Subsurface Heat Flux Research Articles

Measurement of soil‐surface heat flux with a multi‐needle heat‐pulse probe

SummarySoil‐surface heat flux (G0), an important component of the surface energy balance, is often determined by summing soil heat flux (Gz) at a depth (z) below the surface and the rate of change in soil heat storage (ΔS) in the layer above z. The soil heat flux Gz is commonly measured with passive heat flux plates, but self‐calibrating plates or additional corrections are required to obtain accurate data. In some cases, ΔS is neglected because of the difficulty of monitoring the dynamics of volumetric heat capacity (C), which might lead to erroneous estimates of G0. To overcome these limitations, we introduce the heat‐pulse method for measuring G0 with a multi‐needle heat‐pulse probe (HPP). Soil temperature (T) distribution, thermal conductivity (λ) and C of the 0–52‐mm layer were measured hourly on five consecutive days with an 11‐needle HPP, and Gz at 50‐mm depth (G50) and ΔS of the 0–50‐mm layer (ΔS0–50) were determined by the gradient and calorimetric methods, respectively. Independent measurements of G50 with a self‐calibrating heat flux plate and ΔS0–50 calculated with the de Vries model C were used to evaluate the HPP data. With reliable G50 and ΔS0–50 measurements, the HPP‐based G0 data agreed well with those estimated from the independent method (with a mean absolute difference of 4.5 W m−2). Supporting measurements showed that determining Gz at the 50‐mm depth minimized the likelihood of errors from evaporation below the measurement depth. The multi‐needle HPP provides a reliable way to determine G0 in situ. Additional analysis demonstrated that by reducing the number of needles from 11 to 5, the datalogging requirement was reduced by half and G0 was still determined with acceptable accuracy.HighlightsHeat flux at the soil surface (G0) was monitored by the heat‐pulse technique. A multi‐needle heat‐pulse probe (HPP) was used to measure subsurface soil heat flux (Gz) and heat storage concurrently. Appropriate measurement depths of Gz were determined to minimize the effects of subsurface latent heat sink on G0. A simplified calculation reduced the datalogging requirement of the multi‐needle HPP.

Surface energy budget responses to radiative forcing at Summit, Greenland

Abstract. Greenland Ice Sheet surface temperatures are controlled by an exchange of energy at the surface, which includes radiative, turbulent, and ground heat fluxes. Data collected by multiple projects are leveraged to calculate all surface energy budget (SEB) terms at Summit, Greenland, for the full annual cycle from July 2013 to June 2014 and extend to longer periods for the radiative and turbulent SEB terms. Radiative fluxes are measured directly by a suite of broadband radiometers. Turbulent sensible heat flux is estimated via the bulk aerodynamic and eddy correlation methods, and the turbulent latent heat flux is calculated via a two-level approach using measurements at 10 and 2 m. The subsurface heat flux is calculated using a string of thermistors buried in the snow pack. Extensive quality-control data processing produced a data set in which all terms of the SEB are present 75 % of the full annual cycle, despite the harsh conditions. By including a storage term for a near-surface layer, the SEB is balanced in this data set to within the aggregated uncertainties for the individual terms. November and August case studies illustrate that surface radiative forcing is driven by synoptically forced cloud characteristics, especially by low-level, liquid-bearing clouds. The annual cycle and seasonal diurnal cycles of all SEB components indicate that the non-radiative terms are anticorrelated to changes in the total radiative flux and are hence responding to cloud radiative forcing. Generally, the non-radiative SEB terms and the upwelling longwave radiation component compensate for changes in downwelling radiation, although exact partitioning of energy in the response terms varies with season and near-surface characteristics such as stability and moisture availability. Substantial surface warming from low-level clouds typically leads to a change from a very stable to a weakly stable near-surface regime with no solar radiation or from a weakly stable to neutral/unstable regime with solar radiation. Relationships between forcing terms and responding surface fluxes show that the upwelling longwave radiation produces 65–85 % (50–60 %) of the total response in the winter (summer) and the non-radiative terms compensate for the remaining change in the combined downwelling longwave and net shortwave radiation. Because melt conditions are rarely reached at Summit, these relationships are documented for conditions of surface temperature below 0 °C, with and without solar radiation. This is the first time that forcing and response term relationships have been investigated in detail for the Greenland SEB. These results should both advance understanding of process relationships over the Greenland Ice Sheet and be useful for model validation.

