Air-sea Temperature Difference Research Articles

Observed Subdaily Variations in Air–Sea Turbulent Heat Fluxes under Different Marine Atmospheric Boundary Layer Stability Conditions in the Gulf Stream

Abstract Based on data collected from 14 buoys in the Gulf Stream, this study examines how hourly air–sea turbulent heat fluxes vary on subdaily time scales under different boundary layer stability conditions. The annual mean magnitudes of the subdaily variations in latent and sensible heat fluxes at all stations are 40 and 15 W m−2, respectively. Under near-neutral conditions, hourly fluctuations in air–sea humidity and temperature differences are the major drivers of subdaily variations in latent and sensible heat fluxes, respectively. When the boundary layer is stable, on the other hand, wind anomalies play a dominant role in shaping the subdaily variations in latent and sensible heat fluxes. In the context of a convectively unstable boundary layer, wind anomalies exert a strong controlling influence on subdaily variations in latent heat fluxes, whereas subdaily variations in sensible heat fluxes are equally determined by air–sea temperature difference and wind anomalies. The relative contributions by all physical quantities that affect subdaily variations in turbulent heat fluxes are further documented. For near-neutral and unstable boundary layers, the subdaily contributions are O(2) and O(1) W m−2 for latent and sensible heat fluxes, respectively, and they are less than O(1) W m−2 for turbulent heat fluxes under stable conditions. Significance Statement High-resolution buoy observations of air–sea variables in the Gulf Stream provide the opportunity to investigate the physical factors that determine subdaily variations in air–sea turbulent heat fluxes. This study addresses two key points. First, the observed subdaily amplitudes of heat fluxes are related to various processes, including wind fields and air–sea thermal effect differences. Second, the global sea surface heat budget is known to not be in near-zero balance and it ranges from several to tens of watts per square meter. Therefore, consideration of the relatively strong influence of subdaily variability in air–sea turbulent heat fluxes could provide a new strategy for solving the global heat budget balance problem.

The non-uniformity characteristics of the evaporation duct in the South China Sea based on CLDAS data

The non-uniformity of the evaporation duct has a significant impact on the propagation of electromagnetic waves. Based on the near real-time dataset from the China Meteorological Administration Land Data Assimilation System (CLDAS) and the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) datasets, the non-uniformity characteristics of evaporation duct height (EDH) in the South China Sea (SCS) region at different distances of electromagnetic propagation and the main influencing factors of EDH non-uniformity are analyzed. The findings are given as follows. First, the SCS monsoon has a significant impact on the spatial and temporal distribution of EDH, particularly in the western coastal areas of Luzon Island and the eastern coastal areas of the Indochina Peninsula. Secondly, the non-uniformity of EDH is more pronounced in the western part of Luzon Island, the eastern coast of the Indochina Peninsula, and the sea-land boundary area of the Beibu Gulf. Besides, the non-uniformity of EDH in the western part of Luzon Island and the eastern part of the Indochinese Peninsula is greatly influenced by air-sea temperature difference and relative humidity. Finally, it is advisable to consider the influence of the non-uniformity of EDH when the distance of electromagnetic propagation in coastal areas exceeds 50 km. However, it can be disregarded within a range of 100 km on the broad ocean.

Cool Skin Effect as Seen from a New Generation Geostationary Satellite Himawari-8

The cool skin effect refers to the phenomenon where the surface skin temperature of the ocean is always slightly cooler than the temperature of the water directly underneath due to the ubiquitous cooling processes at the ocean surface, especially in the absence of solar radiation. The cool skin effect plays a critical role in the estimation of heat, momentum, and gas exchange between the air and the sea. However, the scarcity of observational data greatly hinders the accurate assessment of the cool skin effect. Here, the matchup data from the new generation geostationary satellite Himawari-8 and in situ sea surface temperature (SST) observations are used to evaluate the performance and dependence on the cool skin effect in the low/mid-latitude oceans. Results show that the intensity of the cool skin effect as revealed by Himawari-8 (−0.16 K) is found to be relatively weaker than previously published cool skin models based on in situ concurrent observations. A considerable amount of warm skin signals has been detected in the high-latitude oceans (e.g., Southern Ocean) under the circumstances of positive air–sea temperature difference and high wind, which may be the main cause of discrepancies with previous thoughts on the cool skin effect.

