‘Evolution of a Storm-driven Cloudy Boundary Layer in the Arctic’

To investigate the processes of development and maintenance of low-level clouds during major synoptic events, the cloudy boundary layer under stormy conditions during the summertime Arctic has been studied using observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and large-eddy simulations (LES). On 29 July 1998, a stable Arctic cloudy boundary-layer event was observed after the passage of a synoptic low pressure system. The local dynamic and thermodynamic structure of the boundary layer was determined from aircraft measurements including the analysis of turbulence, cloud microphysics and radiative properties. After the upper cloud layer advected over the existing cloud layer, the turbulent kinetic energy (TKE) budget indicated that the cloud layer below 200 m was maintained predominantly by shear production. Observations of longwave radiation showed that cloud-top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our LES results demonstrate the importance of the combination of shear mixing near the surface and radiative cooling at the cloud top in the storm-driven cloudy boundary layer. Once the low-level cloud reaches a certain height, depending on the amount of cloud-top cooling, the two sources of TKE production begin to separate in space under continuous stormy conditions, suggesting one possible mechanism for the cloud layering. The sensitivity tests suggest that the storm-driven cloudy boundary layer is possibly switched to the shear-driven system due to the advection of upper clouds or to the buoyantly driven system due to the lack of wind shear. A comparison is made of this storm-driven boundary layer with the buoyantly driven boundary layer previously described in the literature.     

Mixed-phase clouds cause climate model biases in Arctic wintertime temperature inversions

Temperature inversions are a common feature of the Arctic wintertime boundary layer. They have important impacts on both radiative and turbulent heat fluxes and partly determine local climate-change feedbacks. Understanding the spread in inversion strength modelled by current global climate models is therefore an important step in better understanding Arctic climate and its present and future changes. Here, we show how the formation of Arctic air masses leads to the emergence of a cloudy and a clear state of the Arctic winter boundary layer. In the cloudy state, cloud liquid water is present, little to no surface radiative cooling occurs and inversions are elevated and relatively weak, whereas surface radiative cooling leads to strong surface-based temperature inversions in the clear state. Comparing model output to observations, we find that most climate models lack a realistic representation of the cloudy state. An idealised single-column model experiment of the formation of Arctic air reveals that this bias is linked to inadequate mixed-phase cloud microphysics, whereas turbulent and conductive heat fluxes control the strength of inversions within the clear state.     

Resolved Versus Parametrized Boundary-Layer Plumes. Part I: A Parametrization-Oriented Conditional Sampling in Large-Eddy Simulations

A conditional sampling based on the combination of a passive tracer emitted at the surface and thermodynamic variables is proposed to characterise organized structures in large-eddy simulations of cloud-free and cloudy boundary layers. The sampling is evaluated against more traditional sampling of dry thermals or clouds. It enables the characterization of convective updrafts from the surface to the top of the boundary layer (or the top of cumulus clouds), describing in particular the transition from the sub-cloud to the cloud layer, and retrieves plume characteristics, entrainment and detrainment rates, variances and fluxes. This sampling is used to analyze the contribution of boundary-layer thermals to vertical fluxes and variances.     

Interpretation of the positive low-cloud feedback predicted by a climate model under global warming

The response of low-level clouds to climate change has been identified as a major contributor to the uncertainty in climate sensitivity estimates among climate models. By analyzing the behaviour of low-level clouds in a hierarchy of models (coupled ocean-atmosphere model, atmospheric general circulation model, aqua-planet model, single-column model) using the same physical parameterizations, this study proposes an interpretation of the strong positive low-cloud feedback predicted by the IPSL-CM5A climate model under climate change. In a warmer climate, the model predicts an enhanced clear-sky radiative cooling, stronger surface turbulent fluxes, a deepening and a drying of the planetary boundary layer, and a decrease of tropical low-clouds in regimes of weak subsidence. We show that the decrease of low-level clouds critically depends on the change in the vertical advection of moist static energy from the free troposphere to the boundary-layer. This change is dominated by variations in the vertical gradient of moist static energy between the surface and the free troposphere just above the boundary-layer. In a warmer climate, the thermodynamical relationship of Clausius-Clapeyron increases this vertical gradient, and then the import by large-scale subsidence of low moist static energy and dry air into the boundary layer. This results in a decrease of the low-level cloudiness and in a weakening of the radiative cooling of the boundary layer by low-level clouds. The energetic framework proposed in this study might help to interpret inter-model differences in low-cloud feedbacks under climate change.     