Influences on shallow ground temperatures in high flux thermal systems

Ground temperature measurements are a useful indication of subsurface processes and heat flux, particularly in volcanic and hydrothermal systems, but obtaining reliable data at sufficient resolution can be difficult. Investigators commonly use temperature measurements at 1m depths to minimize land surface boundary impacts; however, these measurements are time-consuming and invasive, limiting the number of points that can be surveyed. Alternatively, shallow ground temperature measurements (≤25cm depth) offer a rapid and minimally-invasive way to collect a large number of observations in a target area. Although this method has obvious appeal, changing atmospheric conditions can impact the observed temperatures, and thus may reasonably be expected to influence interpretations arising from the data. Here we examine the impact of precipitation and changing air temperature on shallow ground temperatures in the vicinity of a group of hot springs located in Yellowstone National Park, Wyoming. We find that the mean, the range, and the skewness of the observed temperatures were decreased by changing atmospheric conditions; however, the model variogram representing data taken after several days of moderate precipitation adequately described the spatial correlation of data taken before precipitation. We therefore conclude that the ability to differentiate between high- and low-flux areas may be somewhat reduced by moderate precipitation and changing atmospheric conditions, but that interpretations made on the basis of characteristics of the inferred variograms are likely to be robust to such perturbations in high heat flux environments.

Antarctic and Southern Ocean Surface Temperatures in CMIP5 Models in the Context of the Surface Energy Budget*

Abstract This study examines the biases, intermodel spread, and intermodel range of surface air temperature (SAT) across the Antarctic ice sheet and Southern Ocean in 26 structurally different climate models. Over the ocean (40°–60°S), an ensemble-mean warm bias peaks in late austral summer concurrently with the peak in the intermodel range of SAT. This warm bias lags a spring–summer positive bias in net surface radiation due to weak shortwave cloud forcing and is gradually reduced during autumn and winter. For the ice sheet, inconsistencies among reanalyses and observational datasets give low confidence in the ensemble-mean bias of SAT, but a small summer warm bias is suggested in comparison with nonreanalysis SAT data. The ensemble mean hides a large intermodel range of SAT, which peaks during the summer insolation maximum. In summer on the ice sheet, the SAT intermodel spread is largely associated with the surface albedo. In winter, models universally exhibit a too-strong deficit in net surface radiation related to the downward longwave radiation, implying that the lower atmosphere is too stable. This radiation deficit is balanced by the transfer of sensible heat toward the surface (which largely explains the intermodel spread in SAT) and by a subsurface heat flux. The winter bias in downward longwave radiation is due to the longwave cloud radiative effect, which the ensemble mean underestimates by a factor of 2. The implications of these results for improving climate simulations over Antarctica and the Southern Ocean are discussed.

Monsoon Mixing Cycles in the Bay of Bengal: A Year-Long Subsurface Mixing Record

Based on the first year-long record of mixing collected in the eastern central Bay of Bengal, we quantify the role that subsurface turbulent heat fluxes play in upper-ocean cooling brought on by southwest (SW) and northeast (NE) monsoons. During the NE (dry, or winter) monsoon, atmospheric and subsurface turbulent heat fluxes each contribute about 50% of the net sea surface cooling. During the SW (wet, or summer) monsoon, the atmospheric heat flux varied widely due to “active” and “break” cycles of the monsoon intraseasonal oscillations, but had a net positive seasonal average. The subsurface turbulent heat flux during the SW monsoon led to surface cooling at rates more than three times greater than those measured during the NE monsoon. Since the seasonally averaged atmospheric heat flux was positive, subsurface mixing accounted for nearly all of the cooling during the SW monsoon. During the transition between the NE and SW monsoons, subsurface heat flux was near zero, and atmospheric heating rapidly warmed the sea surface. Following the SW monsoon, passage of Tropical Storm Hudhud led to O(1) m2 s–1 rates of turbulence diffusivity and strong subsurface heat flux, accounting for roughly half of the 1.4°C surface cooling that occurred over a 60-hour period.