Satellite signature of the instantaneous wind response to mesoscale oceanic thermal structures

AbstractThe thermal air–sea interaction mechanism that modulates the atmospheric mixing due to sea‐surface temperature (SST) variability is studied with long‐term consistent satellite records. Statistical analyses of daily and instantaneous wind and SST data are performed over the major western boundary currents (WBCs). This wind–SST coupling, which is mediated by atmospheric mixing, is found to be very relevant on daily, and even shorter, time scales. Co‐located and simultaneous SST and surface wind fields (from Advanced Very High Resolution Radiometer and Advanced Scatterometer data) reveal that the atmosphere responds instantaneously to the presence of SST structures with a larger coupling coefficient with respect to daily and monthly time‐averaged fields. The coupling strength varies seasonally over WBCs in the Northern Hemisphere, with wintertime coupling being the lowest. Reanalysis data show that this behaviour is related to the seasonality of the air–sea temperature difference over the region of interest. Over the Northern Hemisphere WBCs, dry and cold continental air masses drive very unstable conditions, associated with very weak thermal air–sea coupling.

Observed Air–Sea Turbulent Heat Flux Anomalies during the Onset of the South China Sea Summer Monsoon in 2021

Abstract This study presents observational findings of air–sea turbulent heat flux anomalies during the onset of the South China Sea summer monsoon (SCSSM) in 2021 and explains the mechanism for high-resolution heat flux variations. Turbulent heat flux discrepancies are not uniform throughout the basin but indicate a significant regional disparity in the South China Sea (SCS), which also experiences evident year-to-year variability. Based on buoy- and cruise-based air–sea measurements, high-temporal-resolution (less than hourly) anomalies in the latent heat flux during the SCSSM burst are unexpectedly determined by sea–air humidity differences instead of wind effects under near-neutral and mixed marine atmospheric boundary layer (MABL) stability conditions. However, latent heat anomalies are mainly induced by wind speed under changing MABL conditions. The sensible heat flux is much weaker, with its anomalies dominated by sea–air temperature differences regardless of the boundary layer condition. The observational results are used to examine the discrepancies in turbulent heat fluxes and associated air–sea variables in reanalysis products. The comparisons indicate that latent and sensible heat fluxes in the reanalysis are overestimated by approximately 55 and 3 W m−2, respectively. These overestimations are mainly induced by higher estimates of sea–air humidity/temperature differences. The relative humidity is underestimated by approximately 4.2% in the two high-resolution reanalysis products. The higher SST (near-surface specific humidity) and lower air temperature (specific air humidity) eventually lead to higher estimates of sea–air humidity/temperature differences (1.75 g kg−1/0.25°C), which are the dominant factors controlling the variations in the air–sea turbulent heat fluxes. Significance Statement Air–sea interactions are significant in predicting the onset of East Asian monsoon systems, including the SCSSM. During the SCSSM in 2021, four buoys and cruise observations are used to investigate anomalies in the latent and sensible heat fluxes. The physical mechanism of the variations in turbulent heat fluxes under different MABL stability conditions is explored in this study. The humidity and wind speed anomalies play roles under mixed boundary conditions in determining the high-resolution variations in latent heat fluxes. Based on these observational results, the heat fluxes and associated air–sea variables from reanalysis products are compared to identify the differences in the operational systems. These comparison results can help improve the reanalysis to obtain better monsoon predictions.

Spatial and Seasonal Variations of Sea Surface Temperature Threshold for Tropical Convection

Abstract Tropical rainfall variations are of direct societal relevance and drive climate variations worldwide via teleconnections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SSTthr, usually taken to be constant in time and space. We analyze 40-yr monthly observations and find that SSTthr varies by up to 4°C in space and with season. Based on local convective instability, we develop a quantitative theory that largely explains the SSTthr variations using the climatological state of the tropical atmosphere. Although it is often assumed that spatial variations of tropical upper-tropospheric temperature are small and can be neglected, it is shown that lower climatological values favor a lower SSTthr. Similarly, a small increase in climatological surface relative humidity also leads to a decrease in SSTthr, as does a lower climatological air–sea temperature difference. Consequently, efforts to understand and predict natural or forced variations in tropical rainfall must account for, in addition to SST, the temperatures aloft and the near-surface humidity and temperature and requires improved understanding of what controls their distribution in space and time.

