Surface Turbulent Energy Research Articles

Comparison of turbulent structures and energy fluxes over exposed and debris-covered glacier ice

AbstractWe present the first direct comparison of turbulence conditions measured simultaneously over exposed ice and a 0.08 m thick supraglacial debris cover on Suldenferner, a small glacier in the Italian Alps. Surface roughness, sensible heat fluxes (~20–50 W m−2), latent heat fluxes (~2–10 W m−2), topology and scale of turbulence are similar over both glacier surface types during katabatic and synoptically disturbed conditions. Exceptions are sunny days when buoyant convection becomes significant over debris-covered ice (sensible heat flux ~ −100 W m−2; latent heat flux ~ −30 W m−2) and prevailing katabatic conditions are rapidly broken down even over this thin debris cover. The similarity in turbulent properties implies that both surface types can be treated the same in terms of boundary layer similarity theory. The differences in turbulence between the two surface types on this glacier are dominated by the radiative and thermal contrasts, thus during sunny days debris cover alters both the local surface turbulent energy fluxes and the glacier component of valley circulation. These variations under different flow conditions should be accounted for when distributing temperature fields for modeling applications over partially debris-covered glaciers.

On the Increasing Importance of Air-Sea Exchanges in a Thawing Arctic: A Review

Forty years ago, climate scientists predicted the Arctic to be one of Earth’s most sensitive climate regions and thus extremely vulnerable to increased CO2. The rapid and unprecedented changes observed in the Arctic confirm this prediction. Especially significant, observed sea ice loss is altering the exchange of mass, energy, and momentum between the Arctic Ocean and atmosphere. As an important component of air–sea exchange, surface turbulent fluxes are controlled by vertical gradients of temperature and humidity between the surface and atmosphere, wind speed, and surface roughness, indicating that they respond to other forcing mechanisms such as atmospheric advection, ocean mixing, and radiative flux changes. The exchange of energy between the atmosphere and surface via surface turbulent fluxes in turn feeds back on the Arctic surface energy budget, sea ice, clouds, boundary layer temperature and humidity, and atmospheric and oceanic circulations. Understanding and attributing variability and trends in surface turbulent fluxes is important because they influence the magnitude of Arctic climate change, sea ice cover variability, and the atmospheric circulation response to increased CO2. This paper reviews current knowledge of Arctic Ocean surface turbulent fluxes and their effects on climate. We conclude that Arctic Ocean surface turbulent fluxes are having an increasingly consequential influence on Arctic climate variability in response to strong regional trends in the air-surface temperature contrast related to the changing character of the Arctic sea ice cover. Arctic Ocean surface turbulent energy exchanges are not smooth and steady but rather irregular and episodic, and consideration of the episodic nature of surface turbulent fluxes is essential for improving Arctic climate projections.

Evaluating Land–Atmosphere Coupling Using a Resistance Pathway Framework

Abstract This paper presents a methodology for examining land–atmosphere coupling in a regional climate model by examining how the resistances to moisture transfer from the land to the atmosphere control the surface turbulent energy fluxes. Perturbations were applied individually to the aerodynamic resistance from the soil surface to the displacement height, the aerodynamic resistance from the displacement height to the reference level, the stomatal resistance, and the leaf boundary layer resistance. Only perturbations to the aerodynamic resistance from the soil surface to the displacement height systematically affected 2-m air temperature for the shrub and evergreen boreal forest plant functional types (PFTs). This was associated with this resistance systematically increasing the terrestrial and atmospheric components of the land–atmosphere coupling strength through changes in the partitioning of the surface energy balance. Perturbing the other resistances did contribute to changing the partitioning of the surface energy balance but did not lead to systematic changes in the 2-m air temperature. The results suggest that land–atmosphere coupling in the modeling system presented here acts mostly through the aerodynamic resistance from the soil surface to the displacement height, which is a function of both the friction velocity and vegetation height and cover. The results show that a resistance pathway framework can be used to examine how changes in the resistances affect the partitioning of the surface energy balance and how this subsequently influences surface climate through land–atmosphere coupling. Limitations in the present analysis include grid-scale rather than PFT-scale analysis, the exclusion of resistance dependencies, and the linearity assumption of how temperature responds to a resistance perturbation.

