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Michael Tjernström

Abstract

Data from the Arctic Ocean Experiment 2001 (AOE-2001) are used to study the vertical structure and diurnal cycle of the summertime central Arctic cloud-capped boundary layer. Mean conditions show a shallow stratocumulus-capped boundary layer, with a nearly moist neutrally stratified cloud layer, although cloud tops often penetrated into the stable inversion. The subcloud layer was more often stably stratified. Conditions near the surface were relatively steady, with a strong control on temperature and moisture by the melting ice surface.

A statistically significant diurnal cycle was found in many parameters, although weak in near-surface temperature and moisture. Near-surface wind speed and direction and friction velocity had a pronounced cycle, while turbulent kinetic energy showed no significant diurnal variability. The cloud layer had the most pronounced diurnal variability, with lowest cloud-base height midday followed by enhanced drizzle and temporarily higher cloud-top heights in the afternoon. This is opposite to the cycle found in midlatitude or subtropical marine stratocumulus. The cloud layer was warmest (coolest) and more (less) stably stratified midafternoon (midmorning), coinciding with the coolest (warmest) but least (most) stably stratified capping inversion layer.

It is speculated that drizzle is important in regulating the diurnal variability in the cloud layer, facilitated by enhanced midday mixing due to a differential diurnal variability in cloud and subcloud layer stability. Changing the Arctic aerosol climate could change these clouds to a more typical “marine stratocumulus structure,” which could act as a negative feedback on Arctic warming.

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Michael Tjernström

Abstract

The amount of liquid water in stratus clouds or fog is discussed from the point of view of estimating visibility variations in areas with complex terrain. The average vertical profile of liquid water from numerical simulations with a higher-order closure mesoscale model is examined, and runs with the model for moderately complex terrain are utilized to estimate the of low-level liquid water content variability and thus, indirectly, the variations in horizontal visibility along a slope.

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Michael Tjernström

Abstract

The vertical turbulence structure in the marine atmosphere close to a coastline is investigated using airborne measurements. The measurements are from a field experiment close to the coast in the southeast of Sweden, in the Baltic Sea. The Baltic Sea has two main properties that make it particularly interesting to study: significant annual lag in sea surface temperature compared to inland surface temperatures and the fact that it is surrounded by land in all directions within advection distances of from a few hours up to 10–15 hours in normal meteorological conditions.

The present results are mostly from spring or early summer with mainly cool water that is, with a stable or neutral marine boundary layer but with substantial heating of the land area during daytime, thus considerable thermal contrasts. When the daytime inland convective boundary layer is advected out over the cool sea, there is a frictional decoupling in space analogous to the same nocturnal process in time. This sometimes creates a residual layer, a remnant of the inland convective boundary layer, that can be advected for considerable distances over the sea. At the top of this layer, wind shear gives rise to a local increase in turbulent kinetic energy. These layers are used for an analysis of turbulent scales for free shear flow in stable stratification. The analysis is based on different length scales used in numerical model closures for turbulence processes and reveals the asymptotic behavior of different scales in the neutral limit and their functional form, and also illustrates the nonlinear relationship between scales for different properties. The applicability of some often used formulations is also discussed.

The profiles from the aircraft are taken from 25 slant soundings performed in connection to low-level boundary-layer flights. The results are calculated from turbulence data extracted through filtering techniques on instantaneous time (space) series (individual profiles). The calculated turbulence parameters from all profiles are lumped together and finally averaged compositely over all profiles.

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Michael Tjernström and Patrick Samuelsson

Abstract

Wind calculations from a radome gust probe system, in combination with a wander-angle Inertial Navigation System (INS), are used to calculate turbulence (variance and fluxes) by eddy correlation. The difference in time response between the air motion gust probe and the INS is analysed in postprocessing of in-flight data from calibration maneuvers. The effect of this difference on mean winds during accelerated maneuvers and on turbulence spectra and cospectra from low-level turbulence measurement flight legs is demonstrated.

