Water and energy fluxes in the lower Mackenzie valley, 1994/95

Abstract

The 1994/95 water year in the lower Mackenzie Valley was an extraordinary year hydrologically, with the important winter to summer transition being the earliest on record. Unlike more temperate areas, the northern water year is dominated, to a great extent, by this onset of spring which results in the melting of nearly half of the annual precipitation over a period of a few weeks, initiates the thawing of the river and lake ice and the soil active layer, and marks the beginning of the evaporation season. An early winter to summer transition occurred at two small research basins in the Inuvik area and at the East Channel of the Mackenzie River Delta. At the research basins, for example, the spring of 1994/95 had the earliest onset of continuous above‐freezing air temperatures, removal of the snow cover, and initiation of runoff. Consideration of the entire water year at the research basins demonstrates that rain and snow were nearly equal in magnitude, evaporation exceeded runoff, and the annual change in storage was negative to near zero. This negative change in storage was related to the long, snow‐free evaporation season, above‐average air temperatures, and below‐normal precipitation. The unusual winter to summer transition on the Mackenzie River in the eastern portion of the Mackenzie Delta was, in many ways, even more remarkable than that in the research basins. Earlier work had suggested that the timing of the spring breakup was very consistent from year to year. An analysis of the timing of breakup from the early 1960s to the late 1990s, however, shows a trend towards earlier spring breakup, with the mean for the 1990s being nine days earlier than that for the 1960s, and with the 1995 breakup being the earliest on record. Such an early breakup is not only an indication of warm local conditions, but of warm temperatures and an early runoff event over the more southerly areas of the Mackenzie basin. A companion Mackenzie Global Energy and Water Cycle Experiment study illustrates the importance of a high pressure circulation pattern centred east of the basin to this early melt event.     

Atmospheric turbulent fluxes over snow

On March 26, 1971, eddy fluxes of momentum, sensible heat and water vapour were measured over Lake Mendota, Wisconsin, U.S.A., which was covered by an extensive snowfall. An evaporation rate of about 0.7mm day^–1 (2.2 mW cm^–2) was detected. Wind speeds were light and the atmosphere near the surface was highly stable. In these conditions, the average sensible heat transfer and Reynolds stress were -0.9 mW cm^–2 and 0.10 dyn cm^–2, respectively. Comparison with measured gradients of wind speed, temperature and humidity yield a drag coefficient of about 0.54 × 10^–3, and bulk transfer coefficients for sensible and latent heat of 0.41 × 10^–3 and 0.78 × 10^–3, respectively, applied to 10-m data. When corrected for the effect of atmospheric stability, these three coefficients become (in the same order) 1.2 × 10^–3, 0.9 × 10^–3 and 2.5 × 10^–3. The errors in these estimates are such that the drag coefficient is not significantly different from that corresponding to an aerodynamically smooth surface, while the heat coefficients are similar to those normally applied over liquid water surfaces.     

On dynamically consistent eddy fluxes

The role of mesoscale oceanic eddies in driving the large-scale currents is studied in an eddy-resolving, double-gyre ocean model. The new diagnostic method is proposed, which is based on dynamical decomposition of the flow into the large-scale and eddy components. The method yields the time history of the eddy forcing, which can be used as additional, external forcing in the corresponding non-eddy-resolving model of the gyres. The main strength of this approach is in its dynamical consistency: the non-eddy-resolving solution driven by the eddy forcing history correctly approximates the original large-scale flow component. It is shown that statistical decompositions, which are based on space-time filtering diagnostics, are dynamically inconsistent. The diagnostics algorithm is formulated and tested, and the diagnosed eddies are analysed, both statistically and dynamically. It is argued that the main dynamic role of the eddies is to maintain the eastward-jet extension of the subtropical western boundary current (WBC). This is done largely by both the time–mean isopycnal-thickness flux and the relative-vorticity eddy flux fluctuations. The fluctuations drive large-scale flow through the nonlinear rectification mechanism. The relative-vorticity flux contributes mostly to the eastward jet meandering. Finally, eddy fluxes driven by both the eddies and the large-scale flow are found to be important. The latter is typically neglected in the analysis, but here it corresponds to important large-scale feedback on the eddies.     

