Continuous measurement of aerodynamic and alfalfa canopy resistances using the Bowen ratio-energy balance and Penman-Monteith methods

To investigate the alfalfa crop response to environmental factors, a Bowen ratio-energy balance method was used to evaluate short-term alfalfa canopy resistance. Continuous evapotranspiration (ET _a ) and the aerodynamic resistance (r _a ) for an alfalfa crop in each 20-min interval were calculated. Using the calculated ET _a and r _a and the Penman-Monteith approach, the bulk stomatal or actual canopy resistance (r _c ) was evaluated. The continuous 20-min resistances were computed for clear and partially cloudy sky conditions, and different average crop heights. The results show that this technique can satisfactorily be used to study the manner in which the aerodynamic and canopy resistances respond to short-term variations in weather elements such as photosynthetically active radiation (PAR), wind speed and atmospheric saturation vapor deficit.Research Assistant Professor and Assistant Utah State Climatologist, Research Associate Professor and Research Assistant, respectively.     

Transpiration and evaporation from heather Moorland

The energy balance of an upland heath dominated by heather (Calluna vulgaris) was measured in dry and wet weather. Median values of both transpiration and evaporation rates were ca. 2 mm hr^-1. The median Bowen ratio for the dry canopy was 2.0 and for the wet canopy 0.6. On dry days the median value of the saturation deficit was only 3.8 mb and that of the climatological resistance was 30 s m^-1. The bulk stomatal resistance increased from ca. 50 s m ^-2 in the morning to over 290 s m^-1 in the afternoon with an overall median value of 110 s m^-1. Transpiration from the dry canopy was controlled by a combination of small saturation deficits and large stomatal resistances. The median value of the boundary-layer resistance of the canopy was 22 s m^-1 and was low partly because of a large low-level drag coefficient. Saturation deficits on wet days were close to zero and evaporation of intercepted water proceeded at close to the equilibrium rate, being largely limited by the low fluxes of available energy. The water loss from heather was compared with simulated losses from coniferous forest, herbaceous crops and grassland in the same conditions to evaluate the effects of vegetation on water loss from catchments.Laboratorio de Ecologia, Dept. de Biologia Vegetal, Universidade de Brasilia, 70 910 — Brasila — DF, Brazil.     

Sulfur dioxide plume dispersion and subsequent absorption by coniferous forests: Field measurements and model calculations

A Forest SO₂ Absorption Model (ForSAM) was developed to simulate (1) SO₂ plume dispersion from an emission source, (2) subsequent SO₂ absorption by coniferous forests growing downwind from the source. There are three modules: (1) a buoyancy module, (2) a dispersion module, and (3) a foliar absorption module. These modules were used to calculate hourly abovecanopy SO₂ concentrations and in-canopy deposition velocities, as well as daily amounts of SO₂ absorbed by the forest canopy for downwind distances to 42 km. Model performance testing was done with meteorological data (including ambient SO₂ concentrations) collected at various locations downwind from a coal-burning power generator at Grand Lake in central New Brunswick, Canada. Annual SO₂ emissions from this facility amounted to about 30,000 tonnes. Calculated SO₂ concentrations were similar to those obtained in the field. Calculated SO₂ deposition velocities generally agreed with published values.Notation c air parcel cooling parameter (non-dimensional) - E foliar absorption quotient (non-dimensional) - f areal fraction of foliage free from water (non-dimensional) - f _w SO₂ content of air parcel - h height of the surface layer (m) - H height of the convective mixing layer (m) - H _stack stack height (m) - k time level - k _drag coefficient of drag on the air parcel (non-dimensional) - K _z eddy viscosity coefficient for SO₂ (m²·s^–1) - L Monin-Obukhov length scale (m) - L _A single-sided leaf area index (LAI) - n degree-of-sky cloudiness (non-dimensional) - N number of parcels released with every puff (non-dimensional) - PAR photosynthetically active radiation (W m^–2) - Q emission rate (kg s^–2) - r _b diffusive boundary-layer resistance (s m^–1) - r _c canopy resistance (s m^–1) - r _cuticle cuticular resistance (s m^–1) - r _m mesophyllic resistance (s m^–1) - r _s stomatal resistance (s m^–1) - r _exit smokestack exit radius (m) - R normally distributed random variable with mean of zero and variance of t (s) - u _* frictional velocity scale, (m s^–1) - v lateral wind vector (m s^–1) - v _d SO₂ dry deposition velocity (m s^–1) - VCD water vapour deficit (mb) - z _can mean tree height (m) - Z zenith position of the sun (deg) - environmental lapse rate (°C m^–1) - dry adiabatic lapse rate (0.00986°C m^–1) - von Kármán's constant (0.04) - _B vertical velocities initiated by buoyancy (m s^–1) - canopy extinction coefficient (non-dimensional) - ()a denotes ambient conditions - ()can denotes conditions at the top of the forest canopy - ()h denotes conditions at the top of the surface layer - ()H denotes conditions at the top of the mixed layer - ()s denotes conditions at the canopy surface - ()p denotes conditions of the air parcels     