The effect of pavement-watering on subsurface pavement temperatures

Pavement-watering is currently viewed as a potential climate change adaptation and urban heat island mitigation technique. The effects of pavement-watering on pavement temperature measured 5cm deep are presented and discussed. Subsurface temperature measurements could not be used to improve or optimize pavement-watering methods as was seen in previous work on surface temperatures or subsurface pavement heat flux measurements.

Measurements of Net Subsurface Heat Flux in Snow and Ice Media in Dronning Maud Land, Antarctica

Abstract: Sub-surface heat flux plays an important role in the energy balance of snow cover, glaciers and ice sheets, and varies with the density of snow/ice media. In this paper, we report experimental observations of the sub-surface heat flux conducted in different snow and ice media in Antarctica. Experiments were conducted on low density fresh snowpack, wind compacted high density snowpack, and blue ice area in east Dronning Maud Land, Antarctica, in a radius of 10 km from Indian research station Maitri (70°46'03.98" S and 11°41'40.72" E). Direct measurements of net subsurface heat flux were carried out using an experimental setup consisting of heat flux plates. Two heat flux plates along with two temperature sensors were placed horizontally inside the snowpack / ice sheet at a depth of approximately 10 cm from the surface during summer season between December 2011and February 2012. The mean diurnal net subsurface heat fluxes for low density snow (location 1), high density wind compacted snow (location 2), and blue ice area (location 3) observed were 7.36±9.39 W m-2, -0.12±7.6 W m-2 and 3.20±13.32 W m-2, respectively, during the measurements periods. The amplitudes of the mean diurnal variations of net sub-surface heat flux were 14.23 W m-2, 11.42 W m-2 and 18.55 W m-2 at location 1, location 2 and location 3, respectively. Mean net sub-surface heat flux was observed to be comparatively lower for wind compacted high density snow.

Surface ablation model evaluation on a drifting ice island in the Canadian Arctic

A 4-week micro-meteorological dataset was collected by an automatic weather station on a small ice island (0.13km2) adrift off Bylot Island (Lancaster Sound, Nunavut, Canada) during the 2011 melt season. This dataset provided an opportunity to identify the environmental variables and energy fluxes that contribute most to surface ablation during the melt season, as well as test previously developed surface melt (ablation) models. Surface ablation was estimated using energy fluxes calculated using the bulk aerodynamic approach (EBAWS) and three existing surface ablation models. These models included a simple solar radiation model developed for iceberg use (CIS-IB), a more sophisticated energy-balance model developed for ice island use (CIS-II), and a temperature index melt (TIM) model based on an assumed relationship between air temperature, time, and surface ablation. The models were driven by our measured micro-meteorological data (optimal forcing) or regional environmental forecast data from the Global Environmental Multiscale (GEM) Model, which is used for operational iceberg modeling. The sensible heat flux contributed most to the ice surface's available melt energy (47%), followed by net radiation (38%) and the latent heat flux (30%), while the subsurface heat flux removed 15% of available energy. When cumulative surface ablation was predicted with these calculated energy fluxes (EBAWS), observed surface ablation was under-predicted by 38%. Results illustrate the decreased performance of the melt models when run with GEM data versus in-situ micro-meteorological data, which is optimal for model input but not available for operational modeling. The CIS-II model under-predicted cumulative surface ablation by 5.7% (RMSE=1.2cm) with observed micro-meteorological data and over-predicted cumulative surface ablation by 35% when run with GEM model data. This is likely a result of the GEM model wind speed being 57% greater than that recorded on the ice island. Since surface ablation plays a greater relative role in overall deterioration of ice islands than traditional icebergs due to morphological differences (size, surface structure), it must be accurately represented in operational ice island deterioration models. The costs and benefits between parsimonious TIM models and skilled energy-balance models are weighed here for operational modelers to consider, along with the complications caused by the use of the regional environmental data input provided by the GEM model for operational modeling efforts.

