Air-sea Temperature Difference Research Articles

Observed air-sea interface conditions and a monsoon depression during MONEX-79

The air-sea interface properties during the early stages of formation of a depression over East Central Arabian Sea during summer MONEX are examined by analyzing the ship data for air and sea temperatures, sea level pressures, sea state numbers and wind fields. Analysis of the data revealed a pronounced increase in sea-air temperature difference (2–4 °C) and this increase in the value is considered, obviously, to be due to a drop in the air temperature. The variations in the sea-air temperature difference and surface pressure are in opposite phase to each other. Some of the plausible mechanisms for the incipient development and movement of the depression are also discussed.

Factors affecting boundary-layer wind shear over the Indian Ocean during FGGE

Statistics on the vertical wind shear in the boundary layer over the Indian Ocean were examined for the causes of regional and seasonal changes. Low-level cloud motions and surface ship wind reports were used to define the vertical shear. Temperature data from the ship reports were analyzed for boundary-layer stability related to the observed shears. Smaller wind shears were found in areas of large negative air-sea temperature difference (unstable boundary layers). The ‘thermal wind’ effects were very small over most of the tropical Indian Ocean. The largest factor affecting the speed shear was the strength of the wind itself. Larger speed shear was found under high wind conditions. A small reduction in the direction difference between cloud and ship observations also was found under higher speeds. The scatter of cloud-ship comparisons around the mean (dispersion) also decreased for higher wind speeds. Daily gridded cloud motion and ship wind speed data had a correlation coefficient of 0.8 with a scatter of 1.9 m s-1 (r.m.s.) around the mean difference.

Overwater dispersion in coastal regions

Air pollution sources such as oil platforms and ships are sometimes located near coastlines where the plumes can impact population centers. Models of overwater dispersion must account for the stability of the overwater boundary layer. An overwater dispersion model based on the standard Gaussian formula is described which uses measurements of the air-sea temperature difference, the wind speed, and the mixing depth to predict concentration patterns at the coastline. Internal boundary layers and complex terrain at the coastline are accounted for. This new model is evaluated using the results of three tracer experiments in United States coastal zones.

Convection model for stratus cloud over a warm water surface

A numerical model is developed to simulate convective stratus cloud formation over the sea. The model is based on quasi-steady state moist plumes advecting over an area of increasing sea-surface temperature with the sea warmer than the air, but it is also a good approximation for non-steady states whenever the model is matched to air-sea temperature differences. Combining the effects of upward transfer of heat and moisture fluxes as well as adiabatic cooling, stratus cloud forms and spreads downward in this field. The depth of the convective field, the sea surface temperature gradient, the liquid water content at cloud top, and the horizontal pressure gradient are the four controlling parameters for the convective field. Alternatively, the wind speed, air-sea temperature difference, and the mixing ratio of the air, derived variables in this treatment, can be taken, with the depth, as the basic parameters.

The Role of Sea Surface Temperature in Large-Scale Air-Sea Interaction

Data from the 1972–73 El Niño event are used to study relationships between sea surface temperature (SST), surface wind, surface heat fluxes and rainfall in the tropical Pacific cast of the dateline. It is found that the maximum in SST anomaly is not correlated spatially or temporally in any consistent manner with the sum of latent and sensible heat fluxes (QE + QS) or rainfall as estimated from highly reflective cloud. Varying anomalies in wind speed and sea-air temperature and humidity differences all influence the QE + QS anomaly in ways which may be unrelated to the SST anomaly. Factors influencing SST, namely zonal wind stress and net surface heating, are found to vary with longitude. The position of the rainfall maximum and the time variation of rainfall in the near-equatorial convergence zone are more closely related to moisture convergence than to SST. Implications to modeling efforts which equate SST anomalies with diabatic heating anomalies are discussed.

The effect of temperature-dependent exchange coeeficients on poleward heat flux by oceanic gyres

The gravitational stability of the air overlying the sea surface is determined by the air-sea temperature difference. Air-sea exchange coeeficients have been shown to increase when the air is unstable and decrease when the air is stable. This stability dependence of the wind stress has been approximated by a linear law relating wind stress to air-sea temperature difference and it has been shown that this thermal feedback generates a warm, Gulf-Stream-like jet emerging from the western boundary current eastward into the interior. Here, the thermal correction is applied tot he wind stress and to the air-sea heat exchange coefficient to study the effects on the circulation and on the poleward heat flux. The basic dynamical model is linear, wind-driven and has an anticyclonic gyre in the southern half and an equally intense cyclonic half in the northern half basin. The thermodynamic model uses the vertically averaged, convective-diffusive heat equation. Thermal feedback leads to a compression of the anticyclonic gyre to a narrower lattitude band near the western boundary and to a warm Gulf Stream extending eastward into the interior. The redistribution of the dynamical properties in the anticyclonic gyre resembles that due to inertial processes. In the cyclonic gyre the changes are quite unlike those due to inertial effects. The main quantitative change is the poleward heat flux across the gyre boundary, which can be several times the value obtained without feedback and should be important for climate studies.