Estimation of areally-averaged surface fluxes

The concept of blending height is used to estimate areally averaged surface fluxes of momentum and heat in a stratified, horizontally inhomogeneous surface-layer flow. This concept is based on the assumption that at sufficiently large heights above a heterogeneous surface, subsequent surface modifications will not be recognizable in the flow individually, but overall flux and mean profiles will represent the surface condition of a large area. The height at which the flow becomes approximately independent of horizontal position is called blending height according to Wieringa (1986).Here, it is proposed to classify the ground surface in a surface-layer grid box of a larger-scale model into several land-use categories. Surface momentum and heat fluxes should be estimated for each category at the blending height. The grid-averaged surface fluxes are to be obtained by the average of surface fluxes on each land-use surface weighted by its fractional area. The postulate of computing the surface fluxes at the blending height leads to a new formulation of turbulent transfer coefficients.The proposed parameterization has been tested by employing a small-scale numerical model as a surface-layer grid box of a hypothesized larger-scale model. Several quite different flow configurations have been studied in order to investigate the performance of the new parameterization. Generally, the relative errors of estimated averaged surface fluxes are found to be well within ±10%.     

Turbulent fluxes over inhomogeneous landscape

Mesoscale surface turbulent fluxes over a complex terrain surrounded by oceans have been investigated using a 3-D numerical mesoscale model, under conditions with and without synoptic flows. The study indicated that under synoptically calm condition, the allocation and intensity of mesoscale surface turbulent fluxes (MSTFs) were greatly impacted by the thermally forced mesoscale circulation (TFMC) over mesoscale heterogeneous landscape. The max-imum values of sensible (Hs) and latent (LE) heat fluxes were located over the convergent zones and considerably im-pacted by the soil wetness (M), but did not depend strongly on the atmospheric background thermal stability (β0). The simulated results suggested that the sensible heat flux was closely proportional to the square of wind speed in the surface layer. By the action of synoptic flow, the allocation of LE was shifted to downwind, its intensity increased.     

Vertical fluxes in katabatic flows

Katabatic flow is a dynamical process occurring on relatively calm, clear nights above sloping terrain. Its existence is dependent on long-wave radiative transfer, particularly radiative flux divergence within the air itself, for both its generation and (it is concluded here), along with advective warming, for much of its retardation.Utilising sounding data closely spaced in time, a discussion is presented of the importance of surface shear, interfacial shear, advective warming and radiative divergence in a strong katabatic flow. It is concluded that radiative divergence is important in generating static and dynamic instabilities in the flow. The role of radiative cooling in mixing of momentum has largely been ignored so far, and might explain why higher-order models tend to overestimate katabatic speeds on smooth slopes.     

Seasonal Variability of the Yellow Sea／East China Sea Surface Fluxes and Thermohaline Structure

We use the U.S. Navy‘s Master Oceanographic Observation Data Set (MOODS) for the Yellow Sea/East China Sea (YES) to investigate the climatological water mass features and the seasonal and non-seasonal variabilities of the thermohaline structure, and use the Comprehensive Ocean-Atmosphere Data Set (COADS) from 1945 to 1989 to investigate the linkage between the fluxes (momentum, heat, and moisture) across the air-ocean interface and the formation of the water mass features. After examining the major current systems and considering the local bathymetry and water mass properties, we divide YES into five regions: East China Sea (ECS) shelf, Yellow Sea (YS) Basin, Cheju bifurcation (CB) zone,Taiwan Warm Current (TWC) region, Kuroshio Current (KC) region. The long term mean surface heat balance corresponds to a heat loss of 30 W m^-2 in the ESC and CB regions, a heat loss of 65 W m^-2 in the KC and TWC regions, and a heat gain of 15 W m^-2 in the YS region. The surface freshwater balance is defined by precipitation minus evaporation. The annual water loss from the surface for the five subarea sranges from 1.8 to 4 cm month^-1. The fresh water loss from the surface should be compensated for from the river run-off. The entire water column of the shelf region (ECS, YS, and CB) undergoes an evident seasonal thermal cycle with maximum values of temperature during summer and maximum mixed layer depths during winter. However, only the surface waters of the TWC and KC regions exhibit a seasonal thermal cycle. We also found two different relations between surface salinity and the Yangtze River run-off,namely, out-of-phase in the East China Sea shelf and in-phase in the Yellow Sea. This may confirm an earlier study that the summer fresh water discharge from the Yangtze River forms a relatively shallow, low salinity plume-like structure extending offshore on average towards the northeast.     

