1Chapter 7Stability and Cloud DevelopmentAtmospheric Stability2Cloud Development - stable environment• Stable air (parcel) - vertical motion is inhibited – if clouds form, they will be shallow, layered clouds like stratus • Cloud Development - unstable environment– Unstable air (parcel) - vertical motion occurs – commonly produces cumulus, cumulonimbus clouds • A ball in water: – When does it rise?– When does it sink?• An air parcel:– Density– Lighter than environment, it will rise.– Heavier than environment, it will sink. • What determine air density at a given pressure:– Temperature How to determine atmospheric Stability?3Determining Air Parcel Temperature: Rising air parcels and adiabatic cooling• consider a rising parcel of air -->>• As the parcel rises, it will adiabatically expand and cool • adiabatic - a process where the parcel temperature changes due to an expansion or compression, no heat is added or taken away from the parcelConsider a sinking parcel of air -->>Adiabatic lapse rate• As a parcel of air rises, it cools, but at what rate?• Lapse rate– rate of temperature change with height– units of lapse rate are °C km-1• Dry-adiabatic lapse rate– unsaturated parcels cool at a rate of 10°C km-1- this is called the dry-adiabatic lapse rate• Moist Adiabatic Lapse Rate– For a saturated parcel of air, i.e., when its T=Td, then it cools at the moist adiabatic lapse rate = 6°C km-1• Q: Why does the parcel cool at a slower rate (6 °C km-1) when it is saturated than at 10°C km-1when it is unsaturated?4Dry versus Moist-Adiabatic Process• the moist adiabatic lapse rate is less than the dry adiabatic lapse rate because as vapor condenses into water (or water freezes into ice) for a saturated parcel, latent heat is released into the parcel, mitigating the adiabatic cooling Moist adiabatic lapse rate: vary with temperature and pressure5Assessing Atmospheric Stability• The bottom line -– To determine whether or not a parcel will rise or sink in the atmosphere, one must compare the parcels temperature (Tp) with that of the environment (Te) at some altitude:–if Tp> Tewhat will the parcel do? –if Tp= Tewhat will the parcel do? –if Tp< Tewhat will the parcel do? • So, to assess stability, what two pieces of information do we need?Absolute Stability• Generally, notice that Teis always larger than Tspand Tupat any level – Hence, an unsaturated or saturated parcel will always be cooler than the environment and will sink back down to the ground • The condition for absolute stability is: – Γd > Γm > Γe– Γdis the dry adiabatic lapse rate (10°C km-1) – Γmis the moist adiabatic lapse rate (6°C km-1) – Γeis the environmental lapse rateTup: the temperature of an unsaturated parcelTsp: temperature of a saturated parcelTe: environmental temperature6Stability of Inversion Layers• Q: How would you characterize the stability of an inversion layer?– They are absolutely stable• Note that the absolute stability criteria: Γe< Γm< Γd• Q: How do you form stable layers in the atmosphere?– Radiational Cooling -radiation inversion– cold air moving in at low levels– warm air moving over cold groundCold surface air, on this morning, produces a stable atmosphere that inhibits vertical air motions and allows the fog and haze to linger close to the ground.7Formation of Subsidence Inversions• How does the stability change for a descending layer of air?The top of the layer warms more than the bottomNatural Stability• The environmental lapse rate is equal to the dry adiabatic rate.• Or when parcel is saturate: …8Absolute Instability• The condition for absolute instability is:– Γe> Γd> Γm• Hence, an unsaturated or saturated parcel will always be warmer than the environment and will continue to ascendConditional Instability• The condition for conditional instability is:– Γd> Γe> Γm• The unsaturated parcel will be cooler than then environment and will sink back to the ground• The saturated parcel will be warmer than the environment and will continue to ascend9Stability of the environment• To determine the environmental stability, one must calculate the lapse rate for a sounding • lapse rate = -DT/DZ = (T2-T1)/(Z2-Z1) • Since the environment is often composed of layers with different stabilities, it is useful to first identify these layers and then calculate their respective lapse rates • recall the stability criteria: – Γe< Γm- Absolutely stable– Γm< Γe< Γd- Conditional Instability– Γm< Γd< Γe- Absolutely unstableProcesses that destabilize the atmosphere1. By cooling of the air aloft and Warming of the surface airCooling of the air aloft– Cold air moving is aloft• this often occurs when an extra tropical cyclone passes overhead– Clouds or air emitting IR radiation to space Warming of the surface air– Surface Heating - suggests that the atmosphere will be most unstable. When?– Warm air moving in at low levels• this often occurs ahead of a cold front– Cold air moving over a warm surface: • Such as lake effect snow10The warmth from the forest fire heats the air, causing instability near the surface. Warm, less-dense air (and smoke) bubbles upward, expanding and cooling as it rises. Eventually the rising air cools to its dew point, condensation begins, and a cumulus cloud forms.2. Destabilize the atmosphere by mixingCooling the top layerWarming the bottom layer113. Destabilize the atmosphere by liftingThe vertical stretch of the unsaturated layer cools the top of the layer more than the bottom—conditionally unstable.3. Destabilize the atmosphere by liftingThe vertical stretch of a bottom saturated and top unsaturated layer cools the top of the layer more than the bottom—absolutely unstable.Convective instability12Atmospheric Instability and Cloud Development1. How are vertical parcel motions that create clouds generated naturally in the atmosphere?– Surface heating and free convection– Topography– Widespread ascent due to convergence of surface air– Uplift along weather frontsAtmospheric Instability and Cloud Development - lifting mechanisms2. What kind (if any) clouds will you visually observe in different stable environments?In a shallow conditionally unstable or absolutely unstable environment, one may expect clouds to develop, but their vertical growth will be limited, and may observe:cumulus humilis (shallow cumulus)stratocumulusIn
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