Bulk Parameterization of Air-Sea Exchanges of Heat and Water Vapor Including the Molecular Constraints at the Interface

A model is developed for the marine atmospheric surface layer including the interfacial sublayers on both sides of the air-sea interface where molecular constraints on transports are important. Flux-profile relations which are based on the postulation of intermittent renewal of the surface fluid aye matched to the logarithmic profiles and compared with both field and laboratory measurements. These relations enable numerical determination of air-sea exchanges of momentum, heat and water vapor (or bulk transfer coefficients) employing the bulk parameters of mean wind speed, temperature and humidity at a certain height in the atmospheric surface layer, and the water temperature. With increasing wind speed, the flow goes from smooth to rough and the bulk transfer coefficient for momentum also increases. The increase in roughness is associated with increasing wave height which in the present model results in sheltering at the wave troughs. Due to the decrease in turbulent transports, the transfer coefficients of heat and water vapor decrease slightly with wind speed after the wind speed exceeds a certain value. The bulk transfer coefficients are also found to decrease with increasing stability. If the “bucket temperature” which typically gives the water temperature a few centimeters below the surface is used, rather than the interfacial temperature, erroneous results may be obtained when the air-sea temperature difference is small. By including the effects of stability and interfacial conditions in bulk parameterization, the model provides a way to account for physical conditions which are known to affect air-sea exchanges.

MCC and gull flight behavior

Wintertime observations of mesoscale cellular convection (MCC) over the East China Sea have resulted in criteria that have a remarkable similarity to those reported by Woodcock (1975) in the study of thermals and gull flight behavior. It has been determined that the surface wind speed (V) and the air-sea temperature difference (δT) prescribe unique and compatible conditons for both the occurrence of MCC and soaring by sea gulls. Specifically, the onset of MCC when V is between 5 and 9 m s−1 is inversely proportional to δT in the range 5 to 7 °C. Elsewhere, the onset of MCC occurs under conditions of direct proportionality between V and δT. Necessary conditions for the occurrence of MCC due to heating from below are δT ⩾ 5 °C and V ⩾ 5 m s−1. The boundaries of the convective regime for MCC are discussed and interpreted in accordance with the regime for sea-gull soaring and similarity concepts.

Kinematic Analysis and Air-Sea Heat Flux Associated with Mesoscale Cellular Convection during AMTEX 75

Several excellent examples of open and closed mesoscale cellular convection (MCC) were observed in DMSP satellite imagery during the second field period of the Air Mass Transformation Experiment (AMTEX), conducted 14–28 February 1975 over the East China Sea in the vicinity of the Kuroshio current. The kinematic fields of horizontal divergence, vertical motion and vorticity of the large-scale flow producing the MCC and the associated heat flux at the air-sea interface were determined to starch for observational evidence as to the cause of circulation direction in the convection cells. None of these kinematic fields had features unique to open and closed cells, with predominantly downward vertical motion and diverging horizontal flow in the convecting layer. From the study of the kinematic fields determined for 14 cases of open cells and 20 cases of closed Cells a physical depiction has been made of the distribution of the vertical motion in the vertical plane of a mesoscale cell during its formation. Critical values of daily averaged air-sea temperature difference and surface wind speed as necessary conditions for the occurrence of MCC due to heating from below have been determined to be 1) sea surface temperature at least 5°C greater than air temperature and 2) surface wind speed in excess of 5 m s−1. Calculations of the sensible heat flux yielded a threshold value of ∼70 W m−2 for the onset of MCC. Similarly, the threshold value for the total heat flux was ∼200 W m−2. Calculations also showed that both open and closed cells occurred in the cold air outbreak when the sea to air energy flux (latent plus sensible) was either strong (∼1200 W m−2) or weak (∼200 W m−2).