Average fluxes from heterogeneous vegetated regions

Using a surface-layer model, fluxes of heat and momentum have been calculated for flat regions with regularly spaced step changes in surface roughness and stomatal resistance. The distance between successive step changes is limited to 10 km in order to fill the gap between micro-meteorological measurements and meso-scale models. A single-layer big leaf model of the vegetation is compared with a multi-layer model to assess the performance of the former in the determination of surface fluxes in heterogeneous terrain.The sub-models of vegetation and atmosphere are mainly based on well-known theory. However, a modification of the mixing-length closure of atmospheric exchange is included to achieve a more realistic calculation of fluxes near step changes at the surface. Measurements, presented in the literature, are used to determine the mixing-length parameters and to validate the calculated fluxes downwind of a change in vegetation cover.The single-layer model, well validated for homogeneous surfaces, underestimates the effects of local advection upon the surface fluxes as this model neglects air flow across the edges of tall vegetation. Using the multi-layer model, local advection results in an increase of up to 50% in regional momentum flux and smaller changes in regional evaporation. Even widely spaced heterogeneities appear to influence regional fluxes.     

Observations of fluxes over heterogeneous surfaces

This study analyzes data collected from repeated aircraft runs 30 m over alternating regions of irrigated and dry nonirrigated surfaces, each region on the order of 10 km across, during the California Ozone Deposition Experiment (CODE). After studying the scale dependence of the flow, the variables and their fluxes are decomposed into means for sublegs defined in terms of irrigated and nonirrigated regions and deviations from such subleg means. Since the repeated runs were flown over the same track, compositing the eight flight legs for each of the two days allows partial isolation of the influences of surface heterogeneity and transient mesoscale motions.A variance analysis is carried out to quantify the relative importance of surface heterogeneity and transient mesoscale motions on the variability of the turbulence fluxes. The momentum and ozone fluxes are more influenced by transient mesoscale motions while fluxes of heat, moisture and carbon dioxide are more influenced by surface heterogeneity. The momentum field is also influenced by a quasi-stationary mesoscale front and larger scale velocity gradients.For the present case, the mesoscale modulation of the turbulent flux is numerically more important than the direct mesoscale flux. This spatial modulation of the turbulent fluxes leads to extra Reynolds terms which act to reduce the area-averaged turbulent momentum flux and enhance the area-averaged turbulent heat flux.     

Urban-scale variations of turbulence parameters and fluxes

Sensible heat (H) and latent heat (LE) fluxes and turbulence statistics in St. Louis, Missouri and the surrounding region are presented. The urban-scale analyses were derived from a series of aircraft transects at 150 m above ground across the metropolitan area during the afternoon convective period. The results revealed that H varied by a factor of two to four in the region; the largest values were associated with the urban heat island. LE varied across the urban area by about a factor of four, but low values of LE overlaid the urban heat island. Consequently, the Bowen ratio (H/LE) exhibited large spatial variability, with a maximum value greater than 1.5 over the city and values less than 0.2 in nonurban areas. The areas along the Mississippi River and adjacent low lying marshland northeast of the downtown area displayed significantly smaller H and Bowen ratio. The derived surface heat storage term (G) for this area as well as for the urban area exceeded either H or LE.The spatial patterns for the standard deviations of the three velocity components ( _u,v,w ), temperature ( _T ), and absolute humidity ( _q ), are also presented. The patterns of _u,v,w were similar to the pattern of H. the highest values associated with the urban heat island. The correlation coefficient between the vertical velocity and temperature fluctuations was highest over the city, and a noteworthy minimum was observed in the upwind area over the river and marshland in association with low H. The convective similarity relationships for _u,v,w appeared to be approximately valid spatially, as variations were typically less than 10% from theory over the urban area and nonurban region, except for a 40% anomaly in the lowland around the river northeast of the city.Measurements of H from 30-m towers within various land-use areas were contrasted with the aircraft data. Land-use differences in H at the surface were at least as large as those observed at 150 m across the city. This was primarily because of the measurement requirement that the minimum resolvable fetch increases with measurement height.     