Water loss and canopy resistance of a young Sitka spruce plantation

Transpiration of a 7 m-high Stika spruce forest was investigated using measurments of net radiation, sensible heat and ground heat fluxes in an energy balance to give latent heat flux, and hence canopy resistance from the Penman-Monteith equation. Sensible heat flux was measured by the eddy-correlation method using a Fluxatron circuit.During six consecutive days of measurement in July/August, canopy resistance typically followed a decreasing trend from high values (150 s m-) at dawn to around 40 s m^–1 at midday and then returning steadily to > 100 s m^–1 at sunset. Transpiration was 3 mm day^–1 on average over the period studied and changes in the rate within the day were significantly correlated with changes in net radiation.Comparisons are drawn with published data from other forest sites and the conclusion is reached that it is imprudent to generalise about transpiration rates and canopy resistances of different species at different sites from results gathered at one or two places.     

Calculation of sensible and latent heat fluxes,and surface resistance from profile data

A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of net radiation and ground heat flux covering a period of about one and a half years has been analysed to give a corresponding set of complete surface energy balance data on an hourly basis. An analysis of the evaporation data so obtained is given.It is shown that surface resistance r _S exhibits a diurnal trend: values are smallest (ca. 150 s m^-1) a few hours before noon and increase to as much as 800 s m^-1 towards dusk. The minimum values tend to be higher during dry periods when the soil moisture is low. There is also some indication that r _S decreases rapidly soon after rainfall.An exponential relation is found between the fraction of available energy used as evaporative flux, , and r _S for values of r _I/r_S <0.70, where r _I is the climatological resistance. On the other hand, the ratio of r _I to r _S is linearly correlated with , implying that an equilibrium state is established between the grass surface and the atmosphere, at least from mid-morning to mid-afternoon when the leaves are dry. Near-noon values calculated by Stewart and Thom for Thetford Forest also follow a linear trend.The above two regression results (In (r _S) versus r _I/r_S versus ) are combined to obtain an empirical relation of the form r _I=m exp (a-b) which is used to estimate evaporative flux. The estimates are found to be within 20% of calculated values.Guest Scientist from Department of Physics, University of Cape Coast, Cape Coast, Ghana.     

Calibration of a land-surface model using data from primary forest sites in Amazonia

Summary A land-surface model (MOSES) was tested against observed fluxes of heat, water vapour and carbon dioxide for two primary forest sites near Manaus, Brazil. Flux data from one site (called C14) were used to calibrate the model, and data from the other site (called K34) were used to validate the calibrated model. Long-term fluxes of water vapour at C14 and K34 simulated by the uncalibrated model were good, whereas modelled net ecosystem exchange (NEE) was poor. The uncalibrated model persistently underpredicted canopy conductance (g _c ) from mid-morning to mid-afternoon due to saturation of the response to solar radiation at low light levels. This in turn caused a poor simulation of the diurnal cycles of water vapour and carbon fluxes. Calibration of the stomatal conductance/photosynthesis sub-model of MOSES improved the simulated diurnal cycle of g _c and increased the diurnal maximum NEE, but at the expense of degrading long-term water vapour fluxes. Seasonality in observed canopy conductance due to soil moisture change was not captured by the model. Introducing realistic depth-dependent soil parameters decreased the amount of moisture available for transpiration at each depth and led to the model experiencing soil moisture limitation on canopy conductance during the dry season. However, this limitation had only a limited effect on the seasonality in modelled NEE.     

