Thermodynamics of Cloud Formation
Energy Partitioning! Griffith et al. 2008!
Orographic Clouds
Above the PBL
Titan s boundary layer Tokano et al. 2006
Stratiform Clouds!
Convective clouds! Cumulonimbus!
Convective Cloud formation! LFC LCL 2 km Art from Wallace & Hobbs
Another way to make clouds by adiabatic expansion of air?! US Navy Jet! 75 feet above! ocean!
Start with Titan! easier!
Detection of clouds (Griffith et al. (1998) Science, 395, 575) Spectra displaying unusual brightness in windows No effect from surface! Light curve in 2 atmospheric windows
Daily Cloud Variations Spectral variations occur in spectral regions that sense the troposphere, not the surface UKIRT spectra recorded over 2 weeks The difference between each observation and the second observation taken on September 14 (Griffith et al. Science 2000)
Cloud Top Altitudes
Why are the clouds in near Titan s South Pole?
CH 4 Profile is typical! CH 4 in saturation (CH 4 ice)!! CH 4 in saturation (CH 4 -N 2 )!! CH 4 mixed upwards!! CH 4 in equilibrium! Measured Profile! CH 4 -N 2 Saturation!
Schinder et al. 2011!
Thermodynamics of convective clouds! Focus on Titan s tropics, because it s easier! Nearly constant T! Because no lakes in tropics! Expect constant [CH4}!
Middle Tropospheric Convection! Imagine a parcel of air rising from the ground. LCL = (Lifting Condensation Level) is the altitude where the parcel condenses LFC = Level of free convection -- altitude where the parcel becomes buoyant NBL= neutral boundary layer altitude where the parcel becomes neutrally bouyant To make rain on Titan you need a humidity of 60% (Barth & Rafkin 2007)! Griffith et al 2007!
CAPE! The CAPE is the maximum KE acquired by a buoyant parcel from the level of free convection (LFC) to the level of neutral buoyancy (LNB). CAPE = Convective Available Potential Energy!
Fujita scale!
Derivation Potential Temperature! Titan s Potential Temperature
Atmospheric Stability! Parcel raised adiabatically!
Titan s boundary layer Tokano et al. 2006
Effective Potential temperatures, θ e! Imagine a parcel of air rising from the ground. LCL = altitude where condensation begins LFC = altitude where parcel becomes buoyant ~45% ~80% ~50% Griffith et al. 2000
Terrestrial θ Profiles Midwestern thunderstorms Tropical cumuli
Titan vs Earth: Convection Earth Titan LFC 1.5 km (tropics) 5.5 km τ rad 3 months ~130 yrs Surface humidity 70% 50% CAPE (m 2 /s 2 ) 500 (tropics) 2500 (midwest) ~1000 The CAPE is the maximum KE acquired by a buoyant parcel from the level of free convection (LFC) to the level of neutral buoyancy (LNB).
Seasonal Effects on Titan. The radiative time constant of Titan s troposphere (~130 yrs*) exceeds a Titan year (~29 yrs). Titan s troposphere, as a whole, responds to the yearly insolation and meridional transport, and changes only slightly with season**. In contrast, the surface responds to the seasonal insolation. Maximum daily sunlight Yearly average *Geirasch et al. 1970 ** Flasar 1981 The maximum daily sunlight differs dramatically from the average at the poles.
Titan s tropical clouds! Cassini VIMS From 45S to 45N temperature does not change.! There is no appreciable methane on the surface.! ~45% ~80% ~50% Griffith et al. 2009!
Cloud Formation on Titan! Griffith et al., Science 310, 474 (2005)!
Convective Clouds! Clouds rise 2-4 m/s, consistent with convection.!! Clouds fall at time scales of! 20 km/hour, consistent with the fall of millimeter-sized hail!
Level of Free Convection! LCL = altitude where condensation begins! LFC = altitude where parcel becomes buoyant! LNB = Level of Neutral bouyancy!! Griffith 2014
Although this picture cannot explain Titan!
Simplest Thermal Profile! You re in a squall! What is the thermal profile! What are reasonable assumptions?!
Titan s high convective clouds! Cassini VIMS We need a lot of CH4 to fuel the formation of these clouds.! Where does this come from?!
Convective Cloud formation! LFC LCL 2 km Art from Wallace & Hobbs
NEXT CLASS:! Atmospheric dynamics!! Derive the primitive equations in atmospheric sciences,! which are are the five fundamental equations that describe the! evolution of large scale atmospheric motions.!