Modeling subcanopy incoming longwave radiation to seasonal snow using air and tree trunk temperatures

Webster, Clare; Rutter, Nick; Zahner, Franziska; Jonas, Tobias

doi:10.1002/2015jd024099

Cited by 42 publications

(55 citation statements)

References 31 publications

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“…; Webster et al . ). In addition, evaporation though the bark is further limited because birch bark is relatively impermeable to water vapour in comparison to other species (Geurten ).…”

Section: Discussionmentioning

confidence: 97%

Water relations in silver birch during springtime: How is sap pressurised?

et al. 2018

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Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known. We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non-structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period. The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations. Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

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“…; Webster et al . ). In addition, evaporation though the bark is further limited because birch bark is relatively impermeable to water vapour in comparison to other species (Geurten ).…”

Section: Discussionmentioning

confidence: 97%

Water relations in silver birch during springtime: How is sap pressurised?

et al. 2018

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“…During full snow cover conditions, measured net positive downward LWR fluxes during springtime were of similar magnitude (50–60 W/m 2 ) to previous studies (Musselman & Pomeroy, ; Webster et al, ). Magnitudes of peak subcanopy incoming longwave radiation further increased throughout the melt period due to amplified air and canopy temperatures, which supports results by Webster et al (). The cable car system further allowed quantification of snow cover extent over a spatially distributed extent and related it to outgoing longwave radiation.…”

Section: Discussionmentioning

confidence: 99%

Shading by Trees and Fractional Snow Cover Control the Subcanopy Radiation Budget

et al. 2019

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Radiative processes are substantially altered by the presence of forest canopies, further affecting snow energetics during wintertime. In situ measurements of subcanopy radiation can help improve process‐scale understanding of these complex interactions, which are needed to further constrain and improve land surface models. In this study, a custom‐made cable car was used to measure incoming and outgoing, shortwave and longwave radiation below an evergreen forest stand. Hemispherical photographs taken concurrently from the cable car measured view fractions of shaded snow, sunlit snow, and bare ground. With this setup it was possible to quantify diurnal and seasonal radiation patterns together with their potential drivers at high spatiotemporal resolution. Measurements were performed between January and May 2018, along a 48‐m transect in a discontinuous needleleaf forest in the Swiss Alps. Analysis of diurnal radiation patterns revealed a strong linear relationship (R = 0.94) between outgoing shortwave radiation and sunlit snow‐view fraction, highlighting shading as the main control on the subcanopy shortwave radiation budget. Measurements of outgoing longwave radiation were strongly controlled by the snow cover extent, with locations of diminished snow cover showing an increase in outgoing longwave radiation of up to 60 W/m2. The subcanopy radiation budget was shown to be dominated by shortwave radiation when surrounding canopy structure and the position of the sun allowed for direct insolation of the forest floor, but longwave radiation was the dominating component in the absence of direct insolation.

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“…These vertical differences in canopy surface temperatures in the south and east further demonstrate the canopy acting as a cold sink due to the limited solar heating of both the air and canopy, i.e., leading to temperatures in the lower canopy that are cooler than in the upper canopy, shown by Link and Marks [] and in the air temperature data in Webster et al . [].…”

Section: Discussionmentioning

confidence: 99%

“…[] and Webster et al . []. Subsequently, air temperature is an incorrect representation of canopy surface temperatures within equation in these Sun‐lit environments under high solar angles.…”

Section: Discussionmentioning

confidence: 99%

“…These incoming longwave radiation processes, however, are relatively difficult to predict in discontinuous environments compared to shortwave radiation. Longwave radiation in sun‐lit areas of forests has been measured up to 50 W m −2 higher than measurements in the shaded edges of gaps, above the canopy or in adjacent open areas [ Lawler and Link , ; Webster et al ., ]. Forest gaps therefore represent areas with maximum and minimum incoming longwave radiation to the snow surface, which are strongly dependent on direction of forest exposure in relation to the solar path.…”

Section: Introductionmentioning

confidence: 99%

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Improving representation of canopy temperatures for modeling subcanopy incoming longwave radiation to the snow surface

Webster

Rutter

Jonas³

2017

JGR Atmospheres

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A comprehensive analysis of canopy surface temperatures was conducted around a small and large gap at a forested alpine site in the Swiss Alps during the 2015 and 2016 snowmelt seasons (March–April). Canopy surface temperatures within the small gap were within 2–3°C of measured reference air temperature. Vertical and horizontal variations in canopy surface temperatures were greatest around the large gap, varying up to 18°C above measured reference air temperature during clear‐sky days. Nighttime canopy surface temperatures around the study site were up to 3°C cooler than reference air temperature. These measurements were used to develop a simple parameterization for correcting reference air temperature for elevated canopy surface temperatures during (1) nighttime conditions (subcanopy shortwave radiation is 0 W m−2) and (2) periods of increased subcanopy shortwave radiation >400 W m−2 representing penetration of shortwave radiation through the canopy. Subcanopy shortwave and longwave radiation collected at a single point in the subcanopy over a 24 h clear‐sky period was used to calculate a nighttime bulk offset of 3°C for scenario 1 and develop a multiple linear regression model for scenario 2 using reference air temperature and subcanopy shortwave radiation to predict canopy surface temperature with a root‐mean‐square error (RMSE) of 0.7°C. Outside of these two scenarios, reference air temperature was used to predict subcanopy incoming longwave radiation. Modeling at 20 radiometer locations throughout two snowmelt seasons using these parameterizations reduced the mean bias and RMSE to below 10 W m s−2 at all locations.

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Modeling subcanopy incoming longwave radiation to seasonal snow using air and tree trunk temperatures

Cited by 42 publications

References 31 publications

Water relations in silver birch during springtime: How is sap pressurised?

Water relations in silver birch during springtime: How is sap pressurised?

Shading by Trees and Fractional Snow Cover Control the Subcanopy Radiation Budget

Improving representation of canopy temperatures for modeling subcanopy incoming longwave radiation to the snow surface

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