Energy and mass balance of Zhadang glacier surface, central Tibetan Plateau

AbstractClimate variables that control the annual cycle of the surface energy and mass balance on Zhadang glacier in the central Tibetan Plateau were examined over a 2 year period using a physically based energy-balance model forced by routine meteorological data. The modelled results agree with measured values of albedo, incoming longwave radiation, surface temperature and surface level of the glacier. For the whole observation period, the radiation component dominated (82%) the total surface energy heat fluxes. This was followed by turbulent sensible (10%) and latent heat (6%) fluxes. Subsurface heat flux represented a very minor proportion (2%) of the total heat flux. The sensitivity of specific mass balance was examined by perturbations of temperature (±1 K), relative humidity (±20%) and precipitation (±20%). The results indicate that the specific mass balance is more sensitive to changes in precipitation than to other variables. The main seasonal variations in the energy balance were in the two radiation components (net shortwave radiation and net longwave radiation) and these controlled whether surface melting occurred. A dramatic difference in summer mass balance between 2010 and 2011 indicates that the glacier surface mass balance was closely related to precipitation seasonality and form (proportion of snowfall and rainfall).

The Surface Energy Budget in the Accumulation Zone of the Laohugou Glacier No. 12 in the Western Qilian Mountains, China, in Summer 2009

The energy balance of a glacial surface can describe physical melting processes. To expand the understanding of how glaciers in arid regions respond to climate change, the energy budget in the accumulation zone of the Laohugou Glacier No. 12 was measured. Input variables were meteorological data (1 June—30 September 2009) from an automatic weather station located on the accumulation zone at 5040 m above sea level (a.s.l.). Radiative fluxes directly measured, and turbulent fluxes calculated using the bulk aerodynamic approach, were involved in the surface energy budget. Net radiation flux was the primary source of the surface energy balance (72%) and was chiefly responsible for glacial melting, followed by sensible heat flux (28%). Melting energy was the main output of surface energy (48%), and was almost as large as the sum of latent heat flux (32%) and subsurface heat flux (20%). The modeled mass balance was -75 mm water equivalent, which compared well with sonic ranging sensor readings. Albedo varied between 0.52 and 0.88 on the glacial surface, and melting was prevented by high albedo. Under the assumption of neutral atmospheric conditions, turbulent fluxes were overestimated, especially the sensible heat flux by 54%; therefore, a stability correction was necessary.

The Diurnal Behavior of Evaporative Fraction in the Soil–Vegetation–Atmospheric Boundary Layer Continuum

AbstractThe components of the land surface energy balance respond to periodic incoming radiation forcing with different amplitude and phase characteristics. Evaporative fraction (EF), the ratio of latent heat to available energy at the land surface, supposedly isolates surface control (soil moisture and vegetation) from radiation and turbulent factors. EF is thus supposed to be a diagnostic of the surface energy balance that is constant or self-preserved during daytime. If this holds, EF can be an effective way to estimate surface characteristics from temperature and energy flux measurements. Evidence for EF diurnal self-preservation is based on limited-duration field measurements. The daytime EF self-preservation using both long-term measurements and a model of the soil–vegetation–atmosphere continuum is reexamined here. It is demonstrated that EF is rarely constant and that its temporal power spectrum is wide; thus emphasizing the role of all diurnal frequencies associated with reduced predictability in its daylight response. Oppositely, surface turbulent heat fluxes are characterized by a strong response to the principal daily frequencies (daily and semi-daily) of the solar radiative forcing. It is shown that the phase lag and bias between the turbulent flux components of the surface energy balance are key to the shape of the daytime EF. Therefore, an understanding of the physical factors that affect the phase lag and bias in the response of the components of the surface energy balance to periodic radiative forcing is needed. A linearized model of the soil–vegetation–atmosphere continuum is used that can be solved in terms of harmonics to explore the physical factors that determine the phase characteristics. The dependency of these phase and offsets on environmental parameters—friction velocity, water availability, solar radiation intensity, relative humidity, and boundary layer entrainment—is then analyzed using the model that solves the dynamics of subsurface and atmospheric boundary layer temperatures and heat fluxes in a continuum. Additionally, the asymptotical diurnal lower limit of EF is derived as a function of these surface parameters and shown to be an important indicator of the self-preservation value when the conditions (also identified) for such behavior are present.