Stratiform Cloud—Inversion Characterization During the Arctic Melt Season

Data collected during July and August from the Arctic Ocean Experiment 2001 illustrated a common occurrence of specific-humidity (q) inversions, where moisture increases with height, coinciding with temperature inversions in the central Arctic boundary layer and lower troposphere. Low-level stratiform clouds and their relationship to temperature inversions are examined using radiosonde data and data from a suite of remote sensing instrumentation. Two low-level cloud regimes are identified: the canonical case of stratiform clouds, where the cloud tops are capped by the temperature inversion base (CCI—Clouds Capped by Inversion) and clouds where the cloud tops were found well inside the inversion (CII—Clouds Inside Inversion). The latter case was found to occur more than twice as frequently than the former. The characteristic of the temperature inversion is shown to have an influence on the cloud regime that was supported. Statistical analyses of the cloud regimes using remote sensing instruments suggest that CCI cases tend to be dominated by single-phase liquid cloud droplets; radiative cooling at the cloud top limits the vertical extent of such clouds to the inversion base height. The CII cases, on the other hand, display characteristics that can be divided into two situations—(1) clouds that only slightly penetrate the temperature inversion and exhibit a microphysical signal similar to CCI cases, or (2) clouds that extend higher into the inversion and show evidence of a mixed-phase cloud structure. An important interplay between the mixed-phase structure and an increased potential for turbulent mixing across the inversion base appears to support the lifetime of CII cases existing within the inversion layer.     

Mesoscale Variability in the Summer Arctic Boundary Layer

Observations from the summer Arctic Ocean Experiment 2001 (AOE-2001) are analysed with a focus on the interactions between mesoscale and boundary-layer dynamics. Wavelet analyses of surface-pressure variations show daylong periods with different characteristics, some featuring episodes of pronounced high-frequency surface-pressure variability, here hypothesized to be caused by trapped gravity waves. These episodes are accompanied by enhanced boundary-layer turbulence and an enhanced spectral gap, but with only minor influence on the surface stress. During these episodes, mesoscale phenomena were often encountered and usually identified as front-like features in the boundary layer, with a peak in drizzle followed by changing temperature. These phenomena resemble synoptic fronts, though they are generally shallow, shorter-lasting, have no signs of frontal clouds, and do not imply a change in air mass. Based on this analysis, we hypothesize that the root cause of the episodes with high-frequency surface-pressure variance are shallow, mesoscale fronts moving across the pack ice. They may be formed due to local-to-regional horizontal contrasts, for example, between air with different lifetimes over the Arctic or with perturbations in the cloud field causing differential cooling of the boundary layer. Thermal contrasts sharpen as the air is transported with the mean flow. The propagating mesoscale fronts excite gravity waves, which affect the boundary-layer turbulence and also seem to favour entrainment of free tropospheric air into the boundary layer.     

Vertical Heat Transfer in the Lower Atmosphere over the Arctic Ocean During Clear-sky Periods

A diagnostic study of heat transfer within the lower atmosphere and between the atmosphere and the surface of the Arctic Ocean snow/ice pack during clear-sky conditions is conducted using data from the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. Surface heat budgets computed for four cloudy and four clear periods show that, while the net turbulent heat fluxes at the surface are small during the cloudy periods, during the clear-sky periods they are a considerable source of surface heating, balancing significant portions of the conductive heat fluxes from within the snow/ice pack. Analysis of the dynamics and thermodynamics of the lower atmosphere during the clear-sky periods reveals that a considerable portion of the heat lost to the surface by turbulent heat fluxes is balanced by locally strong heating near the atmospheric boundary-layer (ABL) top due to the interaction of subsiding motions with the strong overlying temperature inversions surmounting the ABL. This heat is then entrained into the ABL and transported to the surface by turbulent mixing, maintained by a combination of vertical wind shear and wave-turbulence interactions. The frequency of stable, clear-sky periods, particularly during the winter, combined with these results, suggests that the downward transfer of heat through the lower atmosphere and into the surface represents an important component of the heat budgets of the lower atmosphere and snow/ice pack over the annual cycle     

Observations of a multi-level turbulence structure in a very stable atmospheric boundary layer

Boundary-layer measurements conducted at the Marsta site in Sweden from a winter-time situation (23–25 Feb.) with stable stratification have been analysed. The data comprise wind and temperature profile measurements up to 30 m, turbulence measurements at 2, 6 and 30 m and Doppler acoustic sounder data up to about 150 m. The upwind fetch at the site is flat and free from obstacles to a distance of ca 5 km for the particular sector chosen for the experiment.During the night, a two-layer vertical structure developed. Analysis of power spectra, co-spectra and variances in a shallow and very stable turbulent boundary layer near the ground show that the turbulence is fully developed and follow the universal behaviour.Above, at a height of 30 m, another turbulent layer is produced by increased wind shear near a low-level jet. This turbulent upper layer can be regarded as a layer of free shear flow. At this height, there also exist wave-turbulence interactions at low frequencies which sometimes cause a countergradient heat flux.     