Uncertainties in Estimating Normalized Difference Temperature Index From TOA Radiances

The widely used surface temperature/vegetation index ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T_{s}$</tex> </formula> /normalized difference vegetation index) triangle method provides an effective way to estimate surface turbulent energy fluxes and soil moisture. This type of method mainly relies on the normalized difference temperature index (NDTI), which is usually calculated from land surface temperature (LST). However, retrieval of LST from remote sensing data requires atmospheric correction procedures, which are often difficult and troublesome. Our study investigates the feasibility of determining NDTI using top-of-the-atmosphere (TOA) radiances, instead of satellite-derived LST. A thorough assessment of the uncertainties in NDTI estimates for different atmospheric and surface conditions is performed. It is shown that NDTI can be estimated from TOA radiances with an accuracy of 90% if the spatial variabilities of atmospheric parameters (water vapor and effective atmospheric temperature) and surface emissivity are below 10%, 4 K, and 0.05, respectively. A test study is performed using Moderate Resolution Imaging Spectroradiometer data over a heterogeneous area of the Poyang Lake basin of China for six consecutive image acquisitions. When the spatial variations of the surface emissivity, effective atmospheric temperature, and water vapor are less than 0.01, 1 K, and 0.2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{g}\cdot\hbox{cm}^{-2}$</tex> </formula> , respectively, the TOA-radiance-calculated NDTI value and LST-determined NDTI value are quite close with root-mean-square deviation values and biases varying from 0.033 to 0.051 and from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $-$</tex></formula> 0.004 to 0.014, respectively. The high coefficient of determination <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(R^{2})$</tex></formula> values, ranging from 0.904 to 0.939, indicated that the use of TOA radiances appears to be adequate for calculating NDTI in these studies. Overall, the proposed algorithm requires less a priori information on the atmospheric state while providing NDTI estimates at a similar level of accuracy than obtained using atmospherically corrected LST data products. It therefore provides a useful alternative for determining NDTI from satellite data.

The vertical distribution of climate forcings and feedbacks from the surface to top of atmosphere

The radiative forcings and feedbacks that determine Earth’s climate sensitivity are typically defined at the top-of-atmosphere (TOA) or tropopause, yet climate sensitivity itself refers to a change in temperature at the surface. In this paper, we describe how TOA radiative perturbations translate into surface temperature changes. It is shown using first principles that radiation changes at the TOA can be equated with the change in energy stored by the oceans and land surface. This ocean and land heat uptake in turn involves an adjustment of the surface radiative and non-radiative energy fluxes, with the latter being comprised of the turbulent exchange of latent and sensible heat between the surface and atmosphere. We employ the radiative kernel technique to decompose TOA radiative feedbacks in the IPCC Fourth Assessment Report climate models into components associated with changes in radiative heating of the atmosphere and of the surface. (We consider the equilibrium response of atmosphere-mixed layer ocean models subjected to an instantaneous doubling of atmospheric CO2). It is shown that most feedbacks, i.e., the temperature, water vapor and cloud feedbacks, (as well as CO2 forcing) affect primarily the turbulent energy exchange at the surface rather than the radiative energy exchange. Specifically, the temperature feedback increases the surface turbulent (radiative) energy loss by 2.87 W m−2 K−1 (0.60 W m−2 K−1) in the multimodel mean; the water vapor feedback decreases the surface turbulent energy loss by 1.07 W m−2 K−1 and increases the surface radiative heating by 0.89 W m−2 K−1; and the cloud feedback decreases both the turbulent energy loss and the radiative heating at the surface by 0.43 and 0.24 W m−2 K−1, respectively. Since changes to the surface turbulent energy exchange are dominated in the global mean sense by changes in surface evaporation, these results serve to highlight the fundamental importance of the global water cycle to Earth’s climate sensitivity.

On the Persistence of Cold-Season SST Anomalies Associated with the Annular Modes

Abstract In this study, a simple stochastic climate model is used to examine the impact of the ocean mixed layer depth, surface turbulent energy fluxes, and Ekman currents on the persistence of cold-season extratropical sea surface temperature (SST) anomalies associated with variability in the annular modes of atmospheric circulation in both hemispheres. Observational analysis reveals that during the cold season, SST anomalies associated with the southern annular mode (SSTSAM) persist considerably longer than those associated with the northern annular mode (SSTNAM). Using the simple model, it is shown that the persistence of the cold-season SSTSAM is consistent with the simple stochastic climate paradigm in which the atmospheric forcing is approximated as white noise, and the persistence of SST anomalies can be largely determined by the thermal inertia of the ocean mixed layer. In the North Atlantic, however, the simple climate model overestimates the persistence of the cold-season SSTNAM. It is thought that this overestimate occurs because the NAM-related heat flux forcing cannot be described purely as white noise but must also include a feedback from the underlying SST anomalies.