Regional Study on the Oceanic Cool Skin and Diurnal Warming Effects: Observing and Modeling

The cool skin and diurnal warming effects are important factors affecting the vertical temperature gradient in the upper ocean. Accurately understanding skin effects is of great significance for studying ocean–atmosphere modeling and climate change. The skin models need to be validated for their applicability under different oceanic conditions to improve their accuracy. Shipboard measurements from August 2015 to October 2018 in the Northwest Pacific Ocean are used to validate some of the current models. The results show that the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) cool skin model obtains a mean cool skin value of −0.25 K, which is close to the averaged observed value of −0.23 K. A significant positive correlation between the sea–air temperature difference and the amplitude of the cool skin effect is observed in this study. Three diurnal warming models are discussed and compared. The profiles of ocean surface heating (POSH) model performed the best and was the closest one to the observation. The mean temperature differences bewteen the COARE and POSH models are close to 0 K, while the other model shows the overestimation with a mean temperature difference of 0.21 K. The measurements and validations of the thermal skin effects in this study can be useful for regional research on the air–sea interaction and upper ocean gradients.

A Comparative Study of Rough Sea Surface and Evaporation Duct Models on Radio Wave Propagation

Microwave and millimeter-wave propagations near the sea surface are greatly affected by the vertical atmospheric refractivity profile and the sea surface. To accurately simulate or predict the performance of ship radar systems, the evaporation duct model and rough sea surface models are usually incorporated into the parabolic equation to model electromagnetic wave propagation. The Paulus-Jeske (PJ) model and Naval Postgraduate School (NPS) model are the most widely used to make the vertical atmospheric refractivity profile, and the roughness sea surface is modeled with the effective reflection coefficient of the rough surface or the generation of the rough sea surface. There are four widely used rough sea surface approximations called Ament, Miller-Brown, Shadowed, and Hybrid for rough sea surface modeling. In this study, numerical simulation is first used to compare the different rough sea surface modeling methods. Then, evaporation duct measurement data and C-band propagation experimental data are used to compare the evaporation duct models and the four rough sea surface models that are combined with evaporation duct models for propagation loss prediction. The simulation results show that the shadowing of radio waves by the rough sea surface and sea surface structure are important factors affecting the transmission of radio waves in evaporation ducts, and the difference in the path loss predicted by different rough sea surface approximations may exceed 10 dB at the same location. In particular, the frequency and wind speed will increase. In addition, the experimental data show that the evaporation duct can be better diagnosed using different evaporation duct models at different atmospheric stabilities. When the air-sea temperature difference is positive, the PJ evaporation duct is recommended; otherwise, the NPS model is better, and the results of the Shadowed and Hybrid rough sea surface models are in better agreement with the observations.

The role of sea spray in air-sea fluxes during Typhoon Molave: a study based on drifting buoy observations

The exchange of heat and momentum between the ocean and the atmosphere greatly affects the growth of typhoons. Utilizing the meteorological and oceanic variables observed by a Drifting Air-sea Interface Buoy (DrIB) during Typhoon Molave, a new air-sea turbulent fluxes product (referred to as DrIB product) is developed with the consideration of the thermal and dynamic effects of sea spray in the Coupled Ocean Atmosphere Response Experiment algorithm. The performances of two reanalysis products, ERA5 and MERRA2, under typhoon conditions are evaluated by comparing them to the DrIB observations. In particular, the air-sea turbulent fluxes during Typhoon Molave are systematically studied. The averaged heat (momentum) flux of the DrIB product is ~200% (~30%) higher than the reanalysis. However, the reanalysis products have higher latent heat than the DrIB product, because the reanalysis products have lower wind speed, smaller air-sea temperature difference, and drier atmosphere. The sea spray-induced mean heat (momentum) flux increase is ~1% (8%) in normal weather and is ~5% (17%) at the during-typhoon stage. Sea spray amplifies the dominance of wind speed on heat fluxes and weakens the contribution of air-sea temperature and humidity differences to heat fluxes. Sea spray starts to obviously contribute to the heat fluxes at a 10-m wind speed of ~10 m/s, and it non-linearly accelerates the air-sea heat exchange at a 10-m wind speed of ~20 m/s. When the 10-m wind speed is less than 20 m/s, the basic momentum flux (without sea spray effects) at the air-sea interface is roughly one or two orders of magnitude higher than the sea spray-induced momentum flux. Including the sea spray effects, the maximum momentum flux can even double at the 10-m wind speed of ~30 m/s.

