J. Ian MacPherson

Effects of spatial variability in topography, vegetation cover and soil moisture on area-averaged surface fluxes: A case study using the FIFE 1989 data

A modified version of the simple biosphere model (SiB) of Sellers et al. (1986) was used to investigate the impact of spatial variability in the fields of topography, vegetation cover, and soil moisture on the area-averaged fluxes of sensible and latent heat for an area of 2×15 km (the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) testbed area) located within the FIFE area. This work builds on a previous study of Sellers et al. (1992a) but makes use of a superior data set (FIFE 1989 rather than FIFE 1987) and has a sharper focus on the nonlinear effects of soil wetness on evapotranspiration. The 2×15 km testbed area was divided into 68×501 pixels of 30×30 m spatial resolution, each of which could be assigned topographic, vegetation condition, and soil moisture parameters from satellite and in situ observations gathered in FIFE-89. One or more of these surface fields was area averaged in a series of simulation runs to determine the impact of using large-area means of these initial/boundary conditions on the area-integrated (aggregated) surface fluxes. Prior to these simulations some validation work was done with the model. The results of the study can be summarized as follows: (1) SiB was initialized with satellite and airborne remotely sensed data for vegetation condition and soil wetness, respectively. The surface fluxes calculated by SiB compared well with surface-based and airborne flux observations. (2) Analyses and some of the simulations indicated that the relationships describing the effects of moderate topography on the surface radiation budget are near linear and thus largely scale invariant. The relationships linking the simple ratio (SR) vegetation index, the canopy conductance parameter ∇F, and the canopy transpiration flux are also near linear and similarly scale invariant to first order (see also Sellers et al., 1992a). Because of this it appears that simple area-averaging operations can be applied to these fields with relatively little impact on the calculated surface heat fluxes. (3) The relationships linking surface and root-zone soil wetness to the soil surface and canopy transpiration rates are nonlinear. However, simulation results and observations indicate that soil moisture variability decreases significantly as the study area dries out, which partially cancels out the effects of these nonlinear functions. (4) The near-infrared surface reflectance ρN estimated from atmospherically corrected satellite data may be a better predictor of vegetation condition than a two-band vegetation index, such as the SR, at least for the grasslands represented in the FIFE area. These results support the use of simple averages of topographic and vegetation parameters to calculate surface energy and heat fluxes over a wide range of spatial scales, from a few meters up to many kilometers. Although the relationships between soil moisture and evapotranspiration are nonlinear for intermediate soil wetnesses, the dynamics of soil drying act to progressively reduce soil moisture variability and thus the impacts of these nonlinearities on the area-averaged surface fluxes. These findings indicate that we can use mean values of topography, vegetation condition, and soil moisture to calculate the surface-atmosphere fluxes of energy, heat, and moisture at larger length scales to within an acceptable accuracy for climate-modeling work.

The impact of using area-averaged land surface properties —topography, vegetation condition, soil wetness—in calculations of intermediate scale (approximately 10 km2) surface-atmosphere heat and moisture fluxes

It is commonly assumed that biophysically based soil-vegetation-atmosphere transfer (SVAT) models are scale-invariant with respect to the initial boundary conditions of topography, vegetation condition and soil moisture. In practice, SVAT models that have been developed and tested at the local scale (a few meters or a few tens of meters) are applied almost unmodified within general circulation models (GCMs) of the atmosphere, which have grid areas of 50–500 km2. This study, which draws much of its substantive material from the papers of Sellers et al. (1992c, J. Geophys. Res., 97(D17): 19033–19060) and Sellers et al. (1995, J. Geophys. Res., 100(D12): 25607–25629), explores the validity of doing this. The work makes use of the FIFE-89 data set which was collected over a 2 km × 15 km grassland area in Kansas. The site was characterized by high variability in soil moisture and vegetation condition during the late growing season of 1989. The area also has moderate topography. The 2 km × 15 km ‘testbed’ area was divided into 68 × 501 pixels of 30 m × 30 m spatial resolution, each of which could be assigned topographic, vegetation condition and soil moisture parameters from satellite and in situ observations gathered in FIFE-89. One or more of these surface fields was area-averaged in a series of simulation runs to determine the impact of using large-area means of these initial or boundary conditions on the area-integrated (aggregated) surface fluxes. The results of the study can be summarized as follows: 1. 1. analyses and some of the simulations indicated that the relationships describing the effects of moderate topography on the surface radiation budget are near-linear and thus largely scale-invariant. The relationships linking the simple ratio vegetation index (SR), the canopy conductance parameter (▽F) and the canopy transpiration flux are also near-linear and similarly scale-invariant to first order. Because of this, it appears that simple area-averaging operations can be applied to these fields with relatively little impact on the calculated surface heat flux. 2. 2. The relationships linking surface and root-zone soil wetness to the soil surface and canopy transpiration rates are non-linear. However, simulation results and observations indicate that soil moisture variability decreases significantly as an area dries out, which partially cancels out the effects of these non-linear functions. In conclusion, it appears that simple averages of topographic slope and vegetation parameters can be used to calculate surface energy and heat fluxes over a wide range of spatial scales, from a few meters up to many kilometers at least for grassland sites and areas with moderate topography. Although the relationships between soil moisture and evapotranspiration are non-linear for intermediate soil wetnesses, the dynamics of soil drying act to progressively reduce soil moisture variability and thus the impacts of these non-linearities on the area-averaged surface fluxes. These findings indicate that we may be able to use mean values of topography, vegetation condition and soil moisture to calculate the surface-atmosphere fluxes of energy, heat and moisture at larger length scales, to within an acceptable accuracy for climate modeling work. However, further tests over areas with different vegetation types, soils and more extreme topography are required to improve our confidence in this approach.

