggests that the findings are relevant at practical scales.

 
more » « less
NSF-PAR ID:
10102601
Author(s) / Creator(s):
 ;  
Publisher / Repository:
American Meteorological Society
Date Published:
Journal Name:
Journal of Physical Oceanography
Volume:
49
Issue:
6
ISSN:
0022-3670
Page Range / eLocation ID:
p. 1403-1421
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Quantifying the influence of sea spray on air‐sea fluxes under high‐wind conditions is challenging due to a variety of factors. Among existing models, the so‐called bulk air‐sea flux model is commonly used in meteorological applications due to its simplicity, which often involves strong but untested assumptions on spray‐mediated heat fluxes and feedback effects. For example, a common assumption is to treat each droplet size as an independent contribution; that is, it does not interact with other sizes. Thus, the interactions between different size classes of spray are often neglected. In this study, we focus on the polydispersity of the spray size distribution and investigate the appropriateness of assuming an independent contribution from different spray size classes. We implement direct numerical simulations (DNS) with Lagrangian tracking of spray droplets. Based on DNS results, the bulk spray model fails to capture the interactions between different sizes that are observed directly from the droplet and feedback statistics in DNS. Thus, assuming independent contributions from spray droplets results in significant overestimates on the total heat fluxes. We further test different representative sizes of a spray size distribution. We find that the volume‐weighted representative size is capable of predicting the droplet‐modified temperature and humidity fields and generally captures the vertical profiles of spray‐mediated and interfacial heat fluxes. The results indicate that the computation of spray‐mediated fluxes can be simplified in large‐scale parameterizations.

     
    more » « less
  2. Abstract

    Accurate estimates of air‐sea enthalpy and momentum fluxes are critically important for hurricane intensity predictions. However, calculating these fluxes is challenging due to the nature of the air‐sea transition region. At extreme wind speeds, a substantial amount of sea spray is lofted making it necessary to calculate the sea spray‐mediated enthalpy and momentum fluxes. These calculations rely on microphysical equations, which are sensitive to the details of the local environmental conditions. Here we use a microphysical model to show that there exists a threshold wind speed beyond which the net sea spray‐mediated enthalpy and momentum fluxes are well‐approximated by using the net sea spray mass flux alone. This result supports the hypothesis that at extreme wind speeds, the ratio of the air‐sea exchange coefficients becomes independent of wind speed, implying the air‐sea flux calculations can be substantially simplified.

     
    more » « less
  3. Abstract

    Accurate representation of air‐sea interaction is crucial to numerical prediction of the ocean, weather, and climate. Sea surface temperature (SST) gradients and surface currents in the oceanic mesoscale regime are known to have significant influence on air‐sea fluxes of momentum. Studies based on high‐resolution numerical models and observations reveal that SST gradients and surface currents in the submesoscale regime are much stronger than those in the mesoscale. However, the feedback between the submesoscale processes and the air‐sea turbulent fluxes is not well understood. To quantitatively assess the responses between air‐sea flux of momentum and submesoscale processes, a non‐hydrostatic ocean model is implemented in this study. The inclusion of SST gradients and surface currents in air‐sea bulk fluxes are argued to be significant for modeling accurate wind stress in the submesoscale regime. Taking both into account, this study shows that the linear relationship between wind stress curl/divergence and crosswind/downwind SST gradients existing in the mesoscale regime is not obvious in the submesoscale. Instead, a linear relationship between wind stress curl/divergence and surface current curl/divergence is revealed in the submesoscale. Furthermore, the magnitude of wind stress curl introduced by submesoscale processes is much greater than that presented by mesoscale processes. Another key finding is that tracer subduction and potential vorticity distribution in the submesoscale is susceptible to submesoscale‐modified air‐sea turbulent momentum flux. This study serves as a starting point in investigating the feedbacks between atmospheric and oceanic submesoscale processes.

     
    more » « less
  4. Abstract

    Modeling the shortwave radiation balance over the Southern Ocean region remains a challenge for Earth system models. To investigate whether this is related to the representation of aerosol‐cloud interactions, we compared measurements of the total number concentration of sea spray‐generated particles within the Southern Ocean region to model predictions thereof. Measurements were conducted from a container laboratory aboard the R/VTangaroathroughout an austral summer voyage to the Ross Sea. We used source‐receptor modeling to calculate the sensitivity of our measurements to upwind surface fluxes. From this approach, we could constrain empirical parameterizations of sea spray surface flux based on surface wind speed and sea surface temperature. A newly tuned parameterization for the flux of sea spray particles based on the near‐surface wind speed is presented. Comparisons to existing model parameterizations revealed that present model parameterizations led to overestimations of sea spray concentrations. In contrast to previous studies, we found that including sea surface temperature as an explanatory variable did not substantially improve model‐measurement agreement. To test whether or not the parameterization may be applicable globally, we conducted a regression analysis using a database of in situ whitecap measurements. We found that the key fitting parameter within this regression agreed well with the parameterization of sea spray flux. Finally, we compared calculations from the best model of surface flux to boundary layer measurements collected onboard an aircraft throughout the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES), finding good agreement overall.

     
    more » « less
  5. null (Ed.)
    Abstract Proposals from multiple nations to deploy air–sea flux moorings in the Southern Ocean have raised the question of how to optimize the placement of these moorings in order to maximize their utility, both as contributors to the network of observations assimilated in numerical weather prediction and also as a means to study a broad range of processes driving air–sea fluxes. This study, developed as a contribution to the Southern Ocean Observing System (SOOS), proposes criteria that can be used to determine mooring siting to obtain best estimates of net air–sea heat flux ( Q net ). Flux moorings are envisioned as one component of a multiplatform observing system, providing valuable in situ point time series measurements to be used alongside satellite data and observations from autonomous platforms and ships. Assimilating models (e.g., numerical weather prediction and reanalysis products) then offer the ability to synthesize the observing system and map properties between observations. This paper develops a framework for designing mooring array configurations to maximize the independence and utility of observations. As a test case, within the meridional band from 35° to 65°S we select eight mooring sites optimized to explain the largest fraction of the total variance (and thus to ensure the least variance of residual components) in the area south of 20°S. Results yield different optimal mooring sites for low-frequency interannual heat fluxes compared with higher-frequency subseasonal fluxes. With eight moorings, we could explain a maximum of 24.6% of high-frequency Q net variability or 44.7% of low-frequency Q net variability. 
    more » « less