Observed influence of moist convection and cloudiness on boundary layer wind and momentum flux profiles.

2020 ◽  
Author(s):  
Mariska Koning ◽  
Louise Nuijens ◽  
Fred Bosveld ◽  
Pier Siebesma ◽  
Remco Verzijlbergh ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Wind Shear ◽  
Momentum Flux ◽  
Cloud Cover ◽  
Cloud Layer ◽  
Momentum Transport ◽  
Clear Sky ◽  
Momentum Fluxes ◽  
Wind Profiles

<p>Convective momentum transport (CMT) measurements are scarce, but important to constrain the impacts of CMT on wind profiles, variability of the wind and possibly the large-scale circulation.</p><p>We investigate how wind profiles and momentum fluxes change with cloudiness and convection. With stronger convection, we expect that the wind shear in the lowest 200m, wherein wind turbines are located, reduces. Cumulus days are generally strongly convective and hence well mixed. They are expected to differ from clear-sky days: the boundary layer is deeper, and cumulus may induce a different (thermal) circulation in the sub-cloud layer. Comparing cumulus and other days fairly, we must be mindful of the changes in convection strength with cloud cover, time of the day, seasons, and the wind strength that impacts the wind shear magnitude.</p><p>This study uses nine years of data from the Cabauw observatory, The Netherlands, containing 10-minute averages of wind speed, wind direction, and momentum fluxes from a 200 m tall tower along with cloud-base heights from a ceilometer. Realistic fine-scale Large Eddy Simulation (LES) hindcasts over the same time period and a 5km<sup>3</sup> domain over Cabauw provide insight into the processes at higher altitude. In both observations and LES, days with rooted clouds, which have strong connection to the sub-cloud layer, are separated from clear-sky days and days in which clouds only impact the convection through radiation effects. Days with rooted clouds are subsequently divided into three groups of increasing cloud cover: 5-30% (shallow clouds), 30-70% (somewhat deeper clouds) and >70% (overcast).</p><p>Both observations and LES show that shear in the near-surface wind speed (NSWS) reduces with stronger insolation, which is expected: more insolation causes a more unstable atmosphere, stronger convection, thus more mixing. In a weakly unstable atmosphere, rooted clouds (5-70% cloud cover) generally have better mixed winds (less normalised shear). The NSWS accelerates more from morning to afternoon on these days, indicating that not only the mixing is stronger, but also that downward mixing of higher momentum by the clouds affects the wind in the lowest 200m. If this is true, the assumption of Monin-Obukhov Similarity Theory (MOST) that large convective eddies are not important in the surface layer, does not hold. This possibly has a great impact on surface-flux parametrizations based on MOST, which are used by many numerical models, from local and mesoscale to global models. Analysing surface-layer scaling for momentum, we test whether this assumption is indeed violated in such cases.</p><p>Momentum transport profiles in LES show that when deeper clouds with larger cloud cover are present, transport in the cloud layer is larger. In the cross-wind component of the profile, the four categories show different deceleration in the mixed layer, and different acceleration near the top of the mixed layer. Likely, the stronger inversion-jump in the cross-wind causes this momentum flux character.</p><p>With this study, we provide an overview of the effects that have been observed in different cloudiness and convective conditions and gained understanding of the important processes and implications of the cloud effects on momentum transport.</p>

scholarly journals Surface-layer wind shear and momentum transport from clear-sky to cloudy weather regimes over land

2021 ◽  
Author(s):  
Ada Mariska Koning ◽  
Louise Nuijens ◽  
Fred C. Bosveld ◽  
Pier Siebesma ◽  
Pim A. van Dorp ◽  
...  
Keyword(s):  
Surface Layer ◽  
Wind Shear ◽  
Momentum Transport ◽  
Weather Regimes ◽  
Clear Sky ◽  
Cloudy Weather

scholarly journals The influence of convective momentum transport and vertical wind shear on the evolution of a cold air outbreak

2020 ◽  
Author(s):  
Beatrice Saggiorato ◽  
Louise Nuijens ◽  
A. Pier Siebesma ◽  
Stephan de Roode ◽  
Irina Sandu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Surface Wind ◽  
Cloud Layer ◽  
Momentum Transport ◽  
Small Scale ◽  
Near Surface ◽  
Cold Air ◽  
Cold Air Outbreak ◽  
Convective Momentum Transport

