Surface Turbulent Fluxes Research Articles

Which Bulk Aerodynamic Algorithms are Least Problematic in Computing Ocean Surface Turbulent Fluxes?

Bulk aerodynamic algorithms are needed to compute ocean surface turbulent fluxes in weather forecasting and climate models and in the development of global surface flux datasets. Twelve such algorithms are evaluated and ranked using direct turbulent flux measurements determined from covariance and inertial-dissipation methods from 12 ship cruises over the tropical and midlatitude oceans (from about 5°S to 60°N). The four least problematic of these 12 algorithms based upon the overall ranking for this data include the Coupled Ocean–Atmosphere Response Experiment (COARE) version 3.0 and The University of Arizona (UA) schemes as well as those used at the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Aeronautics and Space Administration (NASA) Data Assimilation Office for version 1 of the Goddard Earth Observing System reanalysis (GEOS-1). Furthermore, the four most problematic of these algorithms are also identified along with possible explanations. The overall ranking is not substantially affected by the use of the average of covariance and inertial-dissipation flux measurements or by taking into consideration measurement uncertainties. The differences between computed and observed fluxes are further evaluated as a function of near-surface wind speed and sea surface temperature to understand the rankings. Finally, several unresolved issues in terms of measurement and algorithm uncertainties are raised.

Characteristics of Heat Transfer over the Ice Covered Sea of Okhotsk during Cold-air Outbreaks

Heat transfer coefficients, and relative importance of factors affecting surface turbulent heat flux in sea-ice covered ocean, were investigated using data obtained by rawinsonde observations over, and around, the southwestern region of the Sea of Okhotsk from Jan. to Feb. in 1998, 1999 and 2000. The range of the fluxes estimated by an atmospheric heat budget analysis was large, associated with the ice concentration and synoptic situation. The turbulent heat fluxes from the open water surrounded by the sea ice are always positive (upward). However, the heat flux through sea-ice surface tends to be negative (downward) over sea-ice area associated with the relationship between the air and ice-surface temperature. Using the obtained turbulent heat fluxes and a bulk formula, relationships of turbulent heat fluxes with ice concentrations and atmospheric parameters were investigated, and a representative value of the bulk heat transfer coefficient was obtained. The estimated conditional bulk heat transfer coefficient (1:33 × 10-3) in the Sea of Okhotsk shows a smaller value than that in the Arctic region, reflecting the difference of the ice conditions, and thermal conditions in the boundary layer. Through an idea of the insulating coefficient, which measures degrees of the insulating effect of sea ice on the heat transfer, we explained that the sea ice works as a more effective insulator in weaker cold-air outbreaks. From a regression analysis, we found that the primary contribution to the amount of turbulent heat flux is the ice concentration, and second is the air temperature in the boundary layer.

A Parameterization Scheme of Surface Turbulent Momentum and Sensible Heat over the Gobi Underlying Surface

The parameterization of surface turbulent fluxes over the Gobi Desert in arid regions is studied by using rationally screened observational data. First, the characteristics of Monin-Obukhov similarity functions are analyzed and their empirical formulae are fitted. The results show that fitted curves of changes of similarity functions of wind speed and temperature with stability parameter differ little from the typical empirical curves and are within the ranges of scatter of the empirical curves, but their values in the neutral condition arc different from the typical values to some extent. Furthermore, average values of momentum and scalar (sensible heat) roughness lengths as well as changes of scalar roughness length with friction velocity are determined by utilizing the data. It is found that the average values of scalar roughness length are about one order smaller than that of the momentum roughness length and decrease with increasing friction velocity, but they are evidently larger than their theoretically forecasted values.

Uncertainties in sea surface turbulent flux algorithms and data sets

An intercomparison of eight bulk sea surface turbulent flux algorithms used in data set generation as well as weather and climate prediction is performed for the tropical Pacific and midlatitude Atlantic. The results show some significant differences in fluxes due to differences in the way the algorithms consider wave spectrum, convective gustiness, and salinity as well as the way the algorithms parameterize roughness lengths and turbulent exchange coefficients. For instance, for sea surface temperature between 27.75°C and 28.25°C, the maximum differences in monthly latent heat flux and wind stress among algorithms over the tropical Pacific are about 23 W m−2 (or 16% relative to the algorithm‐averaged flux) and 0.013 N m−2 (or 19% relative to the algorithm‐averaged value) respectively. Evaluation of these algorithms using 270 hourly samples of observed turbulent flux data over the midlatitude Pacific shows that algorithms are largely consistent with observations. However, some algorithms show significant deviations from observations under certain conditions (e.g., weak wind conditions). Insights from the above intercomparison are then used to evaluate ocean surface turbulent fluxes from two global data sets, one is derived from satellite remote sensing while the other is a reanalysis. Over two buoy sites in the eastern and western tropical Pacific, the mean heat flux differences between the two data sets are primarily caused by differences in bulk variables (e.g., wind speed) rather than by differences in bulk algorithms. However, bulk algorithm differences could contribute up to 17 W m−2 to the long‐term averaged latent heat flux over the tropical Pacific if different algorithms were used. This suggests that both bulk algorithms and the deviation of environmental variables need to be further improved in order to produce ocean surface turbulent fluxes with an accuracy of 10 W m−2.