Environmental Control of Wind Response to Sea Surface Temperature Patterns in Reanalysis Dataset

Abstract Sea surface temperature (SST) is characterized by abundant warm and cold structures that influence the overlying atmospheric boundary layer dynamics through two different mechanisms. First, turbulence and large eddies in the lower troposphere are affected by atmospheric stability, which can be modified by local SST, resulting in enhanced vertical mixing and larger surface winds over warmer waters. Second, the thermodynamic adjustment of air density to the underlying SST structures and the subsequent changes in atmospheric pressure drive secondary circulations. This paper aims to disentangle the effects of these processes and explore the environmental conditions that favor them. Two main environmental variables are considered: the large-scale air–sea temperature difference (proxy for stability) and wind speed. Using 5 years of daily reanalyses data, we investigate the 10-m wind response to SST structures. Based on linear regression between wind divergence and SST derivatives, we show that both mechanisms operate over a large spectrum of conditions. Ten-meter wind divergence is strongly impacted by the local SST via its effect on vertical mixing for midwind regimes in slightly unstable to near-neutral conditions, whereas the secondary circulation is important in two distinct regimes: low wind speed with a slightly unstable air column and high background wind speed with a very unstable air column. The first regime is explained by the prolonged Lagrangian time that the air parcel stays over an SST structure while the second one is related to strong heat fluxes at the air–sea interface, which greatly modify the marine atmospheric boundary layer properties. Location and frequency of the environmentally favorable conditions are discussed, as well as the response in low-cloud cover and rainfall. Significance Statement The main objective of this study is to explore the wind response to thermal structures at the sea surface under different environmental conditions using the latest atmospheric reanalysis. Recent literature suggests that fine-scale air–sea interactions affect a large spectrum of atmospheric dynamics, from seasonal to weather-type regimes. It is thus important to characterize the atmospheric response to ocean surface variability. Our findings describe the environmental conditions for which the two main physical processes through which the atmosphere responds to sea surface temperature structures are active the most and can guide the development of high-resolution observing missions and campaigns in specific geographical locations and seasons to retrieve data that can be used to improve parameterization in models.

The Boundary Layer Characteristics and Development Mechanism of a Warm Advective Fog Event over the Yellow Sea

A large-scale persistent fog event occurred over the Yellow Sea of China from April 27 to May 4, 2015. In this study, we used satellite remote sensing data, ground meteorological observed data, global sounding data, and reanalysis data from the National Centers for Environmental Prediction (NCEP) and sea surface temperature (SST) data from the National Oceanic and Atmospheric Administration (NOAA) to analyze the evolutionary characteristics, the boundary-layer marine meteorological characteristics, and the development mechanism of the sea fog event. The results show that the sea fog event was a warm advective fog process. The Yellow Sea was at the rear of the warm high and the front of the continental low (circulation situation with high in the east and low in the west). The southerly and southeasterly winds transported warm and moist air from the Northwest Pacific northward to the Yellow Sea which served as the water vapor source. A thermal turbulence interface was formed during the sea fog development. The weak vertical wind shear below the interface was conducive to the maintenance and development of sea fog in the area of the temperature inversion in the boundary layer and the formation of a certain thickness of sea fog. The sea fog occurred in the area with water vapor convergence, where the 2-m dew point was slightly higher than the SST. The area with a relative humidity greater than 90% and an air–sea temperature difference from 0 to 2 °C overlapped with the sea fog area, and these two values indicate the extent of the sea fog.

