Kathleen E. Moore

Importance of Low-Frequency Contributions to Eddy Fluxes Observed over Rough Surfaces

Abstract Eddy covariance flux observations at a deciduous temperate forest site (83 days) and at a boreal forest site (21 days) are analyzed for midday periods (1100–1400 LT). Approximate stationarity of the time series is demonstrated, and the ensemble-averaged roughness sublayer cospectra are presented. Spectral and cospectral forms in the roughness sublayer are more peaked than those found in an inertial sublayer. They exhibit similar forms dependent on (z − d)/(h − d), where d is the displacement height and h is the canopy height. The inertial-layer spectral forms are recovered when observations are made where this scaled height is approximately 4. For a sample summer at the midlatitude deciduous forest, large eddies with periods from 4 to 30 min contribute about 17% to surface eddy fluxes of heat, water vapor, and carbon dioxide (CO2). Much larger contributions can occur in light-wind conditions. This effect, likely caused by the passage of convective boundary layer eddies, is not observed when using...

Seasonal Variation in Radiative and Turbulent Exchange at a Deciduous Forest in Central Massachusetts

Abstract Temperate deciduous forests exhibit dramatic seasonal changes in surface exchange properties following on the seasonal changes in leaf area index. Nearly continuous measurements of turbulent and radiative fluxes above and below the canopy of a red oak forest in central Massachusetts have been ongoing since the summer of 1991. Several seasonal trends are obvious. Global solar albedo and photosynthetically active radiation (PAR) albedo both are good indicators of the spring leaf emergence and autumnal defoliation of the canopy. The solar albedo decreases throughout the summer, a change attributed to decreasing near-infrared reflectance since the PAR reflectance remains the same. Biweekly satellite composite images in visible and near-infrared wavelengths confirm these trends. The thermal emissions from the canopy relative to the net radiation follow a separate trend with a maximum in the midsummer and minima in spring and fall. The thermal response number computed from the change in radiation tempe...

Climatic Consequences of Leaf Presence in the Eastern United States

At the time of leaf emergence in deciduous forests, markedly enhanced evapotranspiration leads to a rapid drop in the Bowen ratio. A small fraction of this surface flux alteration converges into the boundary layer, and this can be detected in the mean temperature and humidity daily increments at the surface. A simple technique is presented for identifying this response in surface climate data and extracting time series for the date of spring onset and for the ‘‘spring intensity,’’ a measure of surface energy budget partition change in spring. A tendency Bowen ratio B9 is found from changes in the daily increment of temperature and humidity in multidecadal averages. The spring date determined using this criterion for stations along the U.S. east coast corresponds to the date at which the normalized difference vegetation index (NDVI) reaches 80% of its seasonal maximum. Northward movement of the vernal front is similar to that obtained using Hopkins’ empirical rule; it is linearly related to leaf emergence and flowering dates from the North American lilac phenology network. Spring intensity increases northward; the states from Virginia north exhibit distinctly higher values. There has been a trend in the most recent decades toward earlier spring dates, except for regions in Virginia and North Carolina. The same analyses performed using the small subset of stations with longer-term records indicate that a trend toward an earlier spring date is confined to recent decades. An inverse relationship between the spring date and spring average temperature was found for the Midwest but is inadequate for the northeast. Spring intensity has generally increased in northeastern North America throughout the twentieth century. However, large oscillations with an approximate 20-yr period distinguish the northeastern United States from the Midwest, indicating that the intensity of spring is not a simple function of spring temperature or of forest cover fraction.

Boundary Layer Clouds and Vegetation–Atmosphere Feedbacks

Abstract An analysis of boundary layer cumulus clouds and their impact on land surface–atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (∼1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (∼1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995–98 but reve...

