Ulf Högström

Review of some basic characteristics of the atmospheric surface layer

Some of the fundamental issues of surface layer meteorology are critically reviewed. For the von Karman constant (k), values covering the range from 0.32 to 0.65 have been reported. Most of the data are, however, found in a rather narrow range between 0.39 and 0.41. Plotting all available atmospheric data against the so-called roughness Reynolds number, % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOuaiaabw% gadaWgaaWcbaGaaeimaaqabaGccqGH9aqpcaWG1bWaaSbaaSqaaiaa% cQcaaeqaaOGaamOEamaaBaaaleaacaaIWaaabeaakiaac+cacqaH9o% GBaaa!3FD0!\[{\rm{Re}}_{\rm{0}} = u_* z_0 /\nu \] or against the surface Rossby number, % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOuaiaab+% gadaWgaaWcbaGaaeimaaqabaGccqGH9aqpcaWGhbGaai4laiaadAga% caWG6bWaaSbaaSqaaiaaicdaaeqaaaaa!3DF1!\[{\rm{Ro}}_{\rm{0}} = G/fz_0 \] gives no clear indication of systematic trend. It is concluded that k is indeed constant in atmospheric surface-layer flow and that its value is the same as that found for laboratory flows, i.e. about 0.40.

Analysis of Turbulence Structure in the Surface Layer with a Modified Similarity Formulation for Near Neutral Conditions

Abstract Data from a recent detailed surface layer experiment are critically examined in terms of the turbulent kinetic energy budget and the other second order moment budgets formed by the three velocity components and temperature. In moderately unstable and slightly stable conditions nondimensional terms of all the moment budgets studied agree reasonably well with results reported from the Kansas study (after application of a flow distortion correction). In the near-neutral range, where the present experiment contains a large amount of data, results deviate significantly from previous studies in general and, in particular, for ideal, zero-pressure gradient turbulent boundary layers. Several moments, such as u2W, v2w and W2 are not, as expected, constant in the surface layer, but vary logarithmically with height, making instead their non dimensional vertical gradients constant. Some moments scale with the roughness length and others with a length scale containing the large-scale pressure gradient or, wit...

A case study of air-sea interaction during swell conditions

Air-sea interaction data from a situation with pronounced unidirectional swell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data from the site Ostergarnsholm in the Baltic Sea were used. The situation, which lasted for ∼48 hours, appeared in the aftermath of a gale. The wind direction during the swell situation turned slowly within a 90° sector. Both during the gale phase and the swell phase the over-water fetch was >150 km. The wind speed during the swell phase was typically 4 m s−1. During the swell phase a wind maximum near or below the lowest wind speed measuring level 10 m was observed. The net momentum flux was very small, resulting in CD values ∼0.7 × 10−3. Throughout the lowest 26 m, covered by the tower measurements, turbulence intensities in all three components remained high despite the low value of the kinematic momentum flux -u′w′¯ resulting in a reduction of the correlation coefficient for the longitudinal and vertical velocity from its typical value around −0.35 to between −0.2 and 0 (and with some positive values at the higher measuring levels), appearing abruptly at wave age c0/U10 equal to 1.2. Turbulence spectra of the horizontal components were shown not to scale with height above the water surface, in contrast to vertical velocity spectra for which such a variation was observed in the low-frequency range. In addition, spectral peaks in the horizontal wind spectra were found at a frequency as low as 10−3 Hz. From a comparison with results from a previous study it was concluded that this turbulence is of the “inactive” kind, being brought down from the upper parts of the boundary layer by pressure transport.

Turbulent exchange above a pine forest II. Organized structures

Data from two 100-min runs on the turbulent atmospheric wind, temperature and humidity fields above a pine forest have been analysed using conditional sampling techniques. With the aid of the temperature time series, ramp events were identified and all fields were averaged in order to remove smaller scale turbulence and random low-frequency turbulence, and to map the organized structures revealed in this way. It is shown that the turbulent fluxes of momentum, heat and humidity, determined from these ramp-events, in fact constitute a large part of the total fluxes, about 90% during the actual events.On average, the duration of and the time between ramps were found to be about 35 s and 100 s, respectively. These periods were also found in the uw-, wT- and wq-cospectra, not only in the data analysed in this paper, but in the major part of all day-time data from this forest site (about 80 runs of 60 or 100 min). This indicates that the ramp events described arc a rather common phenomenon. The organized structures also have remarkable resemblance to near-wall structures observed in the laboratory.