Water vapor transfer from a vegetated surface: A numberical study of bulk transfer coefficients and canopy resistances

The semi-analytical model outlined in previous studies (Massman, 1987a, b) to describe momentum and heat exchange between the atmosphere and vegetated surfaces is extended to include water vapor exchange. The methods employed are based on one-dimensional turbulent diffusivities and use numerical solutions to the steady-state diffusion equation. The model formulates stomatal response as a function of vapor pressure deficit and the within-canopy profile of mean photosynthetically-active radiation (PAR). It is then used to assess the influence that foliage structure, density, and sheltering can have upon the bulk transfer coefficient, kB _v ^-1, and the canopy resistance. A general analytical formulation of the canopy resistance based on the mean within-canopy profile of PAR is proposed and found to agree with the model's solutions for canopy resistance to within a few percent.     

Water Loss and Canopy Resistance of a Young Sitka Spruce Plantation

Transpiration of a 7 m-high Stika spruce forest was investigated using measurements of net radiation, sensible heat and ground heat fluxes in an energy balance to give latent heat flux, and hence canopy resistance from the Penman-Monteith equation. Sensible heat flux was measured by the eddy-correlation method using a Fluxatron circuit. During six consecutive days of measurement in July/August, canopy resistance typically followed a decreasing trend from high values (≈150 s m⁻¹) at dawn to around 40 s m⁻¹ at midday and then returning steadily to >100 s m⁻¹ at sunset. Transpiration was 3 mm day⁻¹ on average over the period studied and changes in the rate within the day were significantly correlated with changes in net radiation. Comparisons are drawn with published data from other forest sites and the conclusion is reached that it is imprudent to generalise about transpiration rates and canopy resistances of different species at different sites from results gathered at one or two places.     

Heat energy exchange over a large water body under stable atmospheric conditions

The surface heat budget over the Riband reservoir covering 300 km² is investigated making use of hydrometeorological data collected at a number of stations during May and June 1983. The observations had to be restricted to 0800–1400 hr for operational reasons in this remote part of India. The winds were weaker, and in general the temperature and humidity gradients were stronger at that time of day than during the afternoon.The mean albedo between 0700–1200 hr is found to be about 34% which could be due to the high turbidity of the water. A simple relation of the form, R = (1 – )Q _i – 85 is proposed to estimate net radiation over the water body from the global radiation. This relation is useful for the computation of net radiation since it avoids the computation of effective back radiation, which requires data on humidity, cloud amount and surface water temperature. The overall means of net radiation, latent and sensible heat fluxes were found to be 420, 96 and -11 W/m², respectively. A net heat gain of about 335 W/m² was observed during the study period. The measured effective back radiation agreed reasonably well with the value computed from the theoretical formula.     

Carbon dioxide,water vapor and sensible heat exchanges of a grain sorghum canopy

With the aid of eddy correlation instrumentation, the components of the energy budget and CO₂ flux were measured over grain sorghum grown at Mead, Nebraska. Diurnal patterns of sensible heat, latent heat, CO₂ and momentum flux are examined for typical days. On a mostly clear day when the crop leaf area index was 3.7, net radiation reached a mid-day peak of 560 W m^-2, while sensible and latent heat fluxes peaked at 50 and 460 W m^-2, respectively. The peak CO₂ flux occurring just prior to solar noon was 1.5 mg m^-2(ground area) s^-1. CO₂ flux (respiration from plants, soil and roots) in the early evening was about -0.28 mg m^-2 (ground area) s^-1.A relationship between CO₂ flux and photosynthetically active radiation (PAR) was developed. Except during the late stage of growth (growth stage 8.5, toward the initiation of senescence), the crop showed no evidence of saturation up to PAR 1800 Ei m^-2s^-1. The light compensation point was found to be about 211 Ei m^-2s^-1. Examination of CO₂ flux-PAR relationships for selected days through the season indicated an aging effect in terms of a decrease in photosynthetic activity of the sorghum canopy. Measurements made on two consecutive days demonstrate the effects of weather conditions on CO₂ flux and carbon-water flux ratio (a measure of water use efficiency of the crop). The occurrence of regional sensible heat advection with concommitant high vapor pressure deficit and air temperature-limited CO₂ exchange reduced the carbon-water flux ratio.Published as Paper No. 7717, Journal Series, Nebraska Agricultural Research Division. The work was conducted under Regional Research Project 11-33 and Nebraska Agricultural Research Division Project 27-003.Formerly Post Doctoral Research Associate (now at the University of Connecticut Department of Renewable Natural Resources Storrs CT)     