Observations of turbulent boundary-layer interaction with a thunderstorm outflow — A possible wake region energy source

Laboratory and numerical model simulations of turbulent circulations within the wake regions of thunderstorm outflows have been done with the assumption that there is no turbulence within the ambient airmass. Furthermore, many observational studies have used Doppler radar data that have been filtered so that turbulent structures are reduced in amplitude or eliminated altogether. This study presents unique Doppler radar observations of the collision of a roll-like boundary-layer circulation with a gust flow. The boundary-layer circulation is seen to interact with the circulation within the gust flow head and to reappear within the wake region. It is suggested that the ambient boundary layer may be an energy source for the generation and/or maintenance of turbulence in the wake region.     

Numerical simulation of cold easterly circulations over the Canadian Western Plains using a mesoscale boundary-layer model

Arctic outbreaks over the Canadian Western Plains during the late spring period frequently take the form of a cold east-northeasterly flow over a warmer, sloping surface. A mesoscale numerical model is developed in an attempt to simulate such circulations. Following Lavoie (1972) the atmospheric structure of the cold air mass is represented by three layers: a constant flux layer in contact with the earth's surface, a well-mixed planetary boundary layer capped by an inversion, and a deep stratum of overlying stable air. Averaging the set of governing primitive equations through the depth of the mixed layer yields predictive equations for the horizontal wind components, potential temperature, specific humidity, and the height of the inversion. Time-dependent calculations are limited to this layer by parameterizing the interactions between the mixed layer and both the underlying and overlying layers. Precipitation from limited convective clouds, and latent heat within the layer are included in terms of mesoscale variables.A 47.6-km by 47.6-km grid mesh of 1369 points covering the Canadian Prairie Provinces is used to represent the variables. The governing equations are solved numerically with terrain influences, surface roughness, temperature variations, and moisture fluxes allowed to perturb the mixed layer from its initial conditions until resultant mesoscale boundary-layer weather patterns evolve.The mean spring topographic precipitation pattern is successfully reproduced by the simulated late spring upslope flow with limited convective precipitation. Mesoscale planetary boundary-layer weather patterns appear to exert a dominant control over the location and intensity of perturbations in the spring precipitation pattern. The elimination of surface heating significantly reduces the area and intensity of precipitation. A case study based on observed initial conditions showed that the model could reproduce a persistent limited convective precipitation pattern maintained by upslope flow and that a low-level trough exerts a marked influence on the location and the intensity of the precipitation.     

Marine Boundary-Layer Variability Over The Indian Ocean During Indoex (1998)

The variability in boundary-layerstructure over the Indian Ocean during a north-eastmonsoon and the factors influencing it areinvestigated. This study was made possible as acomponent of the Indian Ocean Experiment (INDOEX),conducted from February 19 to March 30, 1998. The dataused are, surface-layer mean and turbulencemeasurements of temperature, humidity and wind, andvertical soundings of temperature and humidity.Significant spatio-temporal variability was observedin the boundary-layer structure throughout the cruise.The ITCZ was characterized as the region withstrongest winds and maximum surface turbulent fluxesof momentum and heat. One of the important findingsfrom this study was a strong influence of continentalair masses on the boundary-layer structure in theNorthern Hemisphere, even at a distance of 600 km offthe Indian coast. This was generally evident in theform of an elevated plume of dry continental airbetween altitudes of 1500 m and 2700 m. Advection ofcontinental aerosols in this layer presents potentialfor significant entrainment into shallow clouds inthis region, which eventually feed deeper clouds atthe ITCZ. This finding provides an explanation foranomalous higher aerosol concentrations found duringprevious studies. The structure of the marineboundary layer was influenced by various factors suchas proximity to land, an anomalous warm pool in theocean and the ITCZ. In the southern hemisphere, theboundary-layer height was primarily governed bysurface-layer sensible heat flux and was found to behighest in the vicinity of the ITCZ. North of theequator it was strongly influenced by land-air-seainteractions. In addition to this synoptic modulation,there was also a significant diurnal variability inthe boundary-layer height.     

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.     

