Convection2008

Convection2008 - Dry and Moist Convection in the Atmosphere...

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Dry and Moist Convection in the Atmosphere •General principals of convection & examples •Daytime dry convective boundary layer •Shallow and deep cloud convection
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Convection- general principles • Driven by buoyancy – that is air is heavier or lighter than its surroundings • Lighter air rises – heavier air sinks • Dynamically is non-hydrostatic – that is vertical equation of motion involves imbalance between pressure force and force of gravity • Buoyancy from temperature or mass difference from surroundings. • Buoyant air has potential energy that converts to kinetic energy of rising motion.
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General principles-cont. • Convective motion is cellular – sometimes with some organization/pattern. • Cells consist of upward and downward branches and connecting horizontal flow. • Usually kinetic energy generated in only one branch – the active branch – other branch loses kinetic energy to potential energy – the inactive branch. • Net generation of KE to maintain against friction by active branch being narrower, i.e. distribution of vertical motion skewed (w’**3) toward direction of active generation
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Buoyancy • The quantity ρ ’ g / ρ 0 is the vertical acceleration of air per unit mass due to its density being different than that of its surroundings – this is referred to as the buoyancy. Density varies either because of variation of temperature or humidity or liquid water– the first two can be lumped together using the concept of virtual temperature. Another such idea for liquid water.
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Radiative drivers of buoyancy/convection • Daytime surface heating – some goes into heating air – drives large convection cells that are the dominant component of the daytime boundary layer – leading to rapid mixing of air over this layer. • Cloud top cooling – air at top of clouds
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Vertical equation of motion is: ρ dw/dt =- ρ g – dp/dz Acceleration = sinking by gravity + push by pressure from high to low. Assume dry air. Subtract basic state from p and ρ. ρ = ρ − ρ 0 ( z ), p’ = p p 0 ( z ) , where dp 0 /dz = - ρ 0 g (hydrostatic balance), The term dp’/dz is only important for acoustic waves. For boundary layer convection use: ρ 0 dw/dt =- ρ ’g , •Heating directly changes temperature. Use gas law to get density in terms of T– i.e. T’/T 0 = - ρ ’/ ρ 0 •Where moisture aggects density, include through “virtual temperature” T v , i.e. the temperature that would give the density for dry air
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Daytime dry convective boundary layer • Net radiative heating at surface warms air and drives convection have near adiabatic lapse rate and near uniform water vapor (constant virtual potential temperature). • For uniform mixing, can use vertical averages as state variables – temperature and column water vapor content.
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Convection2008 - Dry and Moist Convection in the Atmosphere...

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