Carol Anne Clayson

An improved mixed layer model for geophysical applications

An improved mixed layer model, based on second-moment closure of turbulence and suitable for application to oceanic and atmospheric mixed layers, is described. The model is tested against observational data from different locations in the global oceans, including high latitudes and tropics. The model belongs to the Mellor-Yamada hierarchy but incorporates recent findings from research on large eddy simulations and second-moment closure. The modified expansion of Galperin, Kantha, Hassid and Rosati (1988) that leads to a much simpler and more robust quasi-equilibrium turbulence model is employed instead of the original Mellor and Yamada (1974) model. Findings from ongoing research at the National Center for Atmospheric Research on large eddy simulations of the atmospheric boundary layer are utilized to improve parameterizations of pressure covariance terms in the second-moment closure. Shortwave solar radiation penetration is given careful treatment in the model so that the model is applicable to investigations of biological and photochemical processes in the upper ocean. But by far the major improvement is in the inclusion of the shear instability-induced mixing in the strongly stratified region below the oceanic mixed layer that leads to a more realistic and reliable mixed layer model that is suitable for application to a variety of geophysical mixed layers and circulation problems. The model appears to predict the mixing in the upper ocean well on a variety of time scales, from event scale storm-induced deepening and diurnal scale variability to seasonal time scales. With proper attention to the heat and salt balances in the upper ocean, it should be possible to use it for simulations of interannual variability as well. While the model validation has been primarily against oceanic mixed layer data sets, it is believed that the improvements can be readily incorporated into a model of the atmospheric boundary layer as well.

Clouds, radiation, and the diurnal cycle of sea surface temperature in the tropical western Pacific

Abstract The relationship among clouds, surface radiation flux, and the sea surface temperature (SST) of the tropical western Pacific Ocean over the diurnal cycle is addressed in the context of the Atmospheric Radiation Measurement (ARM) Program scientific objectives for the tropical western Pacific Ocean. An understanding of the relationship between clouds and SST on a variety of time and space scales is needed to understand fully the cloud-radiation feedback in the tropical oceans and the maintenance of the warm pool. Here the diurnal cycle is emphasized. Data from the TOGA COARE Intensive Observation Period is examined and interpreted using an ocean mixed layer model that includes a parameterization of the “skin” temperature, explicit salinity, a surface beat budget that includes the sensible heat flux associated with rain, and the contribution of rain to the surface momentum flux. Using a mix of modeling and observations, three different case studies are examined in detail: clear and calm, clear and w...

On the effect of surface gravity waves on mixing in the oceanic mixed layer

Abstract We apply a one-dimensional mixed layer model, based on second moment closure of turbulence, to study the effects of surface gravity waves on mixing in the oceanic mixed layer. The turbulent kinetic energy injected near the surface by breaking waves, and the kinetic energy input from Langmuir circulations that may exist in the presence of surface gravity waves, are both parameterized and included in the turbulence model. As expected, the wave breaking elevates both the turbulent kinetic energy and its dissipation rate in the upper few meters, well above the classical values expected from similarity theory for shear layers adjacent to a boundary. While there is a significant impact on mixed layer properties near the surface, wave breaking-induced turbulence decays rapidly with distance from the surface and hence the overall effects on the mixed layer are small. On the other hand, the energy input to turbulence from Langmuir cells elevates the turbulent kinetic energy and mixing throughout the mixed layer, and is therefore more effective in deepening the mixed layer. While the changes in sea surface temperature (SST) brought about by the inclusion of Langmuir cells are rather small on diurnal time scales, they can be appreciable over seasonal time scales. Nevertheless, these SST changes are well within the uncertainties in the modeled SST resulting from an imperfect knowledge of the air–sea fluxes used to drive the mixed layer models.

High-Latitude Ocean and Sea Ice Surface Fluxes: Challenges for Climate Research

Polar regions have great sensitivity to climate forcing; however, understanding of the physical processes coupling the atmosphere and ocean in these regions is relatively poor. Improving our knowledge of high-latitude surface fluxes will require close collaboration among meteorologists, oceanographers, ice physicists, and climatologists, and between observationalists and modelers, as well as new combinations of in situ measurements and satellite remote sensing. This article describes the deficiencies in our current state of knowledge about air–sea surface fluxes in high latitudes, the sensitivity of various high-latitude processes to changes in surface fluxes, and the scientific requirements for surface fluxes at high latitudes. We inventory the reasons, both logistical and physical, why existing flux products do not meet these requirements. Capturing an annual cycle in fluxes requires that instruments function through long periods of cold polar darkness, often far from support services, in situations subject to icing and extreme wave conditions. Furthermore, frequent cloud cover at high latitudes restricts the availability of surface and atmospheric data from visible and infrared (IR) wavelength satellite sensors. Recommendations are made for improving high-latitude fluxes, including 1) acquiring more in situ observations, 2) developing improved satellite-flux-observing capabilities, 3) making observations and flux products more accessible, and 4) encouraging flux intercomparisons.

Evaluation of turbulent fluxes at the ocean surface using surface renewal theory

An internally consistent model is presented that can be used to determine the ocean surface fluxes of heat, moisture, and momentum, given bulk sea surface temperature and atmospheric temperature, humidity, and winds measured at a single level within the atmospheric surface layer. This model is based upon surface renewal theory as described by Brutsaert [1975a]. Liu et al. [1979] (hereinafter referred to as LKB) made partial use of this theory, and further improvements to the LKB parameterization have been made by Fairall et al. [1996a]. The present model includes the following improvements relative to the LKB and Fairall et al. bulk flux models: incorporation of a new time-scale parameterization for surface renewal, inclusion of capillary waves in the surface roughness model, derivation of the surface roughness scales of water vapor and heat based solely upon surface renewal theory; and incorporation of a new surface skin temperature model. The model is validated using shipborne observations of surface fluxes and surface meteorology that were obtained in the central Pacific Ocean, the western tropical Pacific, the subtropical Pacific, and the midlatitude North Atlantic. Comparisons of model results with covariance fluxes of latent heat show biases of less than 3% for all locations, with little dependence of error on wind speed; similar results are obtained for sensible heat and momentum flux. An assessment is given of the advantages of the present scheme over the LKB and Fairall et al. schemes. The model results are interpreted in the context of the physical processes involved in determining the surface roughness length and the surface renewal timescale.