Modelling evaporation from wetland tundra

Evapotranspiration is a major component of both the energy and water balances of wetland tundra environments during the thaw season. Reliable estimates of evapotranspiration are required in the analysis of climatological and hydrological processes occurring within a wetland and in interfacing the surface climate with atmospheric processes. Where direct measurements are unavailable, models designed to accurately predict evapotranspiration for a particular wetland are used.This paper evaluates the performance, sensitivity and limitations of three physically-based, one-dimensional models in the simulation of evaporation from a wetland sedge tundra in the Hudson Bay Lowland near Churchill, Manitoba. The surface of the study site consists of near-saturated peat soil with a sparse sedge canopy and a constantly varying coverage of standing water. Measured evaporation used the Bowen ratio energy balance approach, to which the model results were compared. The comparisons were conducted with hourly and daily simulations.The three models are the Penman-Monteith model, the Shuttleworth-Wallace sparse canopy model and a modified Penman-Monteith model which is weighted for surface area of the evaporation sources.Results from the study suggest that the weighted Penman-Monteith model has the highest potential for use as a predictive tool. In all three cases, the importance of accurately measuring the surface area of each evaporation source is recognized. The difficulty in determining a representative surface resistance for each source and the associated problems in modelling without it are discussed.

List of Symbols

Models BREB Bowen ratio energy balance - P-M Penman-Monteith combination - S-W Shuttleworth-Wallace combination - W-P-M Weighted Penman-Monteith combination Other AE Available energy-all surfaces - AE _c Available energy-canopy (S-W, W-P-M) - AE _s Available energy-bare soil (S-W, W-P-M) - AE _w Available energy-open water (W-P-M) - C _p Specific heat of air - D Vapor pressure deficit - DAI Dead area index - FAI Foliage area index - LAI Leaf area index - Q ^* Net radiation - Q _e Latent heat flux-total - Q _ec Latent heat flux-canopy (S-W, W-P-M) - Q _es Latent heat flux-bare soil (S-W, W-P-M) - Q _ew Latent heat flux-open water (W-P-M) - Q _g ground heat flux - Q _h Sensible heat flux - S Proportion of area in bare soil - W Proportion of surface in open water - r _a Aerodynamic resistance (P-M, W-P-M) - r _c Canopy resistance - r _s Generalized optimized surface resistance - r _st Stomatal resistance - r _c ^a Bulk boundary layer resistance (S-W) - r _s ^a Aerodynamic resistance below mean canopy level (S-W) - r _s ^s Soil surface resistance (S-W, W-P-M) Greek Bowen ratio - Psychrometer constant - Air density - Slope of saturation vapour pressure vs temperature curve     

Meteorological results of MONSOON-88 expedition (pre-monsoon period)

Mean atmospheric circulation, moisture budget and net heat exchange were studied during a pre-monsoon period (18th March to 3rd May, 1988), making use of the data collected on board Akademik Korolev in the central equatorial and southern Arabian Sea region. The net heat exchange (R _n ) is found to be about 20 W m^–2 for a small area (0–4° N; 55–60° E), 50% less than the dimatological value. The mean value of net radiation (140 W m^–2) is less than the climatological value, which was due to higher cloud amount. The higher SST enhanced both the latent and sensible heat fluxes.The mean atmospheric circulation obtained from the upper air data is quite convincing. The mean exchange coefficient (C _e ) estimated from the moisture budget is about 1.0 × 10^–3 for a wind speed of 4 m s^–1. This value is slightly lower than that obtained by the usual methods.National Institute of Oceanography, RC, 52-Kirlampudi layout, Visakhapatnam — 530 023.India Meteorological Department, Gauhati.     

