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.
- Award ID(s):
- 1854929
- PAR ID:
- 10332125
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- ISSN:
- 0022-3670
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.
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Reliable estimates of the fluxes of momentum, heat and moisture at the air–sea interface are essential for accurate long-term climate projections, as well as the prediction of short-term weather events such as tropical cyclones. In recent years, it has been suggested that these estimates need to incorporate an accurate description of the transport of sea spray within the atmospheric boundary layer and the drop-induced fluxes of momentum, heat and moisture, so that the resulting effects on atmospheric flow can be evaluated. In this paper we propose a model based on a theoretical and mathematical framework inspired from kinetic gas theory. This approach reconciles the Lagrangian nature of spray transport with the Eulerian description of the atmosphere. In turn, this enables a relatively straightforward inclusion of the spray fluxes and the resulting spray effects on the atmospheric flow. A comprehensive dimensional analysis has led us to identify the spray effects that are most likely to influence the speed, temperature and moisture of the airflow. We also provide an example application to illustrate the capabilities of the model in specific environmental conditions. Finally, suggestions for future work are offered.more » « less
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Abstract Wave breaking under strong wind conditions in tropical cyclones (TCs) can generate sea spray droplets, which, during their suspension in air, release sensible heat due to the air‐sea temperature difference while absorb sensible heat from the environment when they evaporate and release latent heat to the environment. Since the spray mass flux is a function of surface drag coefficient (CD), the effect of spray on TC evolution should depends on CDparameterization, while this has not been addressed so far. This study examines the effects of sea spray on the simulated TC evolution with two different CDparameterizations (the Weather Research and Forecasting (WRF) default scheme and the Donelan scheme). Results show that during the primary intensification stage, the TC with spray effect becomes stronger than that without spray when the WRF CDscheme is used, but becomes weaker when the Donelan CDscheme is used. This occurs because CDis maximum outside the radius of maximum wind (RMW) with the Donelan scheme, which produces relatively large spray‐mediated latent heat flux outside the RMW, which is unfavorable for TC intensification. The difference is enlarged by a feedback between spray and TC intensification involving the inertial stability and surface friction‐induced radial inflow. However, in the mature stage, the simulated TCs with spray become stronger no matter which CDscheme is used. In addition, the spray effect on the TC inner‐core size evolution also weakly depends on the drag parameterization. When CDis relatively greater outside the RMW, the inclusion of the spray effect would lead to the inner‐core size increase.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JPO-D-21-0023.1