Do stratocumulus clouds detrain? FIRE I data revisited

Analyses of aircraft observations of the stratocumulus-topped boundary layer during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE I) show the frequent presence of clear, but relatively moist, air patches near the stratocumulus cloud-top interface. A conditional sampling of measurements in these clear air patches shows that their thermodynamic properties do more resemble boundary-layer air characteristics than those of free troposphere air. From an aircraft leg through cloud tops it is demonstrated that turbulent mixing across the cloud-top interface can lead to the local dissipation of the cloud top. Analogous to the terminology used for shallow cumulus parameterizations this process can be considered as detrainment, with which we mean that after a mixing event across the cloud-top boundaries, mixed unsaturated parcels become part of the clear environment of the cloud.     

The Atmospheric Boundary-Layer Structure Within A Cold Air Outbreak: Comparison Of In Situ, Lidar And Satellite Measurements With Three-Dimensional Simulations

A cold-air outbreak over the Mediterranean, associated with a Tramontane event, has been simulated with the atmospheric non-hydrostatic model Meso-NH using a horizontal resolution of 2 km. Results are compared with in situ aircraft, airborne lidar and satellite measurements. On average, the mean and turbulent parameters simulated in the surface layer and mixed layer compared well with in situ measurements. The model was able to reproduce accurately the Foehn effect in the wake of Cape Creus, as well as the occurence of rolls in the coastal region in connection with cloud streets observed with AVHRR. Over the sea, the threshold value of turbulent kinetic energy defining the height of the atmospheric boundary-layer top in the model (defined as 25% of the maximum turbulent kinetic energy in the profile) enables the simulated atmospheric boundary-layer height to match the one retrieved from lidar measurements. Nevertheless, the model did not handle very well the abrupt gradients of all meteorological parameters observed at the top of the atmospheric boundary-layer. Reasons for this are investigated.     

The sensitivity of k-ɛ model computations of the neutral planetary boundary layer to baroclinicity

While the importance of baroclinicity in determining the structure of the planetary boundary layer (PBL) is well recognized, the actual effect of baroclinicity on the structure is not well understood. Results based on simulations obtained using the turbulent kinetic energy-dissipation rate of turbulent kinetic energy closure model of the turbulent flow in a neutral baroclinic PBL provide additional insight into the role of baroclinicity. The baroclinic PBL is characterized by significant shear production of turbulent kinetic energy throughout the complete boundary-layer depth. The turbulent mixing length is bounded by the presence of a stable temperature inversion layer indicating that the depth of the baroclinic PBL is determined by the inversion height. Significant turbulent shear stresses exist throughout the baroclinic PBL and the air is relatively well-mixed except in the surface layer.     

Simultaneous determinations of the sensible and latent heat transfer coefficients for tree leaves

The heat and mass transfer coefficients for exchange across the fluid dynamic boundary layer over tree leaves were simultaneously determined in a controlled environment chamber. The mass transfer coefficients were calculated from measured values of evaporation, air specific humidity and a value of leaf specific humidity at leaf temperature. The heat transfer coefficients were calculated from measured values of air temperature, leaf temperature and an estimate of the sensible heat flux density calculated as the measured net radiation at the leaf surfaces minus the latent heat flux density. The experiments described in this paper indicate that the equations based on laminar boundary-layer theory can give reasonable estimates of the transfer coefficients of real tree leaves for the velocities most commonly experienced in plant canopies, if they are adjusted by a constant multiplier greater than one. Calculations of local mass transfer coefficients based on temperature measurements at three locations at different distances from the leading edge of the leaves, indicate that the deviation from theory is probably the result of transition to turbulent boundary-layer flow at some distance from the leading edge.     

Modelling the mean and turbulent structure of the summertime Arctic cloudy boundary layer

This paper addresses the problem of modelling the summertime Arctic cloudy boundary layer. Specifically we consider the problem of multi-layered clouds in the boundary layer that includes the decoupling of the turbulence between upper and lower clouds. A high-resolution one-dimensional model with second-order turbulence closure and spectral radiative transfer is used to simulate a case study that was obtained during the 1980 Arctic Stratus Experiment. The effects of radiation, large-scale vertical motion and drizzle are investigated in sensitivity studies. Results of this study show that radiative transfer is important to the maintenance of the multiple cloud layers, and suggest that weak rising vertical motion is the most favorable situation to maintain two separate cloud layers.     