Interannual changes in mass consistent energy budgets from ERA‐Interim and satellite data

Diagnostic comparisons and evaluations of the global atmospheric energy and water budgets have been carried out with the new ERA‐Interim reanalysis product and with satellite‐derived fluxes. In the first part of this paper, we address time discretization, mass imbalance, and spin‐up effects, which affect diagnostics from analysis and forecast fields in reanalysis. We discuss two ways of calculating vertically integrated horizontal energy and moisture flux divergence: (1) direct computation using analyzed fields and (2) indirectly from forecast model fluxes and vertically integrated tendencies. The spatial divergence patterns from the indirect method are found to be more realistic than those from direct computation from 6‐hourly analyses since they are much less affected by temporal sampling errors, but corrections were nevertheless necessary to ensure mass consistency. The second part of this paper is focused on the interannual variability of ERA‐Interim vertical fluxes and on comparisons with satellite flux data. Time series of vertically integrated energy/moisture tendencies and the corresponding analysis increments are also investigated, since shifts in these important quantities indicate artificial jumps in ERA‐Interim, and relationships with vertical fluxes indicate internal consistency of the budgets. All data agree in that the most prominent changes in global budgets are internally forced (El Niño–Southern Oscillation). Broad agreement is found in ERA‐Interim and Hamburg ocean atmosphere parameters and fluxes from satellite data (HOAPS‐3) on a trend in the surface turbulent energy fluxes. Some of this trend may be caused by artificial shifts in ERA‐Interim and especially HOAPS‐3 in 1992 and possibly 2007. A strong increase in tropical land precipitation during the past decade is found in ERA‐Interim and Global Precipitation Climatology Project 2. While large improvements have been made in terms of temporal homogeneity compared with ERA‐40, especially with regard to precipitation, the homogeneity issue still needs to be carefully addressed when interpreting surface flux data from reanalyses or other sources.

Decadal variations of global energy and ocean heat budget and meridional energy transports inferred from recent global data sets

We use the most recent global, decades‐long data sets, consisting of two satellite‐derived top‐of‐atmosphere (TOA) and surface radiative flux data sets from the International Satellite Cloud Climatology Project Flux product (ISCCP‐FD) and the Global Energy and Water Cycle Experiment Surface Radiation Budget project (GEWEX‐SRB), three ocean surface turbulent flux data sets from Goddard Satellite‐based Surface Turbulent Fluxes (GSSTF), Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite data (HOAPS) and Woods Hole Oceanographic Institution Objectively Analyzed air‐sea Fluxes (WHOI) and one ocean heat content (or energy storage rate) data set from Willis et al. to investigate what can be learned about how decadal‐scale variations of the global energy budget at TOA are partitioned between the atmosphere and ocean and their mean meridional heat transports. Although the mean differences among the TOA radiative flux data sets are large enough that direct measurements of planetary energy imbalances are still unreliable, comparison of the interannual anomalies of the ocean heat content with the two (satellite‐derived) planetary energy imbalances converted to accumulated ocean heat content (or equivalently comparison of the anomalies of ocean heat content converted to ocean heat storage rate with the planetary energy imbalances) show excellent quantitative agreement. These data sets essentially show a heating of the upper ocean since the 1990s but different “trends” at the beginning of this century that need further investigation. The comparison of interannual anomalies of total ocean surface energy fluxes converted to accumulated ocean heat content do not show such good agreement, the former generally indicating a cooling over the past decade. The fact that the anomalies in surface net radiative heating are slightly too large suggests that the latent heat flux anomalies are also too large (causing an overall cooling). The interannual anomalies of the mean meridional heat transport by the atmosphere‐ocean system inferred from the two TOA radiative flux data sets all show similar patterns of weakened poleward transport associated with El Niño events. Each event is different in character. The most interesting difference suggested by the latitudinal patterns is that some heat transport anomalies appear in the atmosphere and some in the ocean, but the current quality of the surface turbulent energy flux data sets precludes confirmation. Although not completely successful, we believe that this analysis indicates that these products are somewhat better than might have been expected and that the goal of further work should now be to reduce their uncertainties enough to diagnose the variations of the coupling of the atmosphere and ocean heat exchanges and transports over decadal timescales.

The Sensitivity of Surface Fluxes to Soil Water Content in Three Land Surface Schemes

Abstract Evaporative fraction (EF; the ratio of latent heat flux to the sum of the latent plus sensible heat fluxes) can be measured in the field to an accuracy of about 10%. In this modeling study, the authors try to determine to what accuracy soil moisture must be known in order to simulate surface energy fluxes within this observational uncertainty and whether there is a firm relationship between the variabilities of soil moisture and surface turbulent energy fluxes. A relationship would provide information for planning the future measurement of soil moisture, the design of field experiments, and points of focus for soil model development. The authors look for relationships in three different land surface schemes using results and ancillary integrations in the Global Soil Wetness Project. It is found that the variation of evaporative fraction as a function of soil moisture is consistent among the models and within subsets of vegetation type. In forested areas, there is high sensitivity of EF to soil mo...