Comparisons of xylem sap flow and water vapour flux at the stand level and derivation of canopy conductance for Scots pine

Summary Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus sylvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaign of the international programme REKLIP. Sap flow was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flowmeters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8 cm; max 17.5 cm). Available energy was high during the observation period (5.5 to 6.9 mm.day^–1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7 mm day^–1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h^–1, i.e., 230 W m^–2.Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data set, a good agreement was found between both methods: sap flow = 0.745^* vapour flux,r ² = 0.86. The difference between the two estimates was due to understory transpiration.Canopy conductance (g _c ) was calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations ofg _c were well correlated (r ² = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression forg _c =f (R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.With 6 Figures     

The daily and annual effects of dew, frost, and snow on a non-ventilated net radiometer

The formation of dew, deposition of frost and accumulation of snow mainly on the upper domes of a non-ventilated net radiometer seriously affect the measurement of available energy (net radiation). Net radiometers measure radiation, and energy balances and are widely used for estimation of evapotranspiration throughout the world. To study the effects of dew, frost, and snow on a non-ventilated net radiometer, a radiation station was set up which uses 2 CM21 Kipp & Zonen pyranometers (one inverted), 2 CG1 Kipp & Zonen pyrgeometers (one inverted), along with a Q7.1 net radiometer (Radiation & Energy Balance Systems, Inc.; REBS) in a semi-arid mountainous valley in Logan, Utah, U.S.A. The pyranometers and pyrgeometers were ventilated using 4 CV2 Kipp & Zonen ventilation systems. The net radiometer was not ventilated. The ventilation of pyranometers and pyrgeometers prevents dew and frost deposition and snow accumulation which otherwise would disturb measurements. All sensors were installed at about 3.0 m above the ground, which was covered with natural vegetation during the growing season (May–September). The incoming and outgoing solar or shortwave radiation, the incoming (atmospheric) and outgoing (terrestrial) longwave radiation, and the net radiation have been continuously measured by pyranometers, pyrgeometers and a net radiometer, respectively, since 1995. These parameters have been measured every 2 s and averaged into 20 min. To evaluate the effects of dew, frost, and snow, three days were chosen: 26 April 2004 with early morning dew, 6 January 2005 with an early morning frost, and the snowy day of 24 February 2005. Dew formation, frost deposition, and snow accumulation occurred mainly on the upper dome of the non-ventilated Q7.1 net radiometer on the related days, while the ventilated Kipp & Zonen system was free of dew, frost and snow. Net radiation measured by the non-ventilated net radiometer R_n,unvent. during dew and frost periods of the above-mentioned days was greater than ventilated ones R_n,vent. (− 0.2 MJ m^− 2 vs. − 0.8 MJ m^− 2 during almost 4 h on 26 April 2004, and − 0.2 MJ m^− 2 vs. − 0.7 MJ m^− 2 during almost 6.5 h on 6 January 2005). The reason for higher reading by the non-ventilated net radiometer during dew and frost periods was due to emission of additional longwave radiation from water and ice crystals formed mainly on the upper dome of the Q7.1 net radiometer. In contrast, during the snowy day of 24 February 2005, the R_n,unvent. was less than R_n,vent. (− 4.00 MJ m^− 2 vs. 0.77 MJ m^− 2, mainly from sunrise to sunset). The extremely low R_n,unvent. measured by the non-ventilated net radiometer on 24 February 2005 is due to blocking of the incoming solar radiation (mainly diffuse radiation) by the snow-covered upper dome.     

Model-based estimates of water loss from “patches” of the understory mosaic of the Hartheim Scots pine plantation

Summary During the Hartheim Experiment (HartX) 1992 conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory and the forest floor by several methods. At the vegetation patch level, direct estimates were made with small weighing lysimeters, and water loss was scaled-up to the stand level based on vegetation patchtype distribution. At the leaf level, transpiration flux was determined with a CO₂/H₂O porometer for the dominant understory plant species,Brachypodium pinnatum, Carex alba, andCarex flacca. Measured leaf transpiration was scaled-up to patch level with a canopy light interception and leaf gas exchange model, and then to stand level as in the case of lysimeter data, but with further consideration of patchtype leaf area index (LAI). On two days, total understory latent heat flux was estimated by eddy correlation methods below the tree canopy.The understory vegetation was subdivided into five major patch-types which covered 62% of the ground area and resulted in a cumulative LAI of approx. 1.54 when averaged over total stand ground area and compared to the average tree canopy LAI of 2.8. The remaining 38% of ground area was unvegetated bare soil and/or covered by moss (mainly byScleropodium purum) or litter. The evapotranspiration from the understory and unvegetated areas equaled approx. 20% of total forest stand transpiration during the HartX period. The understory vegetation transpired about 0.4 mm d^–1 (13%) estimated over the period of May 13 to 21, whereas evaporation from moss and soil patches amounted 0.23 mm d^–1 (7.0%). On dry, sunny days, total water vapor flux below the tree canopy exceeded 0.66 mm d^–1. Using the transpiration rates derived from the GAS-FLUX model together with estimates of evaporation from moss and soil areas and a modified application of the Penman-Monteith equation, the average daily maximum conductance of the understory and the forest floor was 1.7 mm s^–1 as compared to 5.5 mm s^–1 for the tree canopy.With 6 Figures     

