The interplay between wind and clouds in the trades

Research output: ThesisDissertation (TU Delft)

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Abstract

Cumulus clouds ('fair-weather clouds') form as a result of atmospheric convection and have vertical extents between a few hundred metres (humilis species) and several kilometres (congestus species). They are a major source of uncertainty in the estimation of climate sensitivity by climate models. In order to reach more agreement in cloud changes due to global warming as predicted by different climate models, a better understanding of the physics of these clouds is needed. Shallow cumulus clouds are particularly common over the oceans of Earth's trade-wind regions, which are situated roughly between the 10° and 30° parallels on both hemispheres and are characterised by steady easterly surface winds. These winds are part of the Hadley cell, a large-scale circulation system in which air flows away from the equator at high altitudes and towards the equator near the surface. As a consequence, vertical shear (i.e. vertical differences in wind speed and direction) is common in this region. While recent studies have shown that (surface) wind speed is an important predictor of cloudiness in this region, little work has been done to elucidate how shear affects clouds. Vice versa, clouds also affect the wind by vertically transporting it. While this convective momentum transport (CMT) undoubtedly plays an important role in the force balance that sets the trade winds, only little is known about the details of how CMT sets the vertical structure of the wind and of the spatial scales of the momentum-transporting eddies. In this thesis, more light is shed on the bidirectional interaction between shallow cumulus convection and the wind. Particular focus is put on the effect of wind shear on convection and on the different spatial scales (convective and turbulent) at which convection affects the wind at different heights. To this end, results from numerical large-eddy-simulation (LES) experiments are utilised in this thesis. Due to their fine horizontal resolution (of hundreds of metres and less), LES is able to numerically resolve clouds and the largest turbulent eddies explicitly. This leads to a high degree of realism in the simulation. Together with the possibility to artificially simplify the experimental set-up as well as the completeness of the output (in terms of time, space and quantities), this makes LES the ideal tool to understand physical mechanisms in the atmosphere. To identify and understand the effect of wind shear on cumulus convection, LES experiments were carried out in which typical conditions of the trades were simulated, while the amount of wind shear was systematically varied. In these idealised LES, vertical wind shear effectively limits the deepening of trade-wind convection. Several mechanisms are responsible for this, which depend on the direction of the shear vector (vertically decreasing or increasing wind speed) as well as the altitude at which shear is present. A situation with easterly surface winds that weaken with height and eventually turn westerly is referred to as backward shear, and the opposite situation with easterlies that strengthen with height is called forward shear. Different directions of wind shear cause different surface winds due to CMT, which in turn affect the surface evaporation: Faster surface winds occur in the presence of forward shear and lead to stronger evaporation of sea water, resulting in deeper convection. Forward shear in the subcloud layer also leads to a spatial separation of precipitative downdrafts and emerging updrafts, as clouds move faster than their subcloud-layer roots; this is favourable for convective development. Conversely, under backward shear, the surface evaporation is weakened and precipitative downdrafts interfere with updrafts, hindering convective deepening. However, once clouds grow to sufficient depths, they may produce precipitation so strong that the associated downdrafts spread out laterally near the surface, forming a distinct circular region of cold air, a cold pool. The spreading of this cold pool can cause uplift at its edges, triggering new convection. Backward shear limits the triggering of such secondary convection at cold-pool fronts, while forward shear facilitates it. Finally, shear of any direction in the cloud layer weakens cloud updrafts through an enhanced downward-oriented pressure perturbation force. The limiting effect of wind shear on cloud depth also affects the thermodynamic properties of the cloud layer: The relative humidity is larger and its decrease near the trade-wind inversion is more distinct if clouds are shallower. Large-domain LES hindcasts of specific days during the NARVAL measurement campaigns (which took place in December 2013 and August 2016 in the North Atlantic trades) give a uniquely realistic and complete view on the momentum balance of the trade winds. The combined effect of advection resolved by the model — which here is interpreted as CMT — and unresolved small-scale turbulence is to decelerate the wind in a layer that extends from the surface up to a height of about 2 km in winter and 1 km in summer. However, the role of each term in the balance depends on the altitude. CMT itself acts to accelerate near-surface winds, and only due to strong small-scale turbulence, there is still an overall frictious force at this height. Halfway into the subcloud layer, CMT starts to act as a frictious force. This friction is strengthened by small-scale turbulence from cloud base upwards and quickly diminishes with height. Thus, the cumulus clouds themselves do not introduce significant friction at the altitude where the zonal trade-wind jet resides, which coincides with cloud base. In fact, combined with momentum transport against the wind gradient (counter-gradient momentum transport), they may help to sustain this jet. Overall, wind shear appears to be an important player in setting the typical structure of the trade-wind atmosphere by modulating the depth of convection and may thus even affect cloud-radiative effects. Conversely, convection and turbulence give rise to an overall frictious force on the trade winds. CMT alone acts to accelerate the winds near the surface, which may weaken the Hadley circulation, while in the cloud layer, CMT hardly affects the wind.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Siebesma, A.P., Supervisor
  • Nuijens, A.A., Advisor
Thesis sponsors
Award date22 Sep 2021
Print ISBNs978-94-6416-719-1
DOIs
Publication statusPublished - 2021

Keywords

  • shallow convection
  • cumulus
  • wind shear
  • trade winds
  • momentum budget
  • convective momentum transport
  • large-eddy simulation

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