The estimated time delay between the two systems is small for most parameters, less than 0.1 s, but is large for ground speed, about 0.1 s, and substantial for true heading, almost 0.5 s. In forced maneuvers (pitching, yawing, and turning), the effect on the calculated mean wind is significant. During typical turbulent measurement conditions, that is, straight and level flight, the effect on the winds and thus on spectra and cospectra is, however, small.

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Michael Tjernström and Darko Koračin

Abstract

An ensemble-average closure model intended for mesoscale studies is applied to a marine stratocumulus-capped PBL. The intention is to test this model, in particular, for cases where cloud and subcloud layers are decoupled. The test is based on one case from the First ISCCP Regional Experiment, where solid cloud-capped and clear sky areas were found in close proximity.

The model results compare favorably both with the measurements and with results from more complex model formulations. They show the response of the entire boundary layer dynamic structure to stratocumulus formation as well as longwave and shortwave radiative heat transfers. The net result is that the entire turbulent layer in the cloud-capped case is more vigorously mixed, more neutrally stratified, and deeper compared to a cloud-free PBL developing under similar conditions. Surface fluxes of sensible and latent heat, from the measurements as well as simulations thus vary relatively little between the areas in spite of the observed substantial sea surface temperature difference.

All simulations presented here reveal cloud decoupling during daytime. The multilayer structure is, however, seen almost only in profiles of second-order moments. The mean profiles indicate one single, deep well-mixed layer, while the turbulence profiles clearly show two separate well-mixed layers. The turbulent flux of water vapor from the surface thus generally never penetrates to the cloud layer during daytime but may eventually cause formation of a shallow layer of cumuli below the main cloud layer.

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Michael Tjernström and Branko Grisogono

Abstract

Fully 3D nonlinear model simulations for supercritical flow along locations at the California coast, at Cape Mendocino, and Point Sur, are presented. The model results are objectively and subjectively verified against measurements from the Coastal Waves 1996 experiment with good results. They are then analyzed in terms of the flow structure, the impact of the local terrain, the atmospheric forcing on the ocean surface, and the momentum budgets. It is verified that the flow is supercritical (Fr > 1) within a Rossby radius of deformation from the coast and that it can be treated as a reduced-gravity, shallow water flow bounded by a sidewall—the coastal mountain barrier. As the supercritical flow impinges on irregularities in the coastline orientation, expansion fans and hydraulic jumps appear. The modeled Froude number summarizes well the current understanding of the dynamics of these events. In contrast to inviscid, irrotational hydraulic flow, the expansion fans appear as curved lines of equal PBL depth and “lens-shaped” maxima in wind speed residing at the PBL slope. This is a consequence of the realistic treatment of turbulent friction. Modeled mean PBL vertical winds in the hydraulic features range ±∼1–2 cm s−1, while larger vertical winds (±∼5–10 cm s−1) are due to the flow impinging directly on the mountain barrier. Local terrain features at points or capes perturb the local flow significantly from the idealized case by emitting buoyancy waves. The momentum budget along straight portions of the coast reveals a semigeostrophic balance modified by surface friction. While being geostrophic in the across-coast direction, the along-coast momentum shows a balance between the pressure gradient force and the turbulent friction. In the expansion fans, the flow is ageostrophic, and the imbalance is distributed between turbulent friction and ageostrophic acceleration according to the magnitude of the former. There is also a good correspondence between the magnitude of the local curl of the surface stress vector and the measured depression in sea surface temperature (SST) in areas where the latter is large and the along-coast flow is relatively weak, implying that a substantial portion of the upwelling is driven locally. Supplying the measured SST in the numerical simulations, with a considerable depression along the coast, had only marginal feedback effects on the character of the flow.