A coupled model on land-atmosphere interactions — Simulating the characteristics of the PBL over a heterogeneous surface

An attempt is made to construct a model, coupling land surface and atmospheric processes in the planetary boundary layer (PBL). A grassland strip in a semi-desert (hereinafter called desert) is presupposed, so as to simulate the case of heterogeneous vegetation cover.Modeling results indicate that every term in the equation of the surface energy balance changes as the air flows over the grassland. The striking contrast of water and energy conditions between the grassland and the desert means that the air over the grassland is cooler and wetter than that over the desert. Consequently, in the heating and dynamic forcing of the air by the underlying surface, heterogeneities arise and are then transferred upward by the turbulent motions. Horizontal differences thus develop in the PBL, resulting in a local circulation. Meanwhile, the horizontal differences affect the free atmosphere through vertical motion at the top of the PBL.List of symbols d ₁,d ₂,d ₃ depths of surface, middle and lower layers of soil - T _c ,T ₁,T ₂,T ₃ temperatures of canopy, surface, middle and lower layers of soil - R _nc net radiation of canopy layer - _c shielding factor of vegetation - Ew, Etc evaporation from wet fraction of foliage and transpiration from dry fraction of foliage - Et ₁,Et ₂ transpiration of foliage water absorbed by the root in the upper and lower soil, respectively - H _c sensible heat of canopy - P _c ,D _c precipitation rate and drainage of canopy - C _s ,C _c ,C _w heat capacity of soil, canopy and water - _w , _s density of water and air near the surface - D hydraulic permeability of soil - _s saturated value of the ratio of volumetric soil moisture - S _g , _g solar radiation and surface reflection - H _g ,R _{L g} turbulent heat flux and long wave radiation of surface - P _g ,E _g precipitation rate and evaporation of soil surface - K _s soil thermal diffusivity - K ^(m),K ^(H),K ^(q) eddy coefficients of momentum, heat and moisture - u, v, w components of wind speed in three directions - air potential temperature - e turbulent kinetic energy - p atmospheric pressure - C _p specific heat of air under constant pressure - R _d gas constant - u _* friction velocity - _* feature temperature - h height of the PBL - f Coriolis parameter - L ₀ Monin-Obukhov length - latent heat of vaporization - q specific humidity - M _c ,M _cm interception water storage of canopy and its maximum - ₀ Exner number of largescale background field - perturbation Exner number - u _g ,v _g components of the geostrophic wind speed Sponsored by the National Natural Science Foundation of China.     

Influences on surface energy fluxes in simulated present and doubled CO2 climates

The surface energy fluxes simulated by the CSIRO9 Mark 1 GCM for present and doubled CO₂ conditions are analyzed. On the global scale the climatological flux fields are similar to those from four GCMs studied previously. A diagnostic calculation is used to provide estimates of the radiative forcing by the GCM atmosphere. For 1 × CO₂, in the global and annual mean, cloud produces a net cooling at the surface of 31 W m^–2. The clear-sky longwave surface greenhouse effect is 311 W m^–2, while the corresponding shortwave term is –79 W m^–2. As for the other GCM results, the CSIRO9 CO₂ surface warming (global mean 4.8°C) is closely related to the increased downward longwave radiation (LW ). Global mean net cloud forcing changes little. The contrast in warming between land and ocean, largely due to the increase in evaporative cooling (E) over ocean, is highlighted. In order to further the understanding of influences on the fluxes, simple physically based linear models are developed using multiple regression. Applied to both 1 × CO₂ and CO₂ December–February mean tropical fields from CSIRO9, the linear models quite accurately (3–5 W m^–2 for 1 × CO₂ and 2–3 W m^–2 for CO₂) relate LW and net shortwave radiation to temperature, surface albedo, the water vapor column, and cloud. The linear models provide alternative estimates of radiative forcing terms to those from the diagnostic calculation. Tropical mean cloud forcings are compared. Over land, E is well correlated with soil moisture, and sensible heat with air-surface temperature difference. However an attempt to relate the spatial variation of LWt within the tropics to that of the nonflux fields had little success. Regional changes in surface temperature are not linearly related to, for instance, changes in cloud or soil moisture.     

