Boundary-Layer Meteorology (BLMet)

Mesoscale Processes

Research Projects

Boundary-layer ventilation by mid-latitude weather systems

• Pollutants are predominately emitted near the Earth's surface into the atmospheric boundary layer, where if they remain can have a negative impact on human health.

  

  Alternatively pollutants can be vented out of the boundary layer into the free troposphere, where they can be transported long distances and have a large affect on regional air quality. Therefore this work develops an understanding of how and where the boundary layer is ventilated, and also how efficient this ventilation is. To do this it is also necessary to understand how the structure of the boundary layer (in terms of depth, stability and surface fluxes) varies beneath different synoptic weather systems, and how the boundary layer structure changes as the systems evolve. Therefore this work also aims to develop a conceptual model of boundary layer structure beneath large-scale weather systems.
  
• A baroclinic lifecycle model, (the Reading Intermediate Global Circulation Model - IGCM) is used to simulate 2 well studied lifecycles, LC1 and LC2. A first-order closure mixing length boundary layer scheme included and the simulations are dry with no convection present. A passive tracer is included to represent pollution. It is initiated at the initial time with a uniform concentration at the lowest model level and then is acted upon by the turbulent motions diagnosed by the boundary layer scheme and the large scale resolved winds.
  
• Results have shown that boundary layer turbulent mixing the first and an important step in boundary layer ventilation. Turbulent motions act to mix the tracer away from the surface source to the mid to upper regions of the boundary layer. Once the tracer has been lifted to these levels large scale motions associated with synoptic systems can then act to re-distribute the tracer.
• It is found that the warm conveyor belt is a efficient process for ventilation the boundary layer. It is found to originate in the boundary layer at ~40°N to 45°N ahead of the cold front in the cyclones warm sector. The warm conveyor belt then transports tracer out of the boundary layer, polewards and up to ~4km. The warm conveyor belt is observed to split into 2 components, one of which curves anticyclonically ahead of the warm front and the other which curves cyclonically back behind the centre of the low.
  
• Animations of the transport of tracer by the baroclinic systems are available at
  LC1 animation
  LC2 animation
• The figure shows the tracer flux (in kg/s) out of the boundary layer. The black lines are surface pressure (contour interval of 4mb) and the while lines are the vertical velocity on the boundary layer top (0.5cm/s and 1cm/s contour shown). This shows the region where most tracer is vented out of the boundary layer is found ahead of the cold front and is mainly between 40 - 45N. The region of maximum ventilation is not directly correlated with the region of maximum ascent but depends also on the availability of tracer in the source regions.
  

Non-frontal ventilation

• It is often assumed that ventilation of the atmospheric boundary layer is weak in the absence of fronts. But is this always true?
• The UK Met Office Unified Model has been used to simulate the transport processes that occurred during a non-frontal day on the 9 May 2005. This was a typical UK summer day with little wind and scattered shallow cumulus convection. The ventilation processes observed include coastal outflow, ventilation by the sea breeze circulation, ventilation by turbulent mixing and large-scale ascent, and ventilation by convection.
• Pollution sources are represented by the constant emission of a passive tracer everywhere over land. Model simulated vertical tracer distribution qualitatively compared well with AMPEP (Aircraft Measurement of chemical Processing Export fluxes of Pollutants over the UK) field measurements in most cases. Therefore we can conclude that numerical weather prediction model output is a useful tool in these situations and can be used to complement observational results in studying transport processes.
• Budget calculations of tracers were performed in order to determine the importance of these ventilation processes. It was found that significant ventilation of the boundary layer can occur in the absence of fronts. Turbulent mixing and convection processes can double the amount of pollution ventilated from the boundary layer.

Frictional processes in mid-latitude cyclone development

In this exciting new area, we have recently looked at the effect of boundary layer processes on mid-latitude cyclonic evolution using a potential vorticity (PV) approach. The picture below shows the ascent of PV on day 8 of a baroclinic life cycle with surface drag.

• This work develops an understanding of the physical mechanisms responsible for the frictional effects observed in cyclone development. To do this, a first-order closure mixing-length boundary layer scheme has been added to a baroclinic life cycle model to accurately represent the frictional processes occurring in cyclone development. Life cycles simulated with the model consist of normal mode baroclinic growth with cyclone development followed by a period of barotropic decay. By considering life cycles where friction is the only diabatic process acting, we find that surface drag reduces rates of baroclinic growth and barotropic decay by around 40%.
• The classical description of frictional effects in cyclogenesis involves the Ekman spindown of a barotropic vortex. We have studied this mechanism by considering the quasi-geostrophic omega-equation with a frictional term. However, these barotropic vortex ideas do not account for the baroclinic processes occurring, especially within the frontal regions. To address these shortcomings, we have adopted a potential vorticity (PV) approach. Large frictionally generated positive PV anomalies form close to developing warm and cold fronts near the surface. This is due to the relative alignment of surface and low level thermal wind vectors. These PV anomalies are advected upwards and polewards along the warm conveyor belt, are fluxed out of the boundary layer near the warm front, and are then advected westwards along a branch of the warm conveyor belt that peels off the main flow and moves cyclonically towards the surface low centre. This is shown in the animation: The blue lines indicate the $10^5s^{-1}$ surface relative vorticity contour, outlining the frontal regions and surface low centre (marked with a cross). The black lines are isentropes (contour interval 5K) and the colour is the PV on day 8 of a baroclinic life cycle with surface drag. Red indicates a positive PV anomaly. The four figures comprising the animation indicate the PV and theta at four levels within and just above the boundary layer. The advection of this frictionally generated PV out of the boundary layer results in a band of positive PV associated with high static stability in the lower troposphere above the surface low centre. Using Rossby edge wave theory, we have proposed a mechanism to explain the reduced baroclinic development observed in terms of this positive PV anomaly. Since the positive PV anomalies are generated within the frontal regions, it follows that the baroclinic dynamics play a crucial role in the frictional modification of cyclone development. The classical notion of Ekman spindown is shown to be of secondary importance. This mechanism by which frictional processes reduce cyclone development is found also to be valid in the presence of sensible and latent heat fluxes.

Publications

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