Glacier subsurface heat-flux characterizations for energy-balance modelling in the Donjek Range, southwest Yukon, Canada

AbstractWe apply a point-scale energy-balance model to a small polythermal glacier in the St Elias Mountains of Canada in order to investigate the applicability and limitations of different treatments of the glacier surface temperature and subsurface heat flux. These treatments range in complexity from a multilayer subsurface model that simulates snowpack evolution, to the assumption of a constant glacier surface temperature equal to 0°C. The most sophisticated model includes dry densification of the snowpack, penetration of shortwave radiation into the subsurface, internal melting, refreezing of percolating meltwater and generation of slush layers. Measurements of subsurface temperature and surface lowering are used for model validation, and highlight the importance of including subsurface penetration of shortwave radiation in the model. Using an iterative scheme to solve for the subsurface heat flux as the residual of the energy-balance equation results in an overestimation of total ablation by 18%, while the multilayer subsurface model underestimates ablation by 6%. By comparison, the 0°C surface assumption leads to an overestimation of ablation of 29% in this study where the mean annual air temperature is about −8°C.

Climate of the Greenland ice sheet using a high-resolution climate model – Part 2: Near-surface climate and energy balance

Abstract. The spatial variability of near-surface variables and surface energy balance components over the Greenland ice sheet are presented, using the output of a regional atmospheric climate model for the period 1958–2008. The model was evaluated in Part 1 of this paper. The near-surface temperature over the ice sheet is affected by surface elevation, latitude, longitude, large-scale and small-scale advection, occurrence of summer melt and mesoscale topographical features. The atmospheric boundary layer is characterised by a strong temperature inversion, due to continuous longwave cooling of the surface. In combination with a gently sloping surface the radiative loss maintains a persistent katabatic wind. This radiative heat loss is mainly balanced by turbulent sensible heat transport towards the surface. In summer, the surface is near radiative balance, resulting in lower wind speeds. Absorption of shortwave radiation and a positive subsurface heat flux due to refreezing melt water are heat sources for surface sublimation and melt. The strongest temperature deficits (>13 °C) are found on the northeastern slopes, where the strongest katabatic winds (>9 m s−1) and lowest relative humidity (<65%) occur. Due to strong large scale winds, clear sky (cloud cover <0.5) and a concave surface, a continuous supply of cold dry air is generated, which enhances the katabatic forcing and suppresses subsidence of potentially warmer free atmosphere air.

Comparison of observed and general circulation model derived continental subsurface heat flux in the Northern Hemisphere

Heat fluxes in the continental subsurface were estimated from general circulation model (GCM) simulations of the climate of the last millennium and compared to those obtained from subsurface geothermal data. Since GCMs have bottom boundary conditions (BBCs) that are less than 10 m deep and thus may be thermodynamically restricted in the continental subsurface, we used an idealized land surface model (LSM) with a very deep BBC to estimate the potential for realistic subsurface heat storage in the absence of bottom boundary constraints. Results indicate that there is good agreement between observed fluxes and GCM simulated fluxes for the 1780–1980 period when the GCM simulated temperatures are coupled to the LSM with deep BBC. These results emphasize the importance of placing a deep BBC in GCM soil components for the proper simulation of the overall continental heat budget. In addition, the agreement between the LSM surface fluxes and the borehole temperature reconstructed fluxes lends additional support to the overall quality of the GCM (ECHO‐G) paleoclimatic simulations.