Source areas for scalars and scalar fluxes

The spatial resolution of meteorological observations of scalars (such as concentrations or temperature) and scalar fluxes (e.g., water-vapour flux, sensible heat flux) above inhomogeneous surfaces is in general not known. It is determined by the surface area of influence orsource area of the sensor, which for sensors of quantities that are subject to turbulent diffusion, depends on the flow and turbulence conditions.Functions describing the relationship between the spatial distribution of surface sources (or sinks) and a measured signal at height in the surface layer have been termed thefootprint function or thesource weight function. In this paper, the source area of levelP is defined as the integral of the source weight function over the smallest possible domain comprising the fractionP of the total surface influence reflected in the measured signal. Source area models for scalar concentration and for passive scalar fluxes are presented. The results of the models are presented as characteristic dimensions of theP=50% source areas (i.e., the area responsible for 50% of the surface influence): the maximum source location (i.e., the upwind distance of the surface element with the maximum-weight influence), the near and the far end of the source area, and its maximal lateral extension. These numerical model results are related directly to non-dimensional surface-layer scaling variables by a non-linear least squares method in a parameterized model which provides a user-friendly estimate of the surface area responsible for measured concentrations or fluxes. The source area models presented here allow conclusions to be made about the spatial representativeness and the localness (these terms are defined in the text) of flux and concentration measurements.     

Validation of parameterized convective fluxes with DIAMOD

Summary A diagnostic model (DIAMOD) for the atmosphere over Europe is use at the University of Vienna. Central parameters in each diagnostic column (horizontal resolution 100 km, time resolution 12 hours) are the vertical moisture plus heat flux (the total convective heat fluxh) and the vertical rain flux (r); both are functions of pressure. In this study DIAMOD is applied to validate the output of a forecast model for the simulation of acid deposition (EURAD) which is in use at the University of Cologne. The basic equations of both DIAMOD and EURAD models are summarized with emphasis on the sub-gridscale hydrologic components.First, the nontrivial problem of validating model output versus observations is discussed. Two different validation techniques based upon the budget equations are indentified. The fully prognostic technique compares the forecast of EURAD for the total verification period with the corresponding DIAMOD output. The semiprognostic validation technique involves only one-time-step tendencies. Neither yields an exact correspondence between EURAD and DIAMOD; however, the semiprognostic technique comes somewhat closer to the ideal of an objective validation. The quantities investigated are: The fields, the time tendencies and the fluxesh andr.Second, EURAD is validated versus DIAMOD with both techniques for the EUMAC Joint Wet Case (the Chernobyl episode) in April 1986; the output fields include selected profiles ofh(p) over France (a moist night situation) and over Greece (a dry day situation). The comparison demonstrates for both that the EURAD forecasts are acceptable for ther-fluxes but are relatively poor for theh-fluxes. Reasons for the differences are discussed.With 11 Figures     

Increased heat fluxes near a forest edge

Summary ?Observations of sensible and latent heat flux above forest downwind of a forest edge show these fluxes to be larger than the available energy over the forest. The enhancement averages to 56 W m⁻², or 16% of the net radiation, at fetches less than 400 m, equivalent to fetch to height ratios less than 15. The enhancement of turbulent energy fluxes is explained by advection and increases with the difference in temperature and humidity of the air over the upwind area as compared to the forest. The relatively high temperature and humidity of the upwind air are not caused by high surface heat fluxes, but are explained by the relatively low aerodynamic roughness of the upwind surface. Although the heat fluxes over forest are enhanced, the momentum fluxes are almost adjusted to the underlying forest. The different behaviour of heat and momentum fluxes is explained by absorption of momentum by pressure gradients near the forest edge. It is concluded that fetch requirements to obtain accurate surface fluxes from atmospheric observations need to be more stringent for scalar fluxes as compared to momentum fluxes. Received November 23, 2001; accepted May 13, 2002