An Assessment of the Surface Turbulent Heat Fluxes from the NCEP–NCAR Reanalysis over the Western Boundary Currents

With the completion of the NCEP–NCAR and ECMWF reanalyses there are now global representations of air–sea surface heat fluxes with sufficient spatial and temporal resolution to be useful in characterizing the air–sea interaction associated with individual weather systems, as well as in developing global-scale oceanic heat and moisture budgets. However, these fluxes are strongly dependent on the numerical models used, and, as a result, there is a clear need to validate them against observations. Accurate air–sea heat flux estimates require a realistic representation of the atmospheric boundary layer, and the implementation of an appropriate surface flux parameterization. Previous work at high latitudes has highlighted the shortcomings of the surface turbulent heat flux parameterization used in the NCEP–NCAR reanalysis during high wind speed conditions, especially when combined with large air–sea temperature differences. Here the authors extend this result through an examination of the air–sea heat fluxes over the western boundary currents of the North Atlantic and North Pacific Oceans. These are also regions where large transfers of heat and moisture from the ocean to the atmosphere take place. A comparison with in situ data shows that the surface layer meteorological fields are reasonably well represented in the NCEP–NCAR reanalysis, but the turbulent heat flux fields contain significant systematic errors. It is argued that these errors are associated with shortcomings in the bulk flux algorithm employed in the reanalysis. Using the NCEP–NCAR reanalysis surface layer meteorological fields and a more appropriate bulk flux algorithm, “adjusted” fields for the sensible and latent heat fluxes are presented that more accurately represent the air–sea exchange of heat and moisture over the western boundary currents.

Parameterizing Scalar Transfer over Snow and Ice: A Review

Abstract Evaluating the profiles of wind speed, temperature, and humidity in the atmospheric surface layer or modeling the turbulent surface fluxes of sensible and latent heat over horizontally homogeneous surfaces of snow or ice requires five pieces of information. These are the roughness lengths for wind speed (z0), temperature (zT), and humidity (zQ) and the stratification corrections for the wind speed and scalar profiles ψm and ψh, respectively. Because over snow and ice the atmospheric surface layer is often stably stratified, the discussion here focuses first on which of the many suggested ψm and ψh functions to use over snow and ice. On the basis of four profile metrics—the critical Richardson number, the Deacon numbers for wind speed and temperature, and the turbulent Prandtl number—the manuscript recommends the Holtslag and de Bruin ψm and ψh functions because these have the best properties in very stable stratification. Next, a reanalysis of five previously published datasets confirms the valid...

A Fractional Approach to the Factor Separation Method

A fractional study of the factor separation (FS) approach is performed. A fractional version of the FS method is discussed. The revised approach allows a determination of role of the acting physical mechanisms as well as that of potential nonlinearity of the modeling system responses. Application of the approach is demonstrated based on the simulations of an eastern Mediterranean weather development during 1‐2 November 1994, performed with the Florida State University global spectral model. Thirteen model simulations with varying intensity of the turbulent surface fluxes in the runs were performed for the analysis. Two locations selected for sensitivity analysis of the model results represent the processes over the Red Sea and the Mediterranean Sea. It is shown that the variation of the FS results obtained in the experiments with different intensity of factors may be in some cases significant. The degree of variability of the FS results obtained in the experiments with varying intensity of the factors under the analysis provides useful information on sufficiency (or insufficiency) of available simulation results for an improved evaluation of the role of the acting factors in a meteorological process.