Latent heat in soil heat flux measurements

The surface energy balance includes a term for soil heat flux. Soil heat flux is difficult to measure because it includes conduction and convection heat transfer processes. Accurate representation of soil heat flux is an important consideration in many modeling and measurement applications. Yet, there remains uncertainty about what comprises soil heat flux and how surface and subsurface heat fluxes are linked in energy balance closure. The objective of this study is to demonstrate the presence of a subsurface latent heat sink, which must be considered in order to accurately link subsurface heat fluxes between depths near and at the soil surface. Measurements were performed under effectively bare surface conditions in a silty clay loam soil near Ames, IA. Soil heat flux was measured with heat-pulse sensors using the gradient heat flux approach at 1-, 3-, and 6-cm soil depths. Independent estimates of the daily latent heat sink were obtained by measuring the change of mass of microlysimeters. Heat flux measurements at the 1-cm depth deviated from heat flux measurements at other depths, even after calorimetric adjustment was made. This deviation was most pronounced shortly after rainfall, where the 1-cm soil heat flux measurement exceeded 400 W m −2. Cumulative soil heat flux measurements at the 1-cm depth exceeded measurements at the 3-cm depth by >75% over a 7-day rain-free period, whereas calorimetric adjustment allowed 3- and 6-cm depth measurements to converge. Latent heat sink estimates from the microlysimeters accounted for nearly all of the differences between the 1- and 3-cm depth heat flux measurements, indicating that the latent heat sink was distributed between the 1- and 3-cm depths shortly after the rainfall event. Results demonstrate the importance of including latent heat when attempts are made to link or extrapolate subsurface soil heat flux measurements to the surface soil heat flux.

Forest understory soil temperatures and heat flux calculated using a Fourier model and scaled using a digital camera

The characterization of the solar radiation environment under a forest canopy is important for both understanding temperature-dependent biological processes and validating energy balance models. A modified sinusoidal model of soil heat conductivity was used to estimate subsurface temperature and heat flux from the uneven but periodic solar heating of the soil surface due to sun flecks from a forest canopy. Using a mobile sensor platform with an infrared thermometer along an 11 m transect, a sunfleck model of soil surface temperature was tested using soil surface temperature maxima, air temperatures, and photodiodes placed on the soil surface to measure sunflecks. A pan-tilt-zoom digital camera on a 10 m tower above the site was then used to capture a time series of panoramic images of sunflecks reflected from the soil surface and to scale the sunfleck temperature model to a wide area. Finally, this image-based model of surface temperatures was combined with the modified sinusoidal model for heat conduction to estimate soil subsurface temperatures and heat flux over a wide area due to sunflecks from a forest canopy.

Surface energy budget over the South Pole and turbulent heat fluxes as a function of an empirical bulk Richardson number

Routine radiation and meteorological data at South Pole Station are used to investigate historical discrepancies of up to 50 W m−2 in the monthly mean surface energy budget and to investigate the behavior of turbulent heat fluxes under stable atmospheric temperature conditions. The seasonal cycles of monthly mean net radiation and turbulent heat fluxes are approximately equal, with a difference of 40 W m−2 between summer and winter, while the seasonal cycle of subsurface heat fluxes is only a few W m−2. For an 8‐month period (the winter of 2001), we calculate two estimates of turbulent heat fluxes, one from Monin‐Obukhov (MO) similarity theory and one as the residual of the surface energy budget (i.e., subsurface heat fluxes minus net radiation, where all fluxes toward the snow surface are positive). The turbulent fluxes from MO theory agree well with the residual of the energy budget under lapse conditions. However, under stable conditions MO theory underestimates turbulent fluxes by approximately 40–60%. The relationship between turbulent heat fluxes as a residual of the energy budget, temperature inversion strength, and wind shear as a function of the bulk Richardson number (Rib) is examined under stable conditions (i.e., positive Rib). The Rib used here is calculated from 10‐m wind speeds and 0‐ to 2‐m temperature inversion strength. No critical value of Rib is found where the turbulent heat fluxes drop to zero. However, a threshold (Rib = 0.05) exists below which 70% of the turbulent energy fluxes can be explained by only the temperature inversion strength. For Rib > 0.05, the relationship between turbulent heat fluxes and temperature inversion strength decreases, while the importance of wind shear to turbulent heat transfer increases. Above Rib = 0.05, a growing linear correlation also exists between atmospheric temperature inversion strength and wind shear. Thus, inversion strength and wind shear are not independent predictors of turbulent heat flux for extremely stable conditions. The exact values of the correlation coefficients and Rib threshold are likely specific to the experimental conditions; however, their implications are probably valid for all stable flows. Knowledge of the time‐varying surface characteristics would help to generalize these parameters.

