The Thermostat Hypothesis Jesse Vannatta Major: Atmospheