Total Turbulent Heat Flux Research Articles

Regional Variation in Extratropical North Atlantic Air‐Sea Interaction 1960–2020

AbstractAir‐sea interaction in late boreal winter is studied over the extratropical North Atlantic (NA) during 1960–2020 by examining the relationship between sea‐surface temperature (SST) and total turbulent heat flux (THF). The two quantities are positively correlated on interannual timescales over the central‐midlatitude and subpolar NA, suggesting the atmosphere on average drives SST and THF variability is independent of SST. On decadal timescales and over the central‐midlatitude NA the correlation is negative, suggesting ocean processes on average drive SST and THF variability is sensitive to SST. The correlation is positive over the subpolar NA. There, interannual and decadal THF variability is governed by the North Atlantic Oscillation (NAO). During the major late 20th and early 21st century SST increase in the subpolar NA diminishing oceanic heat loss associated with a weakening NAO was observed. This study suggests that the atmosphere is more sensitive to SST over the central‐midlatitude than subpolar NA.

Analytical framework based on thermodynamics to estimate spatially distributed surface energy fluxes from remotely sensed radiations

The Limited surface observations of turbulent heat fluxes result in incomplete knowledge about the surface energy balance that drives the climate system. The highly parameterized surface energy balance models suffer from significant uncertainties. Remote sensing information of incoming and outgoing radiation fluxes are important input variables for turbulent heat flux models, though analytical solutions of surface energy budget from these variables are yet to be derived. Here, we developed a novel, purely physics-based analytical method grounded on the thermodynamic principle of maximum power. The approach derives the total turbulent heat flux only from the four inputs of incoming and outgoing longwave and shortwave radiations at the land surface, which are available from remotely sensed satellite data. The proposed approach does not use any parameterization, unlike the existing surface energy balance models, and hence does not suffer from uncertainty due to the same. We validated our methodology with 102 eddy covariance observation stations around the globe with different land use land covers from FLUXNET2015 and available urban datasets. Based on monthly averages of the total turbulent flux estimates for the eddy covariance sites, we observed root-mean-square error (RMSE) of 23.2 ± 10.9 Wm−2, a mean bias error (MBE) of 11.9 ± 13.1 Wm−2 and R2 value of 0.86 ± 0.15. Using the satellite observations of radiation fluxes from CERES at a spatial resolution of 10, we obtained the global flux field of total turbulent flux (QJ) and land surface heat storage (ΔQs) fluxes. On validation of QJ with FLUXNET sites for six grids for different land use land cover, we found RMSE of 20.1 ± 7 Wm−2, MBE of 14.4 ± 9 Wm−2 and R2 value of 0.96 ± 0.02. Further, from the evaporative stress factor of GLEAM, which is based on microwave remotely sensed vegetation optical depth and root zone soil moisture, we have obtained spatially distributed global estimates of Sensible(H) and latent heat (LE). In addition, our analytical estimates address the distribution of residual energy associated with the surface energy balance closure problem driven by the land use land cover. The theoretical estimates of all surface energy balance components from remote sensing based observations will improve our understanding of surface warming for different land use land covers across the globe.

7 February Chamoli (Uttarakhand, India) Rock-Ice Avalanche Disaster: Model-Simulated Prevailing Meteorological Conditions

The present study aims to analyze the high-resolution model-simulated meteorological conditions during the Chamoli rock-ice avalanche event, which occurred on 7 February 2021 in the Chamoli district of Uttarakhand, India (30.37° N, 79.73° E). The Weather Research and Forecasting (WRF) model is used to simulate the spatiotemporal distribution of meteorological variables pre- and post-event. The numerical simulations are carried out over two fine resolution nested model domains covering the Uttarakhand region over a period of 2 weeks (2 February to 13 February 2021). The model-simulated meteorological variables, e.g., air temperature, surface temperature, turbulent heat flux, radiative fluxes, heat and momentum transfer coefficients, specific humidity and upper wind patterns, were found to show significant departures from their usual patterns starting from 72 h until a few hours before the rock-ice avalanche event. The average 2 m air and surface temperatures near the avalanche site during the 48 h before the event were found to be much lower than the average temperatures post-event. In-situ observations and the ERA5-Land dataset also confirm these findings. The total turbulent heat flux mostly remained downward (negative) in the 72 h before the event and was found to have an exceptionally large negative value a few hours before the rock-ice avalanche event. The model-simulated rainfall and Global Precipitation Measurement (GPM, IMERG)-derived rainfall suggest that the part of the Himalayan region falling in the simulation domain received a significant amount of rainfall on 4 February, around 48 h prior to the event, while the rest of the days pre- and post-event were mostly dry. The results presented here might be helpful in further studies to identify the possible trigger factors of this event.

Surface Heat and Moisture Exchange in the Marginal Ice Zone: Observations and a New Parameterization Scheme for Weather and Climate Models

AbstractAircraft observations from two Arctic field campaigns are used to characterize and model surface heat and moisture exchange over the marginal ice zone (MIZ). We show that the surface roughness lengths for heat and moisture over uninterrupted sea ice vary with roughness Reynolds number (; itself a function of the roughness length for momentum, , and surface wind stress), with a peak at the transition between aerodynamically smooth (<0.135) and aerodynamically rough (>2.5) regimes. A pre‐existing theoretical model based on surface‐renewal theory accurately reproduces this peak, in contrast to the simple parameterizations currently employed in two state‐of‐the‐art numerical weather prediction models, which are insensitive to . We propose a new, simple parameterization for surface exchange over the MIZ that blends this theoretical model for sea ice with surface exchange over water as a function of sea ice concentration. In offline tests, this new scheme performs much better than the existing schemes for the rough conditions observed during the ‘Iceland Greenland Seas Project’ field campaign. The bias in total turbulent heat flux across the MIZ is reduced to only 13 W m−2 for the new scheme, from 48 and 80 W m−2 for the Met Office Unified Model and ECMWF Integrated Forecast System schemes, respectively. It also performs marginally better for the comparatively smooth conditions observed during the ‘Aerosol‐Cloud Coupling and Climate Interactions in the Arctic’ field campaign. The new surface exchange scheme has the benefit of being physically‐motivated, comparatively accurate and straightforward to implement, although to reap the full benefits an improvement to the representation of sea ice topography via is required.

Quasi-linear heat transport induced by ITG turbulence in the presence of impurities

The impurity effects on turbulent transport induced by ion temperature gradient (ITG) turbulence are numerically studied in tokamak plasmas, using a gyrokinetic quasi-linear model. The characteristics of the instability and heat fluxes in the presence of impurity ions are investigated for a broad parameter regime, including temperature and density gradients of main and impurity ions, concentration, charge and mass numbers of impurity ions, magnetic shear as well as wave vector spectrum. The heat fluxes are demonstrated to depend not only on the saturation amplitude of the instability but also on the phase shift between and . The peaking factor of temperature/density profile, defined as the ratio of major radius to gradient scale length when the total turbulent heat flux equals zero, is fitted with linear/quadratic functions. In addition, the contributions from diagonal and off-diagonal terms to heat fluxes are identified in detail, i.e. the main ion heat diffusion are proved to be dominated by off-diagonal (diagonal) terms for regions of weak (strong) ITG. In general, steep temperature gradients of main ions as well as hollow density profiles of impurity ions significantly enhance instability and heat fluxes. However, it is interesting to find that the effect of impurity ions with positive density gradient may transit from the enhancement to reduction of the quasi-linear heat flux of main ions in regions of steep ITG, corresponding to transport barriers (e.g. pedestal of H-mode and I-mode plasmas). Both strong and weak positive magnetic shear decrease heat transport.

Retrospective Analysis of Micrometeorological Observations Above an Australian Wheat Crop

We apply well-established flux–gradient relationships to deduce the aerodynamic and radiative properties of a winter wheat crop, using a neglected 1971 dataset (hourly averages), only recently resurrected as part of a historical review of precision CO2 measurements in Australia (Pearman et al. in Hist Rec Aust Sci 28:111–125, 2017). The aerodynamic roughness length (seasonal variation between 0.07 and 0.14 of the mean crop height) and broadband albedo (seasonal variation between 0.13 and 0.23) are consistent with values published in the literature over the past 50 years. Net radiation at night is found to agree with the net longwave flux only when the dry-bulb temperature exceeds 10 °C, probably the result of dewfall on one or both of the two instruments. During the day, the sum of the four individual radiative flux components (upwards and downwards shortwave and longwave)—the composite net radiation—exceeds the directly measured net radiation, from near zero at sunrise to approximately 100 W m−2 at maximum net radiation ≈ 600 W m−2, viz. an underestimate in the directly measured net radiation of close to 15%. Again, this is in line with instrument comparisons made in the USA and Europe 15–25 years ago. A novel approach is used in the analysis of terms in the surface energy budget, viz., normalization of all terms by the downwelling shortwave flux. Normalization reveals, (1) near-normal frequency distributions of both the total turbulent heat flux (sensible plus latent) and the implied total storage (the residual); (2) significant diurnal variations in the total turbulent heat flux, whose standard deviations of individual values about any hourly mean during daytime are reduced significantly on those for either the sensible or latent heat flux; (3) an implied storage term with a well-defined diurnal variation, but with an overall mean value of 1% of the shortwave input. Overall, with the above results in mind, the computed momentum and heat fluxes (and also the CO2 flux) during the daytime, at small to moderate gradient Richardson numbers, provide support for the profile approach when eddy-correlation fluxes are unavailable. Even so, possible errors due to, (1) uncertainties in the zero-plane displacement, and (2) influences of the roughness sublayer, must be borne in mind.

Eddy covariance measurements of turbulent fluxes in the surf zone

Turbulent eddies play a critical role in oceanic flows. Direct measurements of turbulent eddy fluxes beneath the sea surface were taken to study the direction of flux-carrying eddies as a means of supplementing our understanding of vertical fluxes exchange processes and their relationship to tides. The observations were made at 32 Hz at a water depth of ~1.5 m near the coast of Sanya, China, using an eddy covariance system, which mainly consists of an acoustic doppler velocimeter (ADV) and a fast temperature sensor. The cospectra-fit method—an established semi-empirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves—was used in the presence of surface gravity waves to quantify the turbulent eddy fluxes (including turbulent heat flux and Reynolds stress). As much as 87% of the total turbulent stress and 88% of the total turbulent heat flux were determined as being at band frequencies below those of surface gravity waves. Both the turbulent heat flux and Reynolds stress showed a daily successive variation; the former peaked during the low tide period and the later peaked during the ebb tide period. Estimation of roll-off wavenumbers, k0, and roll-off wavelengths, λ0 (where λ0=2π/k0), which were estimated as the horizontal length scales of the dominant flux-carrying turbulent eddies, indicated that the λ0 of the turbulent heat flux was approximately double that of the Reynolds stress. Wavelet analysis showed that both the turbulent heat flux and the Reynolds stress have a close relationship to the semi-diurnal and diurnal tides, and therefore indicate the energy that is transported from tides to turulence.

Land‐atmosphere coupling and climate prediction over the U.S. Southern Great Plains

AbstractBiases in land‐atmosphere coupling in climate models can contribute to climate prediction biases, but land models are rarely evaluated in the context of this coupling. We tested land‐atmosphere coupling and explored effects of land surface parameterizations on climate prediction in a single‐column version of the National Center for Atmospheric Research Community Earth System Model (CESM1.2.2) and an off‐line Community Land Model (CLM4.5). The correlation between leaf area index (LAI) and surface evaporative fraction (ratio of latent to total turbulent heat flux) was substantially underpredicted compared to observations in the U.S. Southern Great Plains, while the correlation between soil moisture and evaporative fraction was overpredicted by CLM4.5. To estimate the impacts of these errors on climate prediction, we modified CLM4.5 by prescribing observed LAI, increasing soil resistance to evaporation, increasing minimum stomatal conductance, and increasing leaf reflectance. The modifications improved the predicted soil moisture‐evaporative fraction (EF) and LAI‐EF correlations in off‐line CLM4.5 and reduced the root‐mean‐square error in summer 2 m air temperature and precipitation in the coupled model. The modifications had the largest effect on prediction during a drought in summer 2006, when a warm bias in daytime 2 m air temperature was reduced from +6°C to a smaller cold bias of −1.3°C, and a corresponding dry bias in precipitation was reduced from −111 mm to −23 mm. The role of vegetation in droughts and heat waves is underpredicted in CESM1.2.2, and improvements in land surface models can improve prediction of climate extremes.

Meteorological buoy observations from the central Iceland Sea

AbstractWe present the first continuous in situ atmospheric observations from the central Iceland Sea collected from a meteorological buoy deployed for a 2 year period between 23 November 2007 and 21 August 2009. We use these observations to evaluate the ERA‐Interim reanalysis product and demonstrate that it represented low‐level meteorological fields and surface turbulent fluxes in this region very well. The buoy observations showed that moderate to strong winds were common from any direction, while wind speeds below 5 ms−1 were relatively rare. The observed low‐level air temperature and surface heat fluxes were related to the wind direction with cold‐air outbreaks most common from the northwest. Mean wintertime turbulent heat fluxes were modest (<60 Wm−2), but the range was substantial. High heat flux events, greater than 200 Wm−2, typically occurred every 1–2 weeks in the winter, with each event lasting on average 2.5 days with an average total turbulent heat flux of ∼200 Wm−2 out of the ocean. The most pronounced high heat flux events over the central Iceland Sea were associated with cold‐air outbreaks from the north and west forced by a deep Lofoten Low over the Norwegian Sea.

A spatially advancing turbulent flow and heat transfer in a curved channel

AbstractDirect numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two‐dimensional curved channel, where one wall was heated to a constant temperature and the other wall was cooled to a different constant temperature. In the simulation, fully developed flow and temperature from the straight‐channel driver was passed through the inlet of the curved‐channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, the thermal features are mainly targeted in this paper. The turbulent heat flux showed trends consistent with a growing process of large‐scale vortices. In the curved part, the wall‐normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large‐scale vortices. In the inner side of the same section, temperature fluctuation was abnormally large compared with the modest fluctuation of the wall‐normal velocity. This was caused by the combined effect of the large‐scale motion of the vortices and the wide variation of the mean temperature; in such a temperature distribution, large‐scale ejection of the hot fluid near the outer wall, which is transported into the near inner‐wall region, should have a large impact on the thermal boundary layer near the inner wall. Wave number decomposition was conducted for various statistics, which showed that the contribution of the large‐scale vortex to the total turbulent heat flux normal to the wall reached roughly 80% inside the channel 135° downstream from the curved‐channel inlet. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20275

Characteristics of the surface heat budget during the ice-growth season in the southern Sea of Okhotsk

AbstractThe heat budget over the ice-covered area of the southern Sea of Okhotsk is estimated from in situ meteorological and ice observation for four years, 1996−99. The data are from about 1 week in early February in each of four years. Ice-thickness distributions required for calculating the heat budget are quantitatively obtained from video analysis. A one-dimensional thermodynamical model is used to calculate the heat flux. The total heat flux is obtained by summing up the area-weighted heat flux of each ice-thickness category. In addition, to determine the characteristics of the heat budget in this region, we also calculated the heat budget in the northern Sea of Okhotsk using Special Sensor Microwave/Imager ice-extent data and European Centre for Medium-range Weather Forecasts meteorological data, and compared the results. Our investigations show the following characteristics in the southern Sea of Okhotsk: (1) Due to relatively thin ice thickness, the average turbulent heat flux is upward. (2) Thin ice and open water contribute significantly to the total turbulent heat flux. (3) Thermodynamic ice growth is limited to about 1 cm d−1 on average. (4) The heat budget is largely characterized by abundant solar radiation. The first, third and fourth results are characteristic of this region located at a relatively low latitude, while the second one is similar to that for polar regions.

オホーツク海南部の海氷域における熱収支の見積もり

From in-situ meteorological and ice observation in early February of 1996 and 1997, we estimate the heat budget in the ice cover of the southern Okhotsk Sea. Ice concentration and ice thickness distribution required for its calculation are obtained quantitatively from video analysis. One dimensional-thermodynamical model is used to calculate heat flux. The total heat flux is obtained by summing up the areally weighted heat flux on each ice thickness. As a result, the following characteristics are found in this region. (1) Due to relatively thin ice thickness, turbulent heat flux (sensible + latent) is upward, showing that the sea ice region gives turbulent heat to atmosphere. (2) Thin ice and open area contribute significantly to the total turbulent heat flux over the whole area. (3) Thermodynamical ice growth is limited to below 1 cm per day on average. The first and third results are the characteristics of this area located at a relatively low latitude, while the second one is generally observed for polar regions. The results also suggest that surface melting occur at daytime mainly due to solar radiation. The salinity of sea ice samples shows the values lower than that of the first-year ice in the polar regions and rather similar to that of the multi-year ice, which is lowered by surface melting during summer; this may support this suggestion.

Derivation of ocean—Atmosphere heat fluxes in a tropical environment using satellite and surface data

AbstractOcean—atmosphere heat fluxes measured at John Brewer Reef (80 km north‐north‐east of Townsville, Australia) were compared with regional heat fluxes estimated from non‐direct surface data and information from a number of satellite sensors. Data from the visible channel of the Japanese Geostationary Meteorological Satellite (CMS) were collected for the location, as well as daily atmospheric soundings from NOAA‐9. Sea‐surface temperatures were obtained from a blend of ship observations and NOAA‐9 data. A relationship was developed that estimated solar radiation from GMS visible reflectance data. Incoming longwave radiation was estimated with the LOWTRAN 6 radiative transfer model applied to profiles of temperature and humidity derived from NOAA‐9 soundings. The bulk aerodynamic formulation was used to form regional estimates of the turbulent heat fluxes, combining wind speed from the Darwin Tropical Wind Analysis, blended sea‐surface temperatures, and near‐surface specific humidity related to satellite‐derived precipitable water vapour. A variety of approaches for estimating the near‐surface specific humidity were examined. The root mean square differences (measured—estimated) between the weekly averaged values of the net solar radiation flux, the net longwave radiation flux, the total turbulent heat flux, and the total ocean—atmosphere heat flux were estimated as 12, 10, 45 and 50 Wm−2 respectively. Time series of the weekly heat fluxes are presented for the period June 1985 to April 1986.

Ordered motion of turbulence in a thermally stratified flow under unstable conditions

Measurements of the streamwise and vertical velocity fluctuations and temperature fluctuation in a thermally stratified flow in an open channel under unstable conditions were made with two laser Doppler velocimeters and a film probe operated as a resistance thermometer. The result indicates that the contribution to the total turbulent momentum flux from the sweep motion of large amplitude becomes greatest in the free surface region, while that from the ejection motion remains largest in the wall region. The ejection motion makes the largest contribution to the total turbulent heat transport over the whole measurement range from the wall region to the free surface region for both weakly and strongly unstable conditions. The outward interaction motion makes increasing contribution to the total turbulent momentum flux as well as to the total turbulent heat flux and is closely connected with the ejection motion under the strongly unstable conditions. The mean durations and the mean periods of occurrences of the respective motions decrease with increasing instability.