Wind stress and fluxes of sensible and latent heat over the Arabian Sea during ISMEX-I973

During the Indo-Soviet Monsoon Experiment (ISMEX) in the year 1973, four Russian research vessels Vocikov, Okean, Shokaliasky and Priliv remained stationary at locations near 00.1 oN, 49 .6°E; 00 .1 oN. 60.0°E; 10.0°N. 60.0°E and 18 .O°N, 67.0°E respectively from 28 June to 2 July 1973. Hourly surface observations were taken on board the ships during this period. From these observations hourly values of shearing stress of wind at the surface and vertical fluxes of sensible and latent heat at the air-sea interface are computed by using the bulk aerodynamic formulae. A variable drag coefficient with wind speed is used as suggested by Deacon and Webb (1962). Hourly values of the Richardson number (Ri) for the four locations are also computed.
 It is found that the surface layers at locations near 00 .1°N, 49.6oE; 10.0°N, 60.0°E and 18.0°N, 67.0°E show near neutral stability conditions. The wind stress values are comparatively smaller at the equatorial locations than those at the northern ones. A marked increase in the wind stress occurs in association with a surge in the monsoon current.
 The sensible heat flux shows a well defined diurnal variation which is in close conformity with the diurnal variations of the air sea temperature difference. Maximum positive flux occurs around mid-night or dawn hours and minimum around mid-day or afternoon hours. An 'actual reversal of direction of the sensible heat flux is found to occur at noon hours particularly at the northern locations near 10.0°N, 60 .O°E and 18.0°N. 67.0°E during the period of study. The latent heat flux remains positive at all times of the day and shows a relative decrease or increase with a decreas3 or increase in the wind speed. While the sensible heat flux shows a marked increase during disturbed weather period, the latent heat flux shows a decrease during the same period.

Estimation of the refractive-index temperature structure parameter over the ocean

The semiempirical equation of Wyngaard, Izumi, and Collins is combined with the bulk aerodynamic formulas to provide equations for the estimation of C(T)(2) (the refractive-index temperature structure parameter) in the atmospheric boundary layer above the sea. The equations relate C(T)(2) to easily measured quantities of average wind speed and air-sea temperature and moisture differences. Comparison with limited C(T)(2) data obtained in the marine boundary layer suggests that the estimation equations are satisfactory.

Discussion of ‘thermals over the sea and gull flight behavior’ by A. H. Woodcock

Woodcock's (1975) refined plot of herring gull flight behavior on a diagram of wind speed versus sea-air temperature difference is shown to agree very satisfactorily with expectations derived from free-convection scaling arguments. However, his conclusion that under certain very weak wind conditions, a flock of gulls floating at sea can take off and quickly create a tall thermal suitable for soaring flight was not supported by a causal mechanism. A different hypothesis is that under those conditions, the gulls while on the sea surface can detect the near approach of a dominant thermal, as by sensing the occurrence of an essentially calm region marking the base of the updraft, and then can take off, circle, and quickly locate the nearby thermal.

Air-sea bulk transfer coefficients in diabatic conditions

On the basis of recent data for the roughness Reynolds number of the sea surface, and using the Owen-Thomson theory on the transfers of heat and mass between a rough surface and the flow above it, the bulk transfer coefficients of the sea surface have been estimated. For a reference height of 10 m, the neutral-lapse transfer coefficient for water vapor is larger by only a few percent than that for sensible heat. When the wind speed at the 10-m height is u10>3 m s−1, the coefficient for sensible heat CH is larger by about 10% than that for momentum CD. For u10<5 m s−1, however, the value of CD exceeds the value of CH, and for u10=15 m s−1 it is shown that CH≈0.8CD. It may be also proposed that 103CD=1.11 to 1.70, 103CE=1.18 to 1.30, and 103CH=1.15 to 1.26 for a range of u10=4 to 20 m s−1. A plot of diabatic transfer coefficients versus wind speed is obtained by using a parameter of the sea-air temperature difference. For practical purposes, the coefficients are approximated by empirical formulae.

Air and Sea Temperatures During Traverse of Hurricane Alma 1966

Abstract Continuous temperature measurements, 4 m above and 4 m below the sea surface, were obtained in the eye and in the inner and outer regions of maximum winds for a weak hurricane (decaying tropical storm). In the inner region of maximum winds the sea-air temperature difference is proportional to r2 and in the outer region it is proportional to r−1, where r is approximately the radial distance along a southeasterly direction from the center of the storm. Also, a sequence of dips in the sea temperature record on the southeastern side of the storm may relate to an internal inertio-gravity wave response or wake generated by the moving storm; but, this conclusion is tentative.