Penumbral and foliage distribution effects on Pinus sylvestris canopy gas exchange

Summary  Tree canopy water use and foliage net CO₂ uptake (NPP) were simulated for a 31-year-old Pinus sylvestris (Scots pine) plantation near Hartheim, in the Upper Rhine Valley, Germany with a mechanistically-based, three-dimensional stand gas-exchange model (STANDFLUX) for a ten-day period during spring 1992. STANDFLUX was formulated to include the effects of penumbra caused by the fine structure of the needles on light distribution within crowns. Good correspondence was found between simulated rates of tree canopy water use when including penumbral effects and eddy-covariance ET and sap flow transpiration measurements. Water use was 8–13% lower and NPP was 10–17% lower in simulations for the ten-day period when penumbral effects were not included. Simulated water use and CO₂ uptake were compared with similar outputs from a simplified layer canopy model (including or not including penumbra) which assumed horizontal homogeneity in canopy structure (GAS FLUX). Our results for the Pinus sylvestris stand indicate that penumbral effects were more important than the degree of model simplification with respect to foliage distribution (three-dimensional vs. layered structure) for estimating stand-level fluxes for these pines. Simulated maximum hourly NPP was similar to rates measured for other Pinus sylvestris stands using other methods. Predicted decreases in tree transpiration due to the modelled response of needle gas exchange to increasing vapour-pressure deficit agreed with measured changes in transpiration, and suggested that stomatal response may have been more important than decreasing soil water availability in controlling water flux to the atmosphere during this period. The overall results of the study demonstrate that current approaches in canopy modelling that separate light into sun versus shade intensities can be effective, but must be applied with caution when attempting to predict long-term water and carbon balances of forests. Received May 1, 1999 Revised November 9, 2000     

Surface-layer scintillation measurements of daytime sensible heat and momentum fluxes

Line-averaged measurements of the structure parameter of refractive index (C _n ² ) were made using a semiconductor laser diode scintillometer above two markedly different surfaces during hours of positive net radiation. The underlying vegetation comprised in the first instance a horizontally homogeneous, pasture sward well-supplied with water, and in the second experiment, a sparse thyme canopy in a semi-arid environment. Atmospheric stability ranged between near neutral and strongly unstable (–20). The temperature structure parameterC _T ² computed from the optical measurements over four decades from 0.001 to 2 K² m^–2/3 agreed to within 5% of those determined from temperature spectra in the inertial sub-range of frequencies. Spectra were obtained from a single fine thermocouple sensor positioned near the midway position of the 100m optical path and at the beam propagation height (1.5m).With the inclusion of cup anemometer measurements, rule-of-thumb assumptions about surface roughness, and Monin-Obukhov similarity theory, path-averaged optical scintillations allow calculation of surface fluxes of sensible heat and momentum via a simple iterative procedure. Excellent agreement was obtained between these fluxes and those measured directly by eddy correlation. For sensible heat, agreement was on average close to perfect over a measured range of 0 to 500 W m^–2 with a residual standard deviation of 30 W m^–2. Friction velocities agreed within 2% over the range 0–0.9 m s^–1 (residual standard deviation of 0.06 m s^–1). The results markedly increase the range of validation obtained in previous field experiments. The potential of this scintillation technique and its theoretical foundation are briefly discussed.     