<p>To study the influence of convective momentum transport (CMT) on wind, boundary layer and cloud evolution in a marine cold air outbreak (CAO) we use Large-Eddy Simulations subjected to different baroclinicity (wind shear) but similar surface forcing. The simulated domain is large enough ( ≈100 × 100 km<sup>2</sup>) to develop typical mesoscale cellular convective structures.  We find that a maximum friction induced by momentum transport (MT) locates in the cloud layer for an increase of geostrophic wind with height (forward shear, FW) and near the surface for a decrease of wind with height (backward shear, BW). Although the total MT always acts as a friction, the interaction of friction-induced cross-isobaric flow with the Coriolis force can develop super-geostrophic winds near the surface (FW) or in the cloud layer (BW). The contribution of convection to MT is evaluated by decomposing the momentum flux by column water vapor and eddy size, revealing that CMT acts to accelerate sub-cloud layer winds under FW shear and that mesoscale circulations contribute significantly to MT for this horizontal resolution (250 m), even if small scale eddies are non-negligible and likely more important as resolution increases. Under FW shear, a deeper boundary layer and faster cloud transition are simulated, because MT acts to increase surface fluxes and wind shear enhances turbulent mixing across cloud tops. Our results show that the coupling between winds and convection is crucial for a range of problems, from CAO lifetime and cloud transitions to ocean heat loss and near-surface wind variability.</p>

scholarly journals Turbulence Adjustment and Scaling in an Offshore Convective Internal Boundary Layer: A CASPER Case Study

2020 ◽  
Vol 77 (5) ◽  
pp. 1661-1681
Author(s):  
Qingfang Jiang ◽  
Qing Wang ◽  
Shouping Wang ◽  
Saša Gaberšek
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Kinetic Energy ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Internal Boundary Layer ◽  
Turbulence Kinetic Energy ◽  
Internal Boundary ◽  
Field Observations ◽  
The Mean

Abstract The characteristics of a convective internal boundary layer (CIBL) documented offshore during the East Coast phase of the Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER-EAST) field campaign has been examined using field observations, a coupled mesoscale model (i.e., Navy’s COAMPS) simulation, and a couple of surface-layer-resolving large-eddy simulations (LESs). The Lagrangian modeling approach has been adopted with the LES domain being advected from a cool and rough land surface to a warmer and smoother sea surface by the mean offshore winds in the CIBL. The surface fluxes from the LES control run are in reasonable agreement with field observations, and the general CIBL characteristics are consistent with previous studies. According to the LESs, in the nearshore adjustment zone (i.e., fetch < 8 km), the low-level winds and surface friction velocity increase rapidly, and the mean wind profile and vertical velocity skewness in the surface layer deviate substantially from the Monin–Obukhov similarity theory (MOST) scaling. Farther offshore, the nondimensional vertical wind shear and scalar gradients and higher-order moments are consistent with the MOST scaling. An elevated turbulent layer is present immediately below the CIBL top, associated with the vertical wind shear across the CIBL top inversion. Episodic shear instability events occur with a time scale between 10 and 30 min, leading to the formation of elevated maxima in turbulence kinetic energy and momentum fluxes. During these events, the turbulence kinetic energy production exceeds the dissipation, suggesting that the CIBL remains in nonequilibrium.

Influence of Swell on Marine Surface-Layer Structure

2020 ◽  
Vol 77 (5) ◽  
pp. 1865-1885 ◽  
Author(s):  
Qingfang Jiang
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Convective Boundary Layer ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Convective Boundary ◽  
Stable Conditions ◽  
Marine Surface Layer ◽  
Marine Surface

Abstract The influence of swell on turbulence and scalar profiles in a marine surface layer and underlying physics is examined in this study through diagnosis of large-eddy simulations (LES) that explicitly resolve the surface layer and underlying swell. In general, under stable conditions, the mean wind and scalar profiles can be significantly modified by swell. The influence of swell on wind shear, turbulence structure, scalar profiles, and evaporation duct (ED) characteristics becomes less pronounced in a more convective boundary layer, where the buoyancy production of turbulence is significant. Dynamically, swell has little direct impact on scalar profiles. Instead it modifies the vertical wind shear by exerting pressure drag on the wave boundary layer. The resulting redistribution of vertical wind shear leads to changes in turbulence production and therefore turbulence mixing of scalars. Over swell, the eddy diffusivities from LES systematically deviate from the Monin–Obukhov similarity theory (MOST) prediction, implying that MOST becomes invalid over a swell-dominated sea. The deviations from MOST are more pronounced in a neutral or stable boundary layer under relatively low winds and less so in a convective boundary layer.

scholarly journals Observations of Boundary Layer Wind and Turbulence of a Landfalling Tropical Cyclone