Vertical extrapolation of Advanced Scatterometer (ASCAT) ocean surface winds using machine-learning techniques

Abstract. The increasing demand for wind energy offshore requires more hub-height-relevant wind information, while larger wind turbine sizes require measurements at greater heights. In situ measurements are harder to acquire at higher atmospheric levels; meanwhile the emergence of machine-learning applications has led to several studies demonstrating the improvement in accuracy for vertical wind extrapolation over conventional power-law and logarithmic-profile methods. Satellite wind retrievals supply multiple daily wind observations offshore, however only at 10 m height. The goal of this study is to develop and validate novel machine-learning methods using satellite wind observations and near-surface atmospheric measurements to extrapolate wind speeds to higher heights. A machine-learning model is trained on 12 years of collocated offshore wind measurements from a meteorological mast (FINO3) and space-borne wind observations from the Advanced Scatterometer (ASCAT). The model is extended vertically to predict the FINO3 vertical wind profile. Horizontally, it is validated against the NORwegian hindcast Archive (NORA3) mesoscale model reanalysis data. In both cases the model slightly over-predicts the wind speed with differences of 0.25 and 0.40 m s−1, respectively. An important feature in the model-training process is the air–sea temperature difference; thus satellite sea surface temperature observations were included in the horizontal extension of the model, resulting in 0.20 m s−1 differences with NORA3. A limiting factor when training machine-learning models with satellite observations is the small finite number of daily samples at discrete times; this can skew the training process to higher-/lower-wind-speed predictions depending on the average wind speed at the satellite observational times. Nonetheless, results shown in this proof-of-concept study demonstrate the limited applicability of using machine-learning techniques to extrapolate long-term satellite wind observations when enough samples are available.

Global and Regional Discrepancies between Early-Twentieth-Century Coastal Air and Sea Surface Temperature Detected by a Coupled Energy-Balance Analysis

Abstract A major uncertainty in reconstructing historical sea surface temperature (SST) before the 1990s involves correcting for systematic offsets associated with bucket and engine-room intake temperature measurements. A recent study used a linear scaling of coastal station-based air temperatures (SATs) to infer nearby SSTs, but the physics in the coupling between SATs and SSTs generally gives rise to more complex regional air–sea temperature differences. In this study, an energy-balance model (EBM) of air–sea thermal coupling is adapted for predicting near-coast SSTs from coastal SATs. The model is shown to be more skillful than linear-scaling approaches through cross-validation analyses using instrumental records after the 1960s and CMIP6 simulations between 1880 and 2020. Improved skill primarily comes from capturing features reflecting air–sea heat fluxes dominating temperature variability at high latitudes, including damping high-frequency wintertime SAT variability and reproducing the phase lag between SSTs and SATs. Inferred near-coast SSTs allow for intercalibrating coastal SAT and SST measurements at a variety of spatial scales. The 1900–40 mean offset between the latest SST estimates available from the Met Office (HadSST4) and SAT-inferred SSTs range between −1.6°C (95% confidence interval: [−1.7°, −1.4°C]) and 1.2°C ([0.8°, 1.6°C]) across 10° × 10° grids. When further averaged along the global coastline, HadSST4 is significantly colder than SAT-inferred SSTs by 0.20°C ([0.07°, 0.35°C]) over 1900–40. These results indicate that historical SATs and SSTs involve substantial inconsistencies at both regional and global scales. Major outstanding questions involve the distribution of errors between our intercalibration model and instrumental records of SAT and SST as well as the degree to which coastal intercalibrations are informative of global trends. Significance Statement To evaluate the consistency of instrumental surface temperature estimates before the 1990s, we develop a coupled energy-balance model to intercalibrate measurements of sea surface temperature (SST) and station-based air temperature (SAT) near global coasts. Our model captures geographically varying physical regimes of air–sea coupling and outperforms existing methods in inferring regional SSTs from SAT measurements. When applied to historical temperature records, the model indicates significant discrepancies between inferred and observed SSTs at both global and regional scales before the 1960s. Our findings suggest remaining data issues in historical temperature archives and opportunities for further improvements.