Growing season boundary layer climate and surface exchanges in a subarctic lichen woodland

Between June and August 1990, observations were made at two surface micrometeorological towers near Schefferville Quebec (54° 52′N, 66° 40.5′W), one in a fen and one in the subarctic lichen woodland, and at four surface climatological stations. Data from these surface stations were supplemented by regular radiosonde launches. Supporting measurements of radiative components and soil temperatures allowed heat and moisture balances to be obtained at two sites. The overall surface meteorological experiment design and results of micrometeorological observations made on a 30-m tower in the lichen woodland are presented here. Seasonal variation in the heat and water vapor transport characteristics illustrate the marked effect of the late summer climatological shift in air mass type. During the first half of the summer, average valley sidewalls only 100 m high are sufficient to channel winds along the valley in the entire convective boundary layer. Channeling effects at the surface, known for some time at the long-term climate station in Schefferville, are observed both at ridge top and in the valley, possibly the response of the flow to the NW-SE orientation of valleys in the region. Diurnal surface temperature amplitude at ridge top (≈ 10° C) was found to be half that observed in the valley. Relatively large differences in precipitation among these stations and the climatological station at Schefferville airport were observed and attributed to the local topography. Eddy correlation observations of the heat, moisture and momentum transports were obtained from a 30-m tower above a sparse (≈ 616 stems/ha) black spruce lichen woodland. Properties of the turbulent surface boundary layer agree well with previous wind tunnel studies over idealized rough surfaces. Daytime Bowen ratios of 2.5–3 are larger than those reported in previous studies. Surface layer flux data quality was assessed by looking at the surface layer heat balance. Diurnal and seasonal scale heat budget imbalances were found. We suggest that unmeasured surface heat storage may be responsible for some of the observed imbalance. The presence of the unexplained residual in this and other studies of energy balance over forests casts a note of caution on the interpretation of energy balance components obtained using heat residual methods.

A season of heat, water vapor, total hydrocarbon, and ozone fluxes at a subarctic fen

High-latitude environments are thought to play several critical roles in the global balance of radiatively active trace gases. Adequate documentation of the source and sink strengths for trace gases requires long time series of detailed measurements, including heat and moisture budgets. A fen near Schefferville, Quebec, was instrumented during the summer of 1990 for the measurement of the surface energy, radiation, and moisture balances as well as for eddy correlation estimates of ozone and methane flux. Despite the limited fetch at this site, analysis of the tower flux“footprint”indicates that at least 80% of the flux observed originates from sources within the fen. Sensible heat fluxes averaged 25% of the daytime net radiation at the site, while the latent heat flux, determined from the energy balance, was 63%; the Bowen ratio varied from 0.2 to 0.8 from day to day, without a seasonal trend to the variation. The competing effects of rooted macrophyte development (with concomitant effects on roughness and transpiration) and the normal shift in synoptic pattern around day 200 to warm, dry conditions results in a lack of net seasonal effect on the energy partitioning. Over the period from days 170 to 230, the evaporation (167 mm) was double the rainfall, while the decline in water level was 107 mm, leaving a net runoff of 0.44 mm/d. The total hydrocarbon flux was 75–120 mg m−2 d−1, following a diurnal pattern similar to heat or moisture flux, while the daytime ozone flux was about −1.11×1011 molecules cm−2 s−1. A period near the end of the experiment, during week 30, produced the strongest total hydrocarbon flux, associated with warmer deep (1 m) soil temperatures, lower fen water levels, and the late summer shift in wind direction at that time. An early summer“flush”of total hydrocarbon was not observed.