2021 ◽  
Author(s):  
Zhongkuo Zhao ◽  
Ruiquan Gao ◽  
Jun A. Zhang ◽  
Yong Zhu ◽  
Chunxia Liu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Radial Distance ◽  
Flux Measurement ◽  
Eddy Diffusivity ◽  
Multiple Level ◽  
A Value ◽  
Momentum Fluxes ◽  
Speed Range ◽  
Wind Profiles

Abstract This study analyzed the atmospheric boundary layer characteristics based on the multiple level observations by a 350-m height tower during the landfall of Super Typhoon Mangkhut (1822). Mean wind profiles showed logarithmic wind profiles at different wind speed ranges suggesting nearly constant flux layers. The height of the constant layer increased with the wind speed and deceased with the radial distance from the storm centre. This behaviour was supported by flux observations. Momentum fluxes and turbulent kinetic energy increased with the wind speed at all flux measurement levels. The drag coefficient (surface roughness) estimated was nearly a constant with a value of 8´10-3 (0.09 m). Both the estimated eddy diffusivity and mixing length varied with height. The eddy diffusivity also varied with the wind speed. Our results supported that the eddy diffusivity is larger over land than over ocean in a same wind speed range.

scholarly journals Twenty-Four-Hour Observations of the Marine Boundary Layer Using Shipborne NOAA High-Resolution Doppler Lidar

2005 ◽  
Vol 44 (11) ◽  
pp. 1723-1744 ◽  
Author(s):  
Volker Wulfmeyer ◽  
Tijana Janjić
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Shear ◽  
Marine Boundary Layer ◽  
Sea Surface ◽  
Doppler Lidar ◽  
Sensible Heat ◽  
Wind Profiles

Abstract Shipborne observations obtained with the NOAA high-resolution Doppler lidar (HRDL) during the 1999 Nauru (Nauru99) campaign were used to study the structure of the marine boundary layer (MBL) in the tropical Pacific Ocean. During a day with weak mesoscale activity, diurnal variability of the height of the convective MBL was observed using HRDL backscatter data. The observed diurnal variation in the MBL height had an amplitude of about 250 m. Relations between the MBL height and in situ measurements of sea surface temperature as well as latent and sensible heat fluxes were examined. Good correlation was found with the sea surface temperature. The correlation with the latent heat flux was lower, and practically no correlation between the MBL height and the sensible heat and buoyancy fluxes could be detected. Horizontal wind profiles were measured using a velocity–azimuth display scan of HRDL velocity data. Strong wind shear at the top of the MBL was observed in most cases. Comparison of these results with GPS radiosonde data shows discrepancies in the wind intensity and direction, which may be due to different observation times and locations as well as due to multipath effects at the ship’s platform. Vertical wind profiles corrected for ship’s motion were used to derive vertical velocity variance and skewness profiles. Motion compensation had a significant effect on their shape. Normalized by the convective velocity scale and by the top of the mixed layer zi, the variance varied between 0.45 and 0.65 at 0.4z/zi and decreased to 0.2 at 1.0z/zi. The skewness ranged between 0.3 and 0.8 in the MBL and showed in almost all cases a maximum between 1.0z/zi and 1.1z/zi. These profiles revealed the existence of another turbulent layer above the MBL, which was probably driven by wind shear and cloud condensation processes.

The Relationships among Wind, Horizontal Pressure Gradient, and Turbulent Momentum Transport during CASES-99

2013 ◽  
Vol 70 (11) ◽  
pp. 3397-3414 ◽  
Author(s):  
Jielun Sun ◽  
Donald H. Lenschow ◽  
Larry Mahrt ◽  
Carmen Nappo
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Pressure Gradient ◽  
Coriolis Force ◽  
Wind Direction ◽  
Momentum Flux ◽  
Momentum Transport ◽  
Horizontal Pressure Gradient ◽  
Horizontal Pressure ◽  
Turbulent Momentum

Abstract Relationships among the horizontal pressure gradient, the Coriolis force, and the vertical momentum transport by turbulent fluxes are investigated using data collected from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). Wind toward higher pressure (WTHP) adjacent to the ground occurred about 50% of the time. For wind speed at 5 m above the ground stronger than 5 m s−1, WTHP occurred about 20% of the time. Focusing on these moderate to strong wind cases only, relationships among horizontal pressure gradients, Coriolis force, and vertical turbulent transport in the momentum balance are investigated. The magnitude of the downward turbulent momentum flux consistently increases with height under moderate to strong winds, which results in the vertical convergence of the momentum flux and thus provides a momentum source and allows WTHP. In the along-wind direction, the horizontal pressure gradient is observed to be well correlated with the quadratic wind speed, which is demonstrated to be an approximate balance between the horizontal pressure gradient and the vertical convergence of the turbulent momentum flux. That is, antitriptic balance occurs in the along-wind direction when the wind is toward higher pressure. In the crosswind direction, the pressure gradient varies approximately linearly with wind speed and opposes the Coriolis force, suggesting the importance of the Coriolis force and approximate geotriptic balance of the airflow. A simple one-dimensional planetary boundary layer eddy diffusivity model demonstrates the possibility of wind directed toward higher pressure for a baroclinic boundary layer and the contribution of the vertical turbulent momentum flux to this phenomenon.