Measurement of O3 and related compounds over southern Nova Scotia: 1. Vertical distributions

As part of the North Atlantic Regional Experiment, the National Research Council of Canada Twin Otter aircraft was used to measure the concentration of O3 and related compounds in the atmosphere over southern Nova Scotia. Forty-eight flights were conducted, primarily over the surface sampling site in Chebogue Point, Nova Scotia, or over the adjoining Atlantic Ocean. A typical flight included one or more vertical profiles from 30 m above the surface to an altitude of 3 or 5 km. We present here O3 measurements and supporting chemical and meteorological data including NOy, CO, accumulation mode aerosol particles, winds, temperature, and dew point. Data are presented in a format which illustrates day-to-day variability and vertical structure. We find that Nova Scotia is impacted by a wide variety of air masses with varying chemical content depending on flow conditions relative to the locations of upwind emission regions. As an aid to understanding the chemical composition of the air, we characterize four types of events: (1) moist continental boundary layer air with high concentrations of O3 and other anthropogenic pollutants which is advected to Nova Scotia in relatively thin vertical layers, usually with a base altitude of several hundred meters; (2) “background” air with concentrations of anthropogenic ingredients much lower than experienced in continental pollution episodes but higher than observed in more remote regions of Canada, suggesting a dilute anthropogenic or biomass burning influence; (3) near-surface air which because of a strong temperature inversion over the Atlantic Ocean, is decoupled from air aloft, with the consequence that near-surface measurements do not give a representative view of the eastward transport of the North American plume; and (4) dry air masses with high O3 concentration in which we have to distinguish between boundary layer and upper atmosphere source regions.

Relationship between satellite-derived vegetation indices and aircraft-based CO2 measurements

The objective of this study was to analyze the relationship between satellite-derived vegetation indices and CO2 uptake, as an initial step in exploring the possibility of using a satellite-derived vegetation index as a measure of net photosynthesis. The study area included the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site located on the Konza prairie and adjacent area as well as a transect between Manhattan and Salina. One third of the transect exhibited vegetation and terrain characteristics similar to those on the FIFE site, whereas cultivated land predominated in the remaining portion of the 75-km-long flight line. In June, July, August, and October 1987, several CO2 data sets were obtained using the National Research Council of Canadas Twin Otter research aircraft. The normalized difference vegetation index (NDVI) and the simple ratio (SR) were computed from NOAA advanced very high resolution radiometer (AVHRR) data acquired as part of FIFE. Aircraft and satellite data were processed to obtain spatially coincident and locally representative flux values. Results show a linear relationship between NDVI and CO2 uptake during a single day; however, a nonlinear relationship emerged when all data sets were combined. The data from FIFE and the regional transect were consistent for one date but differed for other periods. Overall, about 60% of total variability in CO2 flux was accounted for by the NDVI and 74% by the SR.

A comparison of surface sensible and latent heat fluxes from aircraft and surface measurements in FIFE 1987

Surface fluxes of sensible and latent heat over a tall grass prairie in central Kansas, as measured by 22 surface stations during FIFE 1987, are compared with values gained indirectly by simple linear extrapolation of aircraft-measured flux profiles to the surface. The results of 32 such comparisons covering the period June 26 to October 13, 1987, indicate that (1) the flux profiles were generally linear, as expected in a convective boundary layer; 2) in general, aircraft-derived surface fluxes were 30% less than the surface averages for sensible heat and 10% less for latent heat; (3) high-pass filters, with cutoffs at 5 km, acted to reduce the uncertainty of the aircraft flux estimates, without changing the magnitude or sign of the disagreement between the aircraft and the surface fluxes; (4) undersampling of high frequencies by the aircraft could have caused as much as a 15% underestimate of the surface sensible heat fluxes; and (5) the aircraft and surface Bowen ratios also disagreed, with the greatest disagreement occurring during the dry periods with decreased plant activity (IFC 4).