On Turbulence and Mixing in the Free Atmosphere Inferred from High-Resolution Soundings

Abstract Mixing in the free atmosphere above the planetary boundary layer is of great importance to the fate of trace gases and pollutants. However, direct measurements of the turbulent dissipation rate by in situ probes are very scarce and radar measurements are fraught with uncertainties. In this paper, turbulence scaling concepts, developed over the past decades for application to oceanic mixing, are used to suggest an alternative technique for retrieving turbulence properties in the free atmosphere from high-resolution soundings. This technique enables high-resolution radiosondes, which have become quite standard in the past few years, to be used not only to monitor turbulence in the free atmosphere in near–real time, but also to study its spatiotemporal characteristics from the abundant archives of high-resolution soundings from around the world. Examples from several locations are shown, as well as comparisons with radar-based estimations and a typical Richardson number–based parameterization.

High-Resolution Satellite-Derived Dataset of the Surface Fluxes of Heat, Freshwater, and Momentum for the TOGA COARE IOP

Abstract An integrated approach is presented for determining from several different satellite datasets all of the components of the tropical sea surface fluxes of heat, freshwater, and momentum. The methodology for obtaining the surface turbulent and radiative fluxes uses physical properties of the atmosphere and surface retrieved from satellite observations as inputs into models of the surface turbulent and radiative flux processes. The precipitation retrieval combines analysis of satellite microwave brightness temperatures with a statistical model employing satellite observations of visible/infrared radiances. A high-resolution dataset has been prepared for the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP), with a spatial resolution of 50 km and temporal resolution of 3 h. The high spatial resolution is needed to resolve the diurnal and mesoscale storm-related variations of the fluxes. The fidelity of the satellite-derived s...

Variability of Tropical Diurnal Sea Surface Temperature

Abstract A dataset consisting of daily diurnal warming values from 1996 through 2000 covering the global Tropics (30°N through 30°S) at 0.25° × 0.25° resolution has been created using a parameterization for the diurnal warming developed previously. The inputs to the parameterization are the peak shortwave solar radiation [determined from International Satellite Cloud Climatology Project (ISCCP) data] and daily averaged wind speed [determined from Special Sensor Microwave Imager (SSM/I) data]. Comparisons with Tropical Ocean Global Atmosphere (TOGA) Tropical Atmosphere Ocean (TAO) and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) buoys show that the biases are small (mean bias is 0.0012°C; the standard deviation and correlation are 0.26°C and 0.74) and show no discernable geographic bias. The 5-yr average shows that throughout most regions the values are small, with higher values (approaching 1°C) in the northern Indian Ocean, the western equatorial Pacific, the equatorial eastern Pacific, ...

The Effect of Diurnal Sea Surface Temperature Warming on Climatological Air–Sea Fluxes

AbstractDiurnal sea surface warming affects the fluxes of latent heat, sensible heat, and upwelling longwave radiation. Diurnal warming most typically reaches maximum values of 3°C, although very localized events may reach 7°–8°C. An analysis of multiple years of diurnal warming over the global ice-free oceans indicates that heat fluxes determined by using the predawn sea surface temperature can differ by more than 100% in localized regions over those in which the sea surface temperature is allowed to fluctuate on a diurnal basis. A comparison of flux climatologies produced by these two analyses demonstrates that significant portions of the tropical oceans experience differences on a yearly average of up to 10 W m−2. Regions with the highest climatological differences include the Arabian Sea and the Bay of Bengal, as well as the equatorial western and eastern Pacific Ocean, the Gulf of Mexico, and the western coasts of Central America and North Africa. Globally the difference is on average 4.45 W m−2. The...

Sensitivity of a Coupled Single-Column Model in the Tropics to Treatment of the Interfacial Parameterizations

A coupled atmosphere‐ocean single-column model has been developed for testing tropical ocean‐atmosphere feedbacks. The model is evaluated against observational data (both in situ and satellite) during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) intensive observation period. The coupled model is able to successfully reproduce variations in cloud parameters and surface fluxes; the model also overestimates the latent and sensible heat fluxes compared to observations. The overestimation is most likely due to errors in the atmospheric surface layer temperature and specific humidity. The sea surface temperatures produced by the model are reasonable. The mean bias in sea surface temperature as compared to buoy data is 08C; the maximum deviation from the observed temperature is 0.48C. This model is then used to investigate the sensitivity of the ocean and the ocean‐atmosphere system to variations in the included interfacial parameterization in the tropical Pacific. The sensitivity of the model results to the turbulent flux model used in the coupled version is shown to produce daily averaged sea surface temperature variations of over 0.58C. Of equal significance is the variation in model response to temperatures from different depths in the water column. Use of the typically cooler skin temperature as the interfacial temperature rather than the temperature at depth results in strong differences in the atmospheric profiles of heat, moisture, and cloud properties. These differences are not caused solely by the difference in temperature, but are also due to the much-reduced diurnal variation in sea surface temperature at depth. The extent to which a daily averaged sea surface temperature changes the resulting atmospheric profiles depends on whether the diurnal variability was strong; under low-wind conditions the differences are the most dramatic.