scholarly journals Low cloud reduction within the smoky marine boundary layer and the diurnal cycle

2019 ◽  
Author(s):  
Jianhao Zhang ◽  
Paquita Zuidema
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Cloud Cover ◽  
Marine Boundary Layer ◽  
Field Measurements ◽  
Cloud Layer ◽  
Free Troposphere ◽  
Liquid Water Path ◽  
Low Cloud ◽  
Low Cloud Cover

Abstract. Previous observational studies of the southeast Atlantic emphasize an increase in the stratocumulus cloud cover when shortwave-absorbing aerosols are present in the free-troposphere. Recent field measurements at Ascension Island (8° S, 14.5° W) reveal that smoke is also often present in the marine boundary layer, most evident in August when the smoke is highly absorbing of sunlight, the boundary layer is deeper, the cloud-top inversion is weaker, and a climatologically lower cloud fraction eases penetration of the sunlight to the surface, compared to later months. In these conditions, the low cloud cover decreases further with enhanced smoke loadings, reflecting a boundary layer semi-direct effect that is a positive feedback. The low cloud cover reduction is particularly pronounced in the afternoon, although the cloud liquid water path is more strongly reduced at night. The daily-mean surface-based mixed layer is warmer by approximately 0.5 K when more smoke is present in the boundary layer, with a warming peak in the late afternoon when the cloud cover reduction is largest. After sunset, sub-cloud moisture accumulates throughout the night, increasing the moisture stratification with the cloud layer. This increase in boundary layer decoupling is consistent with reduced turbulence. A new observation is that in the sunlit morning hours, the smokier boundary layer deepens by approximately 200 m, and both the liquid water paths and cloud top heights increase. We speculate this reflects radiatively-induced vertical ascent originating from within a well-mixed smoke-filled sub-cloud layer. Overall, the reduction in daytime low cloud decreases the top-of-atmosphere all-sky albedo, despite an increase in the top-of-atmosphere direct aerosol radiation of ~ 6.5 W m2 between the (most-least) smoky tercile composites. A convolving meteorological influence is also apparent near the cloud top, in that, although the free troposphere is also often smoky in August, the associated above-cloud potential temperatures are often cooler, rather than warmer, and better-mixed. The cooling weakens the inversion beyond that expected from the warming of the boundary layer and further encourages entrainment of more smoke into the already smoky boundary layer, increasing the longevity of the boundary layer smoke events. The free-tropospheric winds are also typically stronger and more easterly. More smoke appears to settle into the sub-cloud layer during the day than at night when it is smoky, speculated to reflect a deeper daytime sub-cloud layer facilitating entrainment, when the nighttime stratification does not.

scholarly journals The diurnal stratocumulus-to-cumulus transition over land in southern West Africa

2020 ◽  
Vol 20 (5) ◽  
pp. 2735-2754 ◽  
Author(s):  
Xabier Pedruzo-Bagazgoitia ◽  
Stephan R. de Roode ◽  
Bianca Adler ◽  
Karmen Babić ◽  
Cheikh Dione ◽  
...  
Keyword(s):  
Boundary Layer ◽  
West Africa ◽  
Wind Shear ◽  
Numerical Experiments ◽  
Inversion Layer ◽  
Surface Fluxes ◽  
Cloud Layer ◽  
Cloud Base ◽  
Scale Models ◽  
The Impact

Abstract. The misrepresentation of the diurnal cycle of boundary layer clouds by large-scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary to improve its representation. To this end, the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) measurement campaign gathered a unique dataset of observations of the frequent stratocumulus-to-cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using large eddy simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget. Finally, we study the impact of mean wind and wind shear at the cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from the surface to the cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for the maintenance leading to a net thickening of the cloud layer of about 18 g m−2 h−1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at the cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at the cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence, wind shear at the cloud top accelerates the breakup and transition by about 2 h. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger-scale models.

Sign in / Sign up

Export Citation Format

Share Document