Comparison of Wind Profiler and Aircraft Wind Measurements at Chebogue Point, Nova Scotia

Abstract In August 1993, a 915-MHz boundary layer wind-profiling radar was deployed at Chebogue Point, Nova Scotia, to provide wind, turbulence, and boundary layer structure information for the North Atlantic Regional Experiment summer 1993 intensive campaign. The National Research Council Canada Twin Otter atmospheric research aircraft was also part of that campaign. During the campaign, the Twin Otter flew 29 soundings over Chebogue Point. This paper describes a comparison of the wind speed and direction measured by the profiler and the aircraft. In the height range 300–2000 m above sea level, the random difference between the wind speed measurements is 0.9 m s−1, and the random difference between the wind direction measurements is 9°. There is a small systematic difference in the wind speeds (0.14 m s−1) that is probably due to uncertainty in the zenith angles of the radar beams and extremely good agreement (within 0.5°) in the wind direction. The Kalman filter-smoother technique used to remove drifts ...

Aircraft encounters with strong coherent vortices over the boreal forest

A number of intense low-level vortices were encountered at a height of 30 m over the boreal forest by a Twin Otter atmospheric research aircraft. They imposed strong vertical and lateral accelerations on this aircraft: the most energetic had an updraft speed in excess of 11 m s−1, only 20 m above the forest. All were associated with large sensible heat fluxes, low wind speeds, and deep boundary layers.

The evolution of AVHRR-derived water temperatures over boreal lakes

Abstract Higher spatial resolution in Earth–atmosphere models requires improved information on surface fields with very different thermal characteristics, such as land and water. Remote sensing is a useful tool for this purpose. A method is presented here to determine and compare the temperature evolution of water bodies. Thermal IR observations were extracted from NOAAs advanced very high-resolution radiometer (AVHRR) satellite data over the period from April 1 to August 31, 1994. The IR temperatures were calibrated and adjusted to account for the intervening atmosphere. For each day, 1-km resolution temperature scenes were generated for a 1000×1000-km rectangular region extending from about 60°N, 111°W over the northern part of the Canadian Province of Alberta to about 50°N, 97°W in southern Manitoba. There are 132 water bodies of area larger than 100 km 2 in this study area. The temperature cycle over these water bodies can be decomposed into a straight line with a small positive slope for spring ice break-up, followed by a fitted quadratic curve for the summer temperature variations. The curve fit parameters give information on the transition between ice and ice-free conditions. Data stratification confirms a strong latitudinal influence on the shape of the curves. It was not possible to quantify the effect of water depth, as the depth of most boreal lakes is not known. Instead, a list of fit coefficients for all the water bodies is provided. Once detrended from the quadratic curves, the amplitude of the diurnal cycle and major variations in temperatures of the water bodies can be determined. The detrended water temperature variations are about one-third the magnitude of the detrended air temperature variations from nearby weather stations.

Local meteorological features affecting chemical measurements at a North Atlantic coastal site

One of the foci of the North Atlantic Regional Experiment (NARE) 1993 summer intensive campaign was Chebogue Point, approximately 10 km south of Yarmouth, Nova Scotia. Measurements were taken at this site with a 915-MHz boundary layer wind profiler, the Canadian Twin Otter and National Center for Atmospheric Research King Air aircraft, and a variety of surface instruments. This paper discusses features observed in the meteorological measurements and the implications of those features for the interpretation of chemical measurements. The meteorology of this coastal site is complex. A strong surface-based temperature inversion was almost always present, producing strong layering in the lower atmosphere. As a result, surface chemistry measurements were not often representative of the state of the overlying atmosphere. A low-level jet was also frequently present. A variety of turbulence structures were observed by the profiler, including convective boundary layers and complex layering. Ozone concentrations above 60 parts per billion at the surface occurred on four occasions late in August, in conjunction with strong stability and winds off the Atlantic Ocean (Gulf of Maine). Ozone levels of more than 60 ppbv were observed by the aircraft at altitudes between 400 and 1600 m on five other occasions earlier in the month. Particular (but not necessarily unusual) combinations of transport, mixing, and source conditions appear to be required to produce ozone episodes at Chebogue Point.

Cross‐shore variations of near‐surface wind velocity and atmospheric turbulence at the land‐sea boundary during CASP

Abstract Airborne measurements of mean wind velocity and turbulence in the atmospheric boundary layer under wintertime conditions of cold offshore advection suggest that at a height of 50 m the mean wind speed increases with offshore distance by roughly 20% over a horizontal scale of order 10 km. Similarly, the vertical gust velocity and turbulent kinetic energy decay on scales of order 3.5 km by factors of 1.5 and 3.2, respectively. The scale of cross‐shore variations in the vertical fluxes of heat and downwind momentum is also 10 km, and the momentum flux is found to be roughly constant to 300 m, whereas the heat flux decreases with height. The stability parameter, z/L (where z = 50 m and L is the local Monin‐Obukhov length), is generally small over land but may reach order one over the warm ocean. The magnitude and horizontal length scales associated with the offshore variations in wind speed and turbulence are reasonably consistent with model results for a simple roughness change, but a more sophistic...