Spectra, variances and length scales in a marine stable boundary layer dominated by a low level jet

A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was characterized by stable stratification and the presence of a pronounced low level jet at very low height, 30–150 m, above the surface of the sea. The atmospheric surface layer was apparently extremely shallow; thus the non-dimensional wind gradients and temperature gradients derived from measurements at 8 m do not show adherence to Monin-Obukhov similarity, in sharp contrast to findings from the same site at similar stability conditions but with no low level jet. Instead these quantities are shown to be governed by scales characteristic of stable shear flow away from the surface. The height to the jet centre appears to be an important quantity. Thus, for the cases with the lowest jet height values (30–50 m), some turbulent characteristics of the flow (non-dimensional velocity standard deviations and the correlation between the longitudinal and vertical velocity) have values similar to those found for the zero pressure-gradient laboratory boundary layer over a flat plate (the so called ‘canonical’ boundary layer) rather than the typical values found in atmospheric boundary-layer flow. It was inferred that the large scale fluctuations known as ‘inactive’ turbulence, as well as gravity waves, were suppressed in this case.

Critical test of the validity of Monin-Obukhov similarity during convective conditions

A recent study of convective boundary layer characteristics performed with large eddy simulation technique (LES) has demonstrated unexpected influence of the depth of the boundary layer on surface layer characteristics. The present study tests some of the predictions from these simulations with field measurements from a summertime experiment in Sweden, which includes in addition to regular surface layer data also airborne measurements and numerous radio soundings, which enable accurate determination of boundary layer depth. It is found that the measurements strongly support most of the conclusions draws from the LES study and give additional information over a wider stability range. Thus, the normalized wind gradient fm is found to depend on both z/L, where z is height above the ground and L is the Monin‐Obukhov length, and zi/L, where zi is the height of the convective boundary layer. This additional dependence on zi/L explains much of the scatter between experiments encountered for this parameter. In the case of the normalized temperature gradient fh, the experimental data follow the generally accepted functional relation with z/L, but with an additional, slight ordering according to zi/L. Analyses of nondimensional variances show (i) the horizontal velocity variance scales on mixed layer variables and is a function only of zi/L, in agreement with the LES results and with previous measurements; (ii) the normalized vertical velocity variance depends on the large-scale pressure gradient length scale for slight instability and is primarily a function of z /L for moderate and strong instability; (iii) the normalized temperature variance is a function of z/L, with a possible slight dependence on zi/L; and (iv) whereas mean temperature gradient is characterized by local shear scales, temperature variances are normalized by local buoyancy-driven scales.

Turbulence characteristics in a near neutrally stratified urban atmosphere

Turbulence measurements from the city of Uppsala, Sweden, are analysed. Measurements were taken at two sites: one in the central area, ca. 6 m above roof level, the average building height being ca. 15 m; the other at ca. 8 and 50 m above the ground on a tower situated 100 m downwind of a sharp discontinuity between the densely built-up urban area and flat grass-covered land. The average stability was close to neutral, the range being -0.2 < z/L < 0.2. The main emphasis of the study is on the non-dimensional standard deviations of the velocity components Σi/u*t and on the corresponding non-dimensional energy spectra, u*t being a local velocity scale defined as √Τi/ρ(Τl is the local momentum flux). Comparison with results obtained from surface-layer measurements at ‘ideal sites’ (with u*, being the ordinary friction velocity) shows good general agreement. The most complete agreement is found for the tower 50 m measurements, a result which is notable as this measurement point is found to be within a distinctly transitional zone between the urban and post-urban boundary layers. The results from the central city measurement point are also fairly close to the ‘ideal’ results, the deviations found being small in view of the fact that the site is probably inside the layer in which the roughness elements (the buildings) have direct influence. The measurements at the tower 8 m level show certain distinct deviations from ideal results: all three Σi/u*l, are higher by ca. 10%, the excessive energy being found at the low frequency end of the spectrum. Arguments are presented for this feature to be due to a spectral lag effect.