One year measurement and simulation of turbulent surface heat fluxes over the Baltic Sea

One of the aims of the Pilot Study of Evaporation and Precipitation in the Baltic Sea (PEP in BALTEX) is the improvement of the parameterisation of evaporation over the Baltic Sea in models. The atmospheric regional climate model REMO is used here to simulate evaporation over the Baltic Sea for one year. The turbulent surface fluxes of latent and sensible heat are determined by a bulk formula using prescribed sea surface temperature (SST) values from a coarser grid. Comparison with measurements at four coastal or island sites which can be considered to be marine sites during onshore winds turns out to be problematic: Deviations of the prescribed SST from measured SST are great at certain times and systematic due to seasonal heating or cooling of the water near coasts. A test with realistic SSTs at one site for a 5-days period strongly reduces the heat fluxes. Comparisons with measured fluxes during the periods with strongest evaporation - cold air outbreak with easterly wind in autumn - were not possible due to the specific location of the measuring sites. Three parameterisation schemes are tested in REMO and the effect of e.g. the wind speed dependence of the transfer coefficient C HN is not as great as expected from tests with artificial data. For future comparisons of regional model results with single-point measurements, measured data in rather homogeneous areas, which are representative for the gridbox area, should be taken. In inhomogeneous areas, mesoscale models may serve as a bridge between the gridbox of the regional model and the measuring point. Application of a fully coupled atmospheric-oceanographic model should improve the SST resolution both in space and time.

A Lagrangian investigation of vertical turbulent heat fluxes in the upper ocean during Tropical Ocean–Global Atmosphere/Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE)

We investigated the diurnal cycle of the near‐surface ocean temperatures in the western Pacific Warm Pool, using temperature data from drifters with 20 m thermistor chains, during the Coupled Ocean‐Atmosphere Response Experiment (COARE) Intensive Observation Period (December 1992 to February 1993). We estimated turbulent heat fluxes at 20 m as a residual in the hourly heat budget along each drifter trajectory using hourly surface fluxes. Fortnightly averaged turbulent heat fluxes at 20 m were upward during periods of sustained winds, indicating a warming of the daily thermocline by the deeper layers. During calm periods the inferred turbulent heat fluxes were downward. We also calculated the day‐to‐night differences of the turbulent heat fluxes at 20 m as a residual in the budget of the daily cycle of heat content in the upper 20 m. This method largely avoids the uncertainties of surface turbulent heat fluxes. The residual averaged over the 2 month period was negative for the drifters that experienced strong westerly wind conditions (with heat fluxes positive downward). These surprising results for the warm pool region are at odds with what was found earlier from moored measurements in the central Pacific. Interestingly, however, periods characterized by a westerly wind burst during December 1992 to March 1993 contrasted with the periods with weaker winds: during westerly wind bursts the difference was negative, whereas it was positive during calm or warming periods. This is consistent with our analysis of the daily budget and also with other studies in the COARE domain. It confirms that the nighttime mixed layer was usually much deeper than 20 m during this period. However, our estimates are different for drifters deployed north of 1°N than for drifters near or south of the equator, indicating marked spatial variability in the heat budget of the warm pool.

The Impact of Surface Flux– and Circulation-Driven Feedbacks on Simulated Madden–Julian Oscillations

Abstract The intent of this study is to measure the total contribution of individual wind- and moist flux–induced feedback mechanisms on the generation and maintenance of simulated Madden–Julian oscillations (MJOs). This task is accomplished through the use of an idealized GCM that is also employed to examine the impact of sea surface temperatures on the robustness of the MJO signal. Among the mechanisms investigated are Conditional Instability of the Second Kind (CISK), Wind Induced Surface Heat Exchange (WISHE), and the feedback due to specific humidity differences between the surface and the boundary layer. A series of numerical experiments was conducted in which one or more of these feedbacks were suppressed. The resulting structure and periodicity of the simulated mode was analyzed using eigenvector and spectral analysis methods. The relative phases of surface wind, turbulent flux, and boundary layer moisture anomaly fields were then contrasted. It was found that the model produced a far stronger sig...

A Comparison of Surface Layer and Surface Turbulent Flux Observations over the Labrador Sea with ECMWF Analyses and NCEP Reanalyses