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Stefan Söderberg and Michael Tjernström

Abstract

In this study, a three-dimensional hydrostatic mesoscale model is used to address the transient behavior of supercritical along-coast flow. A control experiment and several sensitivity tests are performed in order to investigate the diurnal cycle of flow characteristics. An idealized representation of the northern California terrain is used, and the model results are interpreted within the reduced-gravity shallow water concept. In two preceding studies by the authors, this theory appeared to be violated since the flow accelerated along the coastline upstream of the change in coastline orientation, even though the flow was supercritical. Here, it is shown that the criticality of the flow for typical summertime conditions along the California coast actually varies diurnally. The gradual acceleration of the flow along the upstream coastline is established during a subcritical phase of the simulation; thus, the shallow water concept is not violated. Because the along-coast jet is primarily driven by the cross-coast baroclinicity, there will be a continuous variation in the strength of the jet. This in turn will affect the flow criticality and thus the flow is not only spatially, but also temporally, transcritical. The results here suggest that observations of quasi-steady-state supercritical flow in reality are not likely; transcritical flow along mountainous coastlines should be more prevalent.

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Joseph Sedlar and Michael Tjernström

Abstract

Measurements from spaceborne sensors have the unique capacity to fill spatial and temporal gaps in ground-based atmospheric observing systems, especially over the Arctic, where long-term observing stations are limited to pan-Arctic landmasses and infrequent field campaigns. The AIRS level 3 (L3) daily averaged thermodynamic profile product is widely used for process understanding across the sparsely observed Arctic atmosphere. However, detailed investigations into the accuracy of the AIRS L3 thermodynamic profiles product using in situ observations over the high-latitude Arctic are lacking. To address this void, we compiled a wealth of radiosounding profiles from long-term Arctic land stations and included soundings from intensive icebreaker-based field campaigns. These are used to evaluate daily mean thermodynamic profiles from the AIRS L3 product so that the community can understand to what extent such data records can be applied in scientific studies. Results indicate that, while the mid- to upper-troposphere temperature and specific humidity are captured relatively well by AIRS, the lower troposphere is susceptible to specific seasonal, and even monthly, biases. These differences have a critical influence on the lower-tropospheric stability structure. The relatively coarse vertical resolution of the AIRS L3 product, together with infrared radiation through persistent low Arctic cloud layers, leads to artificial thermodynamic structures that fail to accurately represent the lower Arctic atmosphere. These thermodynamic errors are likely to introduce artificial errors in the boundary layer structure and analysis of associated physical processes.

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Leif Enger and Michael Tjernström

Abstract

The effects on the regional precipitation climate by the construction of an artificial lake, in a semiarid region are studied. The study is performed using a mesoscale model to identify the larger-scale meteorological conditions when precipitation enhancement is to be expected and to estimate the amount of precipitation enhancement in such situations. The model results are combined with a ten-year synoptic observations dataset to estimate the mean annual increase in precipitation for the region. The results show a significant increase in precipitation partly over the artificial lake itself, due to land/sea-breeze type circulations during periods when the large-scale wind is weak and the surface temperature of the sea is higher than that of the surrounding areas, and partly over the mountains north of the area in cases with sufficiently strong southerly winds causing the air to be lifted. Being aware of the uncertainty in the quantitative estimates presented here, this paper shows how a combination of mesoscale model “sensitivity runs” and climatological data can be used to estimate at least the order of magnitude change in precipitation and also identify the areas where it is likely to occur.

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Michael Tjernström and Carl A. Friehe

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A “radome gust probe” system was installed on a twin-jet aircraft for the purpose of boundary-layer research. This system provided a useful relatively low-cost method for air motion and turbulence measurements on an aircraft already equipped with an inertial navigation system (INS) and a data acquisition system. An error analysis was made for the wind measurements and gave the limitations for the present system with an unmodified airliner-type INS, The major factors that limit the precision of the horizontal wind are the resolutions and accuracy of the aircraft ground speed components and the true heading. A simple method was devised to improve the heading resolution. From in-flight maneuvers, it was determined that the mean horizontal airspeed vector was accurate to <0.5 m s−1—limited by the long-term drift and oscillation errors from the INS-and that pitch and yaw contamination of the wind was less than 5%. The in-flight data indicate that there probably are unknown time lags within the INS, which degrade the wind vector measurement for anything but straight and level flight. Some intercomparisons were obtained from fly-by(s) past an instrumented 140-m tower. These showed general agreement between mean, variance and high-frequency spectral measurements of the velocity components and temperature. Flux or covariance comparisons were not as good, probably due both to the short flight tracks and a complex boundary layer structure at the tower.

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