The role of radiation penetration in the energy budget of the snowpack at Summit, Greenland

Abstract. Measurements of the summer surface energy balance at Summit, Greenland, are presented (8 June–20 July 2007). These measurements serve as input to an energy balance model that searches for a surface temperature for which closure of all energy terms is achieved. A good agreement between observed and modelled surface temperatures was found, with an average difference of 0.45°C and an RMSE of 0.85°C. It turns out that penetration of shortwave radiation into the snowpack plays a small but important role in correctly simulating snow temperatures. After 42 days, snow temperatures in the first meter are 3.6–4.0°C higher compared to a model simulation without radiation penetration. Sensitivity experiments show that these results cannot be reproduced by tuning the heat conduction process alone, by varying snow density or snow diffusivity. We compared the two-stream radiation penetration calculations with a sophisticated radiative transfer model and discuss the differences. The average diurnal cycle shows that net shortwave radiation is the largest energy source (diurnal average of +61 W m−2), net longwave radiation the largest energy sink (−42 W m−2). On average, subsurface heat flux, sensible and latent heat fluxes are the remaining, small heat sinks (−5, −5 and −7 W m−2, respectively), although these are more important on a subdaily timescale.

Adaptation of a leaf wetness model to estimate dewfall amount on a roof surface

Most leaf wetness and dew studies are rural, however, there is growing scientific interest in urban dew. This study investigates the feasibility of adapting an existing leaf wetness (dew) model for urban use. The approach predicts dewfall amount for (a) a maple leaf, and (b) an asphalt-shingle roof, using a computed surface energy balance (which includes a roof subsurface heat flux), latent heat fluxes, and an inferred surface water balance. Simulations are run and verified to a first-order using data collected in Vancouver, BC, Canada, in 1996. Too few data are available to adequately test the ‘leaf’ model. The skill of the ‘roof’ model to predict dewfall amount is confirmed by several statistical indices, e.g. a root mean square error of 0.04 mm per night, and Willmott's index of agreement of 0.87. Skill is seen also in the timing of dewfall onset and maxima. It seems feasible to estimate dewfall on an artificial surface, such as a roof, using a ‘leaf wetness’ approach, with suitable modifications. Approaches could readily be applied to other surfaces, and are a first step towards a comprehensive urban dew model.

On the role of subsurface heat conduction in glacier energy-balance modelling

AbstractWe discuss the inclusion of the subsurface heat-conduction flux into the calculation of the energy balance and ablation at the glacier–atmosphere interface. Data from automatic weather stations are used to force an energy-balance model at several locations on alpine glaciers and at one site in the dry Andes of central Chile. The heat-conduction flux is computed using a two-layer scheme, assuming that 36% of the net shortwave radiation is absorbed by the surface layer and that the rest penetrates into the snowpack. We compare simulations conducted with and without subsurface heat flux. Results show that assuming a surface temperature of zero degrees leads to a larger overestimation of melt at the sites in the accumulation area (10.4–13.3%) than in the ablation area (0.5–2.8%), due to lower air temperatures and the presence of snow. The difference between simulations with and without heat conduction is also high at the beginning and end of the ablation season (up to 29% for the first 15 days of the season), when air temperatures are lower and snow covers the glacier surface, while they are of little importance during periods of sustained melt at all the locations investigated.