A study of the seasonal migration of ITCZ and the quasi-periodic oscillations in a simple monsoon system using an energy balance model

Summary A zonally averaged global energy balance model with feedback mechanisms was constructed to simulate (i) the poleward limits of ITCZ over the continent and over the ocean and (ii) a simple monsoon system as a result of differential heating between the continent and the ocean. Three numerical experiments were performed with lower boundary as (1) global continent, (2) global ocean and (3) continent-ocean, with freezing latitudes near the poles. Over the continent, midlatitude deserts were found and the ITCZ migrates 25° north and south with seasons. Over a global swamp ocean results do not show migration of ITCZ with time but once the ocean currents are introduced the ITCZ migrates 5° north and south with seasons. It was found that the seasonal migration of ITCZ strongly depends on the meridional distribution of the surface temperature. It was also found that continent influences the location of the oceanic ITCZ. In the tropics northward progression of quasi-periodic oscillations called events are found during the pre- and post-monsoon periods with a period of 8 to 15 days. This result is consistent with the observed quasi-periodic oscillations in the tropical region. Northward propagation of the surface temperature perturbation appears to cause changes in the sensible heat flux which in turn causes perturbations in vertical velocity and latent heat flux fields.List of Symbols vertical average - ₀ zonal average - vertical mean of the zonal average - _0s zonal average at the surface - _0a zonal average at 500 mb level - latitude We now define the various symbols used in the model rate of atmospheric heating due to convective cloud formation (K/sec) - dp/dt (N/m²/sec) - density - potential temperature (K) - rate of rotation of the earth (rad/sec) - empirical constant - humidity mixing ratio - ^* saturated humidity mixing ratio - opacity of the atmosphere - ₁,₂ factors for downward and upward effective black body long wave radiation from the atmosphere - Stefan-Boltzmann constant - emissivity of the surface - _D subsurface temperature (K) - a specific volume - _0xs ,_0ys eastward and northward components of surface frictional stress - ^* vertical velocity at the top of the boundary layer (N/m²/sec) - P Thickness of the boundary layer (mb) - nondimensional function of pressure - P pressure - P _a pressure of the model atmosphere (N/m²) - P _s pressure at the surface (N/m²) - t time (sec) - U eastward wind speed (m/sec) - V northward wind speed (m/sec) - surface water availability - T absolute temperature (K) - heat addition due to water phase changes - g acceleration due to gravity (m²/sec) - a radius of the earth (m) - R gas constant for dry air (J/Kg/K) - C _p specific heat of air at constant pressure (J/Kg/K) - k R/C _p - L latent heat of condensation (J/Kg) - f coriolis parameter (rad/sec) - H _s H _0s ⁽¹⁾ +H _0s ⁽²⁾ +H _0s ⁽³⁾ +H _0s ⁽⁴⁾ +H _0s ⁽⁵⁾ (J/m²/Sec)=sum of the rates of vertical heat fluxes per unit surface area, directed toward the surface - H _a H _0a ⁽¹⁾ +H _0a ⁽²⁾ +H _0a ⁽³⁾ +H _0a ⁽⁴⁾ (J/m²/Sec)=sum of the rates of heat additions to the atmospheric column per unit horizontal area by all processes - H _0s ⁽¹⁾ ,H _0a ⁽¹⁾ heat flux due to short wave radiation - H _0s ⁽²⁾ ,H _0a ⁽²⁾ heat flux due to long wave radiation - H _0s ⁽³⁾ ,H _0a ⁽³⁾ heat flux due to small scale convection - H _0s ⁽⁴⁾ heat flux due to evaporation - H _0a ⁽⁴⁾ heat flux due to condensation - H _0s ⁽⁵⁾ heat flux due to subsurface conduction and convection - e ^* saturation vapor pressure - R solar constant (W/m²) - r _a albedo of the atmosphere - r _s albedo of the surface - b ₂ empirical constant (J/m²/sec) - c ₂ empirical constant (J/m²/sec) - e ₂ nondimensional empirical constant - f ₂ empirical constant (J/m₂/sec) - factor proportional to the conductive capacity of the surface medium - a _s constant used in Sellers model - b _s positive constant of proportionality used in the Sellers model (kg m²/J/sec²) - K _HT coefficient for eddy diffusivity of heat (m²/sec) - K _HE exchange coefficient for water vapor (m²/sec) - h depth of the water column (m) - z height (m) - V _0ws meridional component of surface current (m/sec) - n cloud amount - G _0,n long wave radiation form the atmosphere for cloud amount n (W/m²) - B ₀ long wave radiation from the surface (W/m²) - S _0,n short wave radiation from the atmosphere for cloud amount n (W/m²) - A _n albedo factor for a cloud amount n - R _f1 large scale rainfall (mm/day) - R _f2 small scale rainfall (mm/day) With 22 Figures