A process oriented characterization of tropical oceanic clouds for climate model evaluation, based on a statistical analysis of daytime A-train observations

This paper aims at characterizing how different key cloud properties (cloud fraction, cloud vertical distribution, cloud reflectance, a surrogate of the cloud optical depth) vary as a function of the others over the tropical oceans. The correlations between the different cloud properties are built from 2?years of collocated A-train observations (CALIPSO-GOCCP and MODIS) at a scale close to cloud processes; it results in a characterization of the physical processes in tropical clouds, that can be used to better understand cloud behaviors, and constitute a powerful tool to develop and evaluate cloud parameterizations in climate models. First, we examine a case study of shallow cumulus cloud observed simultaneously by the two sensors (CALIPSO, MODIS), and develop a methodology that allows to build global scale statistics by keeping the separation between clear and cloudy areas at the pixel level (250, 330?m). Then we build statistical instantaneous relationships between the cloud cover, the cloud vertical distribution and the cloud reflectance. The vertical cloud distribution indicates that the optically thin clouds (optical thickness <1.5) dominate the boundary layer over the trade wind regions. Optically thick clouds (optical thickness >3.4) are composed of high and mid-level clouds associated with deep convection along the ITCZ and SPCZ and over the warm pool, and by stratocumulus low level clouds located along the East coast of tropical oceans. The cloud properties are analyzed as a function of the large scale circulation regime. Optically thick high clouds are dominant in convective regions (CF?>?80?%), while low level clouds with low optical thickness (<3.5) are present in regimes of subsidence but in convective regimes as well, associated principally to low cloud fractions (CF?<?50?%). A focus on low-level clouds allows us to quantify how the cloud optical depth increases with cloud top altitude and with cloud fraction.     

The Summer Arctic Boundary Layer during the Arctic Ocean Experiment 2001 (AOE-2001)

Boundary-layer measurements made from the Swedish icebreaker Oden during the Arctic Ocean Experiment 2001 (AOE-2001) are analysed. They refer mainly to ice drift in the central Arctic during the period 2–21 August 2001. On board Oden a remote sensing array with a wind profiler, cloud radar and a scanning microwave radiometer, and a regular weather station operated continuously; soundings were also released during research stations. Turbulence and profile measurements on an 18-m mast were deployed on the ice, along with two sodar systems, a microbarograph array and a tethered sounding system. Surface flux and meteorological stations were also deployed on nearby ice floes. There is a clear diurnal cycle in radiation and also in wind speed, cloud base and visibility. It is absent in temperature and humidity, probably due to the very strong control by melting/ freezing ice and snow. In the advection of warm air, latent heat of melting maintains the surface temperature at 0 °C, while with a negative energy balance the latent heat of freezing of the salty ocean water acts to maintain the surface temperature > −2 °C. The constant presence of water at the surface maintains a relative humidity close to 100%, and this is also often facilitated by an increasing specific humidity through the capping inversion, making entrainment a moisture source. This ensures cloudy conditions, with low cloud and fog prevailing most of the time. Intrusions of warm and moist air from beyond the ice edge are frequent, but the local Arctic boundary layer remains at a relatively constant temperature, and is shallow and well mixed with strong capping inversions. Power spectra of surface-layer wind speed sometimes show large variance at low frequency. A scanning radiometer provides a monitoring of the vertical thermal structure with a spatial and temporal resolution not seen before in the Arctic. There are often two inversions, an elevated main inversion and a weak surface inversion, and occasionally additional inversions occur. Enhanced entrainment across the main inversion appears to occur during frontal passages. Variance of the scanning radiometer temperatures occurs in large pulses rather than varying smoothly, and the height to the maximum variance appears to be a reasonable proxy for the boundary-layer depth.     

The Relation Between Ageostrophy and Boundary-Layer Structure in the Western-Pacific Trades (Wake Island)

Eight years (1980–1987) of Wake Island rawinsonde data are used to derive atmospheric boundary layer (ABL) depth, integrated boundary-layer moisture, and a measure of boundary-layer ageostrophy. The variability in these processes controls the accumulation of moisture and heat in the tradewind regions and their transport to regions of intense convection. Preliminary analyses using different methods reveal quasi-periodic signals in these data in the 30–60 days range. Cross correlation calculations in this intraseasonal range show that these ABL variables are coherent with each other and with the low-level flow. The integrated ABL variables and the ABL height exhibit local in-phase relationships. At higher frequencies, the analyses show intense diurnal variation of boundary-layer height but only a weak diurnal signal in integrated ABL properties. At the lower frequency range, the analyses show a significant reduction in the amplitude of the seasonal and intraseasonal variation in ageostrophy during the strong El-Niño event of 1982/1983. The results clearly establish a relationship between integrated water vapour and divergent ABL processes (Ekman pumping/suction) in which shallower (deeper) ABLs are associated with mass and moisture divergence (convergence) and higher (lower) sea-level pressure. A possible interpretation in terms of a remote dynamic response of the trade inversion and ABL processes to equatorial deep convection is suggested.