Comparisons are made between a time series of meteorological surface layer observational data taken on board the R/V Knorr, and model analysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction (NCEP). The observational data were gathered during a winter cruise of the R/V Knorr, from 6 February to 13 March 1997, as part of the Labrador Sea Deep Convection Experiment. The surface layer observations generally compare well with both model representations of the wintertime atmosphere. The biases that exist are mainly related to discrepancies in the sea surface temperature or the relative humidity of the analyses. The surface layer observations are used to generate bulk estimates of the surface momentum flux, and the surface sensible and latent heat fluxes. These are then compared with the model-generated turbulent surface fluxes. The ECMWF surface sensible and latent heat flux time series compare reasonably well, with overestimates of only 13% and 10%, respectively. In contrast, the NCEP model overestimates the bulk fluxes by 51% and 27%, respectively. The differences between the bulk estimates and those of the two models are due to different surface heat flux algorithms. It is shown that the roughness length formula used in the NCEP reanalysis project is inappropriate for moderate to high wind speeds. Its failings are acute for situations of large air–sea temperature difference and high wind speed, that is, for areas of high sensible heat fluxes such as the Labrador Sea, the Norwegian Sea, the Gulf Stream, and the Kuroshio. The new operational NCEP bulk algorithm is found to be more appropriate for such areas. It is concluded that surface turbulent flux fields from the ECMWF are within the bounds of observational uncertainty and therefore suitable for driving ocean models. This is in contrast to the surface flux fields from the NCEP reanalysis project, where the application of a more suitable algorithm to the model surface-layer meteorological data is recommended

Role of Tropical Clouds in Surface and Atmospheric Energy Budget

In this paper diagnostic estimates of cloud radiative forcing (CRF) and clear-sky radiation budget at the surface and in the atmosphere, based on satellite-observed radiation budget at the top of the atmosphere (TOA) and empirical parameterizations derived from radiation models and field observations, are presented. This analysis is restricted to the tropical Pacific. High clouds over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), and the warm pool (WP) exert a positive CRF of about 70 W m22 within the atmosphere and a negative CRF of about 2 70 Wm 22 at the surface, although with a negligible net CRF at the TOA. On the other hand, low clouds over the eastern subtropical Pacific and the equatorial cold tongue exert a negative CRF of about 2 20 Wm 22 at the surface as well as in the atmosphere. The spatial gradients of the clear-sky radiation budget at the surface and in the atmosphere are small. In particular, it is shown that the clear-sky radiative cooling in the atmosphere is larger over the ITCZ, the SPCZ, and the WP, when compared with that over the subtropics and the cold tongue. Next, based on these diagnostic estimates and available surface turbulent heat flux data, the role of atmospheric CRF in the large-scale atmospheric moist static energy (MSE) transport is quantified. It is found that the atmospheric CRF provides the major energy source for balancing the divergence of MSE transport (from the ITCZ, the SPCZ, and the WP to the subtropics and the cold tongue) by the large-scale atmospheric circulation. On the other hand, the clear-sky radiative flux convergence and the surface turbulent heat fluxes have just the reverse spatial pattern and hence cannot satisfy the large-scale atmospheric MSE transport requirements.

An intercomparison of cloud‐resolving models with the atmospheric radiation measurement summer 1997 intensive observation period data

AbstractThis paper reports an intercomparison study of midlatitude continental cumulus convection simulated by eight two‐dimensional and two three‐dimensional cloud‐resolving models (CRMs), driven by observed large‐scale advective temperature and moisture tendencies, surface turbulent fluxes, and radiative‐heating profiles during three sub‐periods of the summer 1997 Intensive Observation Period of the US Department of Energy's Atmospheric Radiation Measurement (ARM) program. Each sub‐period includes two or three precipitation events of various intensities over a span of 4 or 5 days. The results can be summarized as follows.CRMs can reasonably simulate midlatitude continental summer convection observed at the ARM Cloud and Radiation Testbed site in terms of the intensity of convective activity, and the temperature and specific‐humidity evolution. Delayed occurrences of the initial precipitation events are a common feature for all three sub‐cases among the models. Cloud mass fluxes, condensate mixing ratios and hydrometeor fractions produced by all CRMs are similar. Some of the simulated cloud properties such as cloud liquid‐water path and hydrometeor fraction are rather similar to available observations. All CRMs produce large downdraught mass fluxes with magnitudes similar to those of updraughts, in contrast to CRM results for tropical convection. Some inter‐model differences in cloud properties are likely to be related to those in the parametrizations of microphysical processes.There is generally a good agreement between the CRMs and observations with CRMs being significantly better than single‐column models (SCMs), suggesting that current results are suitable for use in improving parametrizations in SCMs. However, improvements can still be made in the CRM simulations; these include the proper initialization of the CRMs and a more proper method of diagnosing cloud boundaries in model outputs for comparison with satellite and radar cloud observations. Copyright © 2002 Royal Meteorological Society

Observations of the Afternoon Transition of the Convective Boundary Layer

The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.