Observations of the eddy fluxes of heat and vapour over the sea

The results are reported of 61 observations of eddy fluxes of heat and vapour at different heights above the sea surface in the trade wind régime off the eastern Australian coast at latitude 16.51°S. The flux gradients were found to be consistent with observed temporal changes in temperature and mixing ratio as the air moved over the sea. The measured fluxes were found to agree with those calculated by bulk aerodynamic methods on the assumption of a 2°C sea-air temperature difference. The evaporation rate deduced from the vapour flux measurements is about 7 mm day −1.

ON THE RELATIONSHIP BETWEEN GEOSTROPHIC AND SURFACE WIND AT SEA

Abstract The ratio between observed surface and geostrophic wind speed has been investigated from observations at the German Bight, taking geostrophic wind and the air-sea temperature difference as parameters. The ratio decreases with increasing geostrophic wind and increasing stability. While stability is an important parameter for light to moderate winds, variation of the ratio with geostrophic wind speed cannot be neglected, taking the full range of geostrophic wind speeds into consideration. From the Navier-Stokes equations, such a variation is to be expected. For light winds, the (local) surface wind may exceed the (mesoscale) geostrophic wind. Both effects together can be described approximately by a linear relation between the surface wind and geostrophic wind, with a slope of 0.56 and a constant term b>0 varying with stability. The residual error was 2 m/s. Variation with latitude is inferred from the Navier-Stokes equations.

Measurements of vertical air temperature gradients with dipmeter

By means of measuring the dips of horizon with a Pulfrich dipmeter, vertical air temperature gradients near the sea surface were obtained with an accuracy of ca. ±0.01°C/m, much better than a conventional method with an array of temperature sensors. The additional advantage of this technique was that the measurements were made on board a ship underway, consequently data from wide oceanic areas could be accumulated rather quickly and easily. From the results of dip measurements at 125 stations on two cruises of the T/V Oshoro Maru of the Hokkaido University, extending from the Bering Sea to the south of Australia, the temperature gradients were computed with a new formula with an assumption that the refractive index of air varied with a height only. It was found that, in the northern North Pacific Ocean, vertical air temperature gradients were positive, while in the subtropical and tropical Pacific Ocean both the negative and positive gradients were observed. Generally, in the same sea region, the temperature profiles had the similar form, irrespective of air-sea temperature differences.

寒気進出時の湾流上の大気海洋間の熱交換の一例

Heat transfer from the sea to the air over the Gulf Stream off the East Coast of the United States is treated by use of the data at an occasion of cold air outbreak on December 11th through 13th in 1957. Sverdrup's formula gives extreme values of sensible heat transfer by convection, because the Bowen ratio exceeded 0.5 owing to the extraordinary large air-sea temperature differences. The total heat transfer computed from heat and vapor exchange formulae at the sea surface is compared with the one estimated from the reconstructed atmospheric soundings along two trojectories across the Gulf Stream. The results by the two methods reasonably agree with each other. The change of vertical temperature profiles in a cold air mass moving into the warm water is determined by solving a classical diffusion equation and is compared with the observed profiles.

The accuracy of wind‐speed estimates at sea

Most wind reports at sea come from merchant ships without anemometers. The Beaufortscale is used, the units of which are now denned principally in terms of sea disturbance. A study of 1315 pairs of measured and estimated winds made on U. S. weather ships shows that the estimated speeds corresponded to the measured within 1–3 knots (same Beaufort number) in 6pct of the cases; 2pct were underestimated and 1pct overestimated. The greater frequency of underestimates than of overestimates is apparently owing mainly to the fact that the anemometers (at 20–30 m) on the weather ships are higher than the (10 m) elevation for which the speed equivalents of the Beaufort scale were established, but partly also to the extra mileage of the anemometer owing to the rolling and pitching of the ship.Changes in wind speed occur so often that lag in the reaction of the sea is the most common cause of error in estimates. Another source of trouble is the sea—air temperature difference, a cold wind relative to the sea producing larger waves than a warm wind. Decreasing winds that are also cold are especially likely to be overestimated, and increasing ones that are warm, underestimated.

Non-frontal Crachin and the Cool Season Clouds of the China Seas

The crachin of the coastal regions of south China and northern Indochina develops during the cool season whenever relatively moist air is cooled sufficiently by contact with the cold coastal waters to lower its condensation level below the level of turbulent mixing. Inter-mensal and diurnal variation of crachin are discussed as well as its coincidence with low-level winds veering with height. Forecasting procedures are listed. Over the China Seas beyond the crachin zone, convective cloud predominates, but as sea-air temperature differences steadily decrease through the latter part of the cool season, cloud amounts also diminish.