Air–sea gas exchange in a seagrass ecosystem – results from a 3He ∕ SF6 tracer release experiment

Abstract. Seagrass meadows are some of the most productive ecosystems in the world and could help to mitigate the increase of atmospheric CO2 from human activities. However, understanding the role of seagrasses in the global carbon cycle requires knowledge of air–sea CO2 fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocities were determined using the 3He and SF6 dual tracer technique in a seagrass ecosystem in south Florida, Florida Bay, near Bob Allen Keys (25.02663∘ N, 80.68137∘ W) between 1 and 8 April 2015. The observed gas transfer velocity, normalized for CO2 in freshwater at 20 ∘C, k(600), was 4.8 ± 1.8 cm h−1, which was lower than that calculated from published wind speed/gas exchange parameterizations. The deviation in k(600) from other coastal and offshore regions was only weakly correlated with tidal motion and air–sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in the study area. A new wind speed/gas exchange parameterization is proposed (k600=0.143u102), which might be applicable to other seagrass ecosystems and wind-fetch-limited environments.

Utilizing the Random Forest Method for Short-Term Wind Speed Forecasting in the Coastal Area of Central Taiwan

The Taiwan Strait contains a vast potential for wind energy. However, the power grid balance is challenging due to wind energy’s uncertainty and intermittent nature. Wind speed forecasting reduces this risk, increasing the penetration rate. Machine learning (ML) models are adopted in this study for the short-term prediction of wind speed based on the complex nonlinear relationships among wind speed, terrain, air pressure, air temperature, and other weather conditions. Feature selection is crucial for ML modeling. Finding more valuable features in observations is the key to improving the accuracy of prediction models. The random forest method was selected because of its stability, interpretability, low computational cost, and immunity to noise, which helps maintain focus on investigating the essential features from vast data. In this study, several new exogenous features were found on the basis of physics and the spatiotemporal correlation of surrounding data. Apart from the conventional input features used for wind speed prediction, such as wind speed, wind direction, air pressure, and air temperature, new features were identified through the feature importance of the random forest method, including wave height, air pressure difference, air-sea temperature difference, and hours and months, representing the periodic components of time series analysis. The air–sea temperature difference is proposed to replace the wind speed difference to represent atmosphere stability due to the availability and adequate accuracy of the data. A random forest and an artificial neural network model were created to investigate the effectiveness and generality of these new features. Both models are superior to persistence models and models using only conventional features. The random forest model outperformed all models. We believe that time-consuming and tune-required sophisticated models may also benefit from these new features.

Observed Opposite Fall‐to‐Winter Variations in the Air‐Sea Latent Heat Flux Between the Western Boundary Currents and Coastal Seas

AbstractThis study presents observations of opposite fall‐to‐winter variations in the air‐sea latent heat fluxes (LHs) between the western boundary currents (WBCs) and coastal seas obtained using eight air–sea buoys in the Northern Hemisphere. Different from the WBCs, the wintertime LH along the coast is strangely reduced by approximately 60 Wm−2 compared to that in fall. The reduced sea–air humidity differences account for these opposite seasonal variations, although the sea–air temperature differences are enhanced, while the roles played by winds and the nonlinear effects are minor. The boundary layer stability also varies in terms of the air‐sea parameters from fall to winter. However, the conditions of the WBCs meet the traditional expectation: that is, the LH in winter is 60 Wm−2 higher than that in fall with near‐neutral/unstable boundary layer stability. Enhanced winds in winter determine the seasonal enhancement of the LH over the WBCs, indicating a specialized regional character.

The onset process of the 2018/2019 Indonesian–Australian summer monsoon: The importance of the air–sea interaction

The Madden–Julian oscillation (MJO) and its associated air–sea interactions during the onset process of the 2018/2019 Indonesian–Australian summer monsoon (IASM) are investigated based on the in situ data from a moored buoy off the coast of northwest Australia, along with ERA5 reanalysis and satellite data. The results verify that the IASM onset in mid-December 2018 was triggered by the first-branch eastward-propagating MJO (FEMJO) originating from the tropical Indian Ocean. However, the strong negative SST anomaly (SSTA) was evident off northwest Australia, which weakens FEMJO over northern Australia and shifts the convective center further northward. The mixed layer heat budget analysis based on the buoy observations reveals that the increased latent heat loss that occurred before the arrival of the FEMJO convection was primarily attributed to the large air–sea temperature difference and strong winds, resulting in the pronounced SSTA.