The Near-Neutral Marine Atmospheric Boundary Layer with No Surface Shearing Stress: A Case Study

Abstract Data from a marine coastal experiment over the Baltic Sea, comprising airborne measurements and mast measurements, have been used to highlight the turbulence dynamics of a case with most unusual flow characteristics. The boundary layer had a depth of about 1200 m. The thermal stratification was near neutral, with small positive heat flux below 300 m and equally small negative heat flux above. The entire situation lasted about 6 hours. Turbulence levels were unexpectedly high in view of the fact that momentum flux was negligible (in fact positive) in the layers near the surface, and buoyancy flux was also small. The turbulence was found to scale with the height of the boundary layer, giving rise to velocity spectra having the shape of those characteristic of convectively mixed boundary layers. Analysis of the turbulence budget for the entire planetary boundary layer (PBL) revealed that energy was produced from shear instability in the uppermost parts of the PBL and was distributed to the lower par...

Turbulent exchange above a pine forest. I: Fluxes and gradients

Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fluxes have been carried out over a sparse pine forest at Jädra»s in Sweden. In order to ascertain that correct gradient estimates were obtained, two independent measuring systems were employed: one system with sensors at 10 fixed levels on a 51 m tower and another with reversing sensors for temperature and humidity, covering the height interval 23 to 32 m. Turbulent fluxes were measured at three levels simultaneously. Data from three field campaigns: in June 1985, June 1987 and September 1987 have been analyzed. The momentum flux is found on the average to be virtually constant from tree top level, at 20 to 50 m. The average fluxes of sensible and latent heat are not so well behaved. The ratio of the non-dimensional gradients of wind and temperature to their corresponding values under ‘ideal conditions’ (low vegetation) are both found to be small immediately above the canopy (about 0.3 for temperature and 0.4 for wind). With increasing height, the ratios increase, but the values vary substantially with wind direction. The ratios are not found to vary systematically with stability (unstable stratification only studied). The ratio of the non-dimensional humidity gradient to the corresponding non-dimensional potential temperature gradient (equivalent to kh/kw) is found to be unity for (z − d)/Lv less than about −0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large.

Analysis of the turbulence structure of a marine low-level jet

Four aircraft measurement sets made in late May 1989 within low level jets over the Baltic Sea have been analyzed to estimate the turbulence energy budget. It is concluded that the jets had the same origin as found in an earlier study from the same general area: inertial oscillation caused by frictional decoupling when relatively warm air flows out over much colder water.In order to combine budget estimates from the four flights to form a representative average, self-preservation similarity was assumed. When the terms were made nondimensional with the proper scale combination, the largest terms in all four runs were of order one, indicating that the scaling is physically sound.Three terms were found to dominate the turbulence energy budget: shear production, dissipation and pressure transport. The latter was obtained as remainder term, since local time rate of change and advection terms were found to be of negligible magnitude. Shear production was found in a narrow layer above the jet core and in a much deeper layer below it. The pressure transport term was a gain in this layer as well, helping to keep the layer below the jet well mixed. This is in agreement with results from aircraft measurements in the low level jet and monsoon boundary layer over the Arabian Sea.It is concluded that development of the inertial jet downwind of a coastline is of fundamental importance for exchange of momentum at the sea surface in conditions when relatively warm air is advected over cold water. The jet produces turbulence that promotes mixing in the lower layers, which sharpens the shear below the jet core, so that mixing becomes even more effective. Turbulence brought down to the surface by the pressure transport term is likely to be of the ‘inactive’ type, which does not produce shear stress. Through the above-mentioned process it is, however, instrumental in promoting the mechanism that eventually produces ‘active turbulence’, the carrier of momentum.