Feedback between the Land Surface Energy Balance and Atmospheric Boundary Layer Diagnosed through a Model and Its Adjoint

Abstract In this paper, the differences in (daytime) sensitivity of surface turbulent fluxes to states and parameters for a coupled land–atmospheric boundary layer model in coupled and uncoupled modes are investigated using an adjoint framework. The adjoint approach yields diagnostic insight into sensitivity pathways that is not readily obtained when using a traditional forward sensitivity framework. It is shown that the corresponding uncoupled adjoint model is a special case of the coupled adjoint model in which the atmospheric feedback pathways are “closed.” These closed sensitivity pathways are directly responsible for the differences in sensitivity between the uncoupled and coupled models. Integrating the adjoint models about a consistent and common trajectory provided by the forward model yields the sensitivities of the surface turbulent fluxes to all of the model states and parameters, which are shown to be significantly different because of the boundary layer feedbacks. Furthermore, the adjoint sen...

Observations Of The Morning Transition Of The Convective Boundary Layer

The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.

Analytical Solution of Surface Layer Similarity Equations

Turbulent exchange between the surface and the atmosphere strongly depends on the stability of the surface layer. If surface radiometric temperature, rather than aerodynamic temperature, is used to parameterize the surface turbulent fluxes, the solution of the stability parameter is related to the thermal roughness length zT, which is generally not identical to the aerodynamic roughness length z0. This note derives the exact solution of the stability parameter equation for a stable surface layer and proposes an approximate analytical solution for an unstable surface layer. The solution can improve the computational efficiency of flux parameterization and is applicable in a wide range of z/z0 (50–104) and z0/zT (from less than 1 to greater than 104).

Sensitivity of Simulated Regional Surface Thermal Fluxes during Warm Advection Snowmelt to Selection of the Lowest Model Level Height

Under strong warm advection, sensible and latent heat fluxes may provide larger energy for surface snowmelt than does net radiation flux. With these thermally stable conditions, the height of the first model level may be well above the surface-layer depth and thus outside the range of applicability of the surface-layer similarity theory on which the models’ surface thermal flux computation is based. This situation can strongly affect the magnitude of simulated surface thermal fluxes and snowmelt. To explore this issue, the impact of selected heights of the first model level on the simulated surface fluxes and snowmelt under stable surface stratification conditions was investigated. Simulations using a mesoscale atmospheric model considering two extreme contrasts in surface roughness were performed. Setting the first model level to 3 or 10 m, which typically was within the stable surface layer, yielded nearly the same contribution of simulated surface turbulent thermal fluxes to snowmelt. When the first model level height was set at about 40 m, as is used in many regional model simulations, it exceeded the depth of the stable surface layer over the snow cover. The surface turbulent thermal flux contribution in this case was smaller (by about 40%), with a directly proportional effect on the snowmelt. Pending observational support, results presented in this study imply that setting a model’s lowest level to 10 m or less will likely improve simulated snowmelt accuracy during warm advection.

Modelling the local surface exchange over a grass-field site under stable conditions

Observations of near-surface variables are used with the Met Office Surface Exchange Scheme to calculate the exchange of heat and moisture between the surface and the atmosphere for a grass-field site at the Met Office, Cardington, UK. Different methods for modelling the stability near the surface are investigated with the surface temperatures and turbulent heat fluxes compared with data collected from the site. The results show that, whilst using Monin-Obukhov similarity theory can give good results in stable conditions, the form of the stability functions can significantly affect the accuracy in certain stability regimes. The use of log-linear stability functions can cause the turbulent fluxes of heat and moisture to cut off at stabilities which are too close to neutral compared with the observations. The results also show that unrealistically high surface temperatures are obtained in clear-sky nocturnal conditions with the standard surface energy balance. An alternative energy balance which includes a canopy of vegetation above the underlying soil gives better agreement with the observations in these conditions.

Modelling the local surface exchange over a grass‐field site under stable conditions

AbstractObservations of near‐surface variables are used with the Met Office Surface Exchange Scheme to calculate the exchange of heat and moisture between the surface and the atmosphere for a grass‐field site at the Met Office, Cardington, UK. Different methods for modelling the stability near the surface are investigated with the surface temperatures and turbulent heat fluxes compared with data collected from the site. The results show that, whilst using Monin‐Obukhov similarity theory can give good results in stable conditions, the form of the stability functions can significantly affect the accuracy in certain stability regimes. The use of log‐linear stability functions can cause the turbulent fluxes of heat and moisture to cut off at stabilities which are too close to neutral compared with the observations. The results also show that unrealistically high surface temperatures are obtained in clear‐sky nocturnal conditions with the standard surface energy balance. An alternative energy balance which includes a canopy of vegetation above the underlying soil gives better agreement with the observations in these conditions.