Some Refinements to the Most Recent Simple Time-Dependent Theory of Tropical Cyclone Intensification and Sensitivity

Abstract Several key issues in the simple time-dependent theories of tropical cyclone (TC) intensification developed in recent years remain, including the lack of a closure for the pressure dependence of saturation enthalpy at sea surface temperature (SST) under the eyewall and the definition of environmental conditions, such as the boundary layer enthalpy in TC environment and the TC outflow-layer temperature. In this study, some refinements to the most recent time-dependent theory of TC intensification have been accomplished to resolve those issues. The first is the construction of a functional relationship between the surface pressure under the eyewall and the TC intensity, which is derived using the cyclostrophic wind balance and calibrated using full-physics axisymmetric model simulations. The second is the definition of TC environment that explicitly includes the air–sea temperature difference. The third is the TC outflow-layer temperature parameterized as a linear function of SST based on global reanalysis data. With these refinements, the updated time-dependent theory becomes self-contained and can give both the intensity-dependent TC intensification rate (IR) and the maximum potential intensity (MPI) under given environmental thermodynamic conditions. It is shown that the pressure dependence of saturation enthalpy at SST can lead to an increase in the TC MPI and IR by about half of that induced by dissipative heating due to surface friction. Results also show that both MPI and IR increase with increasing SST, surface enthalpy exchange coefficient, environmental air–sea temperature difference, and decreasing environmental boundary layer relative humidity, but the maximum IR is insensitive to surface drag coefficient. Significance Statement A new advancement in the recent decade is the development of simple time-dependent theories of tropical cyclone (TC) intensification, which can provide quantitative understanding of TC intensity change. However, several key issues in these simple time-dependent theories remain, including the lack of a closure for the pressure dependence of saturation enthalpy at sea surface temperature under the eyewall and the definition of environmental conditions. These are resolved in this study with several refinements, which make the most recent time-dependent theory of TC intensification self-contained and practical.

The Diurnal Variation of the Evaporation Duct Height and Its Relationship With Environmental Variables in the South China Sea

Based on data from the European Centre for Medium-Range Weather Forecasts (ECMWF) ECWMF ERA5 Reanalysis (ERA-5) and the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) datasets, the diurnal variation of the evaporation duct height (EDH) in the South China Sea (SCS) and its relationship with environmental variables using the Naval Postgraduate School (NPS) model are analyzed, discussed, and presented. The findings are given as follows. First, the EDH diurnal variations are primarily influenced by the sea-land thermal contrast, with the major influencing variables being the relative humidity (RH) and the near-surface wind speed (WS), followed by the air-sea temperature difference (ASTD). Secondly, the areas where the EDH and the RH are highly correlated are mostly in northern SCS, and the two variables are negatively correlated. Besides, in southeastern SCS, the EDH has a greater and positive correlation with the WS. Seasonal changes can be found in areas where the EDH is highly correlated with the ASTD, primarily to the northwest of Luzon Island in winter and the east coast of Vietnam in summer. Finally, the spatial patterns of the EDH first mode in the SCS present a monopolar structure with the strong loading in northern SCS.

Simulations of sea fog case impacted by air–sea interaction over South China Sea

A sea fog event in the South China Sea was simulated using a coupled ocean–atmosphere model (WRF for the atmosphere and ROMS for the ocean). Offshore and onshore visibility, liquid water content, air temperature, humidity, and wind speed observations and MICAPS data were utilized to validate the model results. The results of the coupled model were also compared with those of the uncoupled atmosphere model. Sea fog duration in the coupled model was closer to offshore and onshore observations, but the uncoupled model emptily forecasted offshore fog, and underreported onshore fog. Air–sea temperature difference played an important role in regulating the formation and dissipation of sea fog. The decrease of sea surface temperature in the coupled model cooled the low-level atmosphere, promoted the condensation of low-level water vapor, and increased the low-level water vapor. The decrease of air–sea temperature difference strengthened the low-level stable stratification, which weakened the horizontal wind speed and favored the formation and development of sea fog. Rising wind speed was the major driver of fog dissipation.