2Review of last lectureWater Vapor Basics (names of different phase changes, latent heat)Humidity indices (there are 6 total). Saturation vapor pressure increases non-linearly with temperatureTwo methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds.
3The most common atmospheric circulation structure Radiation ConvectionCoolingor No HeatingHeatingLatent/SensibleConductionHLImbalance of heatingImbalance of temperatureImbalance of pressure Wind
4CloudsClouds are instrumental to the Earth’s energy and moisture balances, and constitute a wild card for climate change
5Satellite observation of clouds NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites Now, cloud top height and optical depth.NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.
8Cloud Properties 1. Cloud top height/pressure 2. Cloud thickness (optical depth)3. Cloud coverageWhen clouds comprise more than 9/10th of the sky = overcastWhen coverage is between 6/10th and 9/10th = brokenWhen coverage is between 1/10th and 6/10th = scatteredCloud coverage less than 1/10th = clear
9NASA’s International Satellite Cloud Climatology Project (ISCCP) Cloud Classification - commonly used in climate research
10Why do clouds constitute a wildcard for climate change? Strongerwarming effectClouds are both good reflectors of solar radiation (cooling effect) and good absorbers of earth emitted longwave radiation (warming effect).The net effect (cooling or warming) depends on the type of cloudIn a changing climate, increases in high thin clouds would promote warming, while increases in low thick clouds would cause coolingClimate models have difficulties in simulating clouds, especially low thick clouds (stratocumulus)Conclusion: Clouds cause the largest uncertainty in model simulations of future climate.Stronger cooling effect
12Formation of cloudsMost clouds form as air parcels in boundary layer are lifted and cooled to saturation.The air parcels could be lifted by mountains, meeting of different air masses, surface convergence, and local convection
13Lifting by local convection Static stability – refers to atmosphere’s susceptibility to being displacedStability related to buoyancy force determined by density difference btw parcel and environment (FB= ρenvg – ρparcelg) determined by temperature difference btw parcel and environment (ρ = P/TR)When an air parcel rises, the cooling rate of the parcel (adiabatic lapse rate or ALR) relative to the cooling rate of surrounding atmosphere (environmental lapse rate or ELR) determines the “stability” of a parcel.EnvironmentParcelΔp/Δz=ρenvgρparcelg
14The three types of stability When comparing the temperature btw parcel and environment, there are 3 possible outcomes:EnvironmentParcelParcelParcelEnvironmentEnvironmentAbsolutelyStableAbsolutelyUnstableConditionallyUnstableConvection happens when:boundary layer air is warm and moistEnvironmental air above boundary layer is cold
15When convection happens: Rising up of air parcel (called updraft)Formation of clouds and sometimes precipitationHeating up the environment because parcel temperature is warmer than the environment
16What stops ‘unstable’ air masses from rising indefinitely ? 1) EntrainmentTurbulent mixing of ambient air into parcelLeads to evaporation along cloud boundariesEvaporation uses latent heat, cooling the cloud reduces buoyancyCourtesy Russ Dickerson, U. Maryland2) Encountering a layer of stable air (inversion)a rising parcel may reach a stable upper air environmentthe parcel cooling rate will exceed that of the ambient airthe parcel will slowly cease ascension and come to rest at some equal temperature levelthree types: radiation, frontal, subsidence
17In convection, an updraft is often associated with a downdraft – Overturning of the troposphere Air can be cooled down by radiation, evaporation of raindrops, melting of snowflakes, etc.Air that is cooler than its environment tends to sink, leading to the formation of downdraftsSometimes precipitation drag enhances the downdraftsDowndrafts cool down the environment (generally the lower troposphere)Downdrafts (also called downbursts) can cause significant damage at the ground
18Low stratocumulus clouds Generated by convection inside boundary layerConvection is driven by cloud-top longwave cooling and evaporative cooling
19Summary 3 cloud properties, 9 ISCCP cloud types Why do clouds constitute a wildcard for climate change? Competition between greenhouse effect and albedo effectConvection: 3 types of stability. Two factors limiting the height of clouds