Turbulent transports over tundra

Measurements of the surface fluxes of heat, momentum, and water vapor using the eddy correlation technique were made from instruments on a 12-m tower at a remote site in the western Alaskan tundra between July 13 and August 12, 1988. Except for advection, all of the terms in the heat and moisture balances were measured directly. Diurnal averages of the heat budget over periods of several days balance to within 5 W/m2. The observed Bowen ratio was approximately 1. Maximum values of sensible heat flux approached 150 W/m2 during midafternoon. At 0.13 m below the surface, the soil heat flux ranged from 15 to 20 W/m2. Surface exchange of heat and moisture over tundra during the summer is limited by a strong resistance to water vapor transfer from the upper soil layer through the ground cover (200 s/m). Though July 1988 was anomalously warm and dry in the region and August was close to normal temperature and rainfall, there was no appreciable difference in the canopy resistance between the periods. During the dry, sunny period at the end of July, the observed evaporation rate was 1.9 mm/d. The depth of seasonal permafrost melt in the region was between 0.3 and 0.4 m during the latter half of the growing season, and the permafrost edge at the field site descended at approximately 1.6 mm/d. There was no change in the albedo or in the analogous ratio for photosynthetically active radiation during the experimental period.

Growing season water balance at a boreal jack pine forest

Measurements of energy and CO2 fluxes were made over the growing seasons of 1994 and 1996 in a northern jack pine forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Simultaneous measurements of soil water potential and content, rainfall, leaf wetness, and air specific humidity by our group and others allowed us to construct a complete water balance at this site for the 1994 season. The longer-term (spanning weeks) rate of loss of soil moisture in the upper 0.25 m of soil matched the total evapotranspiration, measured by eddy covariance. Evapotranspiration (measured as QE, latent heat flux) was just 24% of the net radiation, a result that has been found in many boreal forest types, regardless of the canopy coverage. Low canopy conductances (typically 2–4 mm s−1) probably represent an adaptation to the extremely low soil moistures and poor nutrient status of the site. Net radiation was the best single-variable predictor of evapotranspiration, having a correlation coefficient of 0.8 with QE for the 1994 season. Afternoons with sustained (>4 continuous hours) cloudless conditions resulted in water stress detectable as reduced QE relative to what would be predicted from the net radiation alone. The open canopy at our site promoted the role of the lichen-covered surface in the overall water vapor exchange; subcanopy QE was 26% of the total.

How well can regional fluxes be derived from smaller‐scale estimates?

Regional surface fluxes are essential lower boundary conditions for large-scale numerical weather and climate models and are the elements of global budgets of important trace gases (Stewart et al., 1989). Surface properties affecting the exchange of heat, moisture, momentum and trace gases vary with length scales from 1 m to hundreds of kilometers. A classical difficulty is that fluxes have been measured directly only at points (towers) or along lines (from aircraft). The process of “scaling up” observations limited in space and/or time to represent larger areas has been done by assigning properties to surface types and combining estimated or calculated fluxes using an area-weighted average. Because of nonlinear influences, such as the effect of internal boundary layers, it is not clear that a simple area-weighted average is sufficient to produce the large scale from the small scale, nor is it known how important the uncertainty is to large-scale model outcomes. Simultaneous aircraft and tower data obtained in the relatively simple terrain of the western Alaskan tundra were used to determine the extent to which surface type variation can be related to regional-scale fluxes of heat, moisture, and other properties. Surface type was classified as lake or land with an aircraft-borne infrared thermometer, and flight-level heat and moisture fluxes were related to surface type. The magnitude and variety of sampling errors inherent in eddy correlation flux estimation place limits on how well any flux can be known even in simple geometries. Because of the presence of intrinsic and site-specific uncertainties, regional-scale flux of heat and moisture using aircraft observations in our study area can be reasonably verified to be estimated correctly from linear combinations of smaller-scale estimates only to within a factor of 1.5. Flights at lower levels or in a more comprehensive or systematic pattern might be able to resolve the contributions from individual surface types better, but an experiment to test any scaling-up hypothesis is difficult to devise.

BOREAS TF-8 NSA-OJP Tower Flux, Meteorological, and Soil Temperature Data

The BOReal Ecosystem-Atmosphere Study Tower Flux (BOREAS TF-3) team collected tower flux, surface meteorological, and soil temperature data at the BOREAS Northern Study Area-Old Black Spruce (NSA-OBS) site continuously from the March 1994 through October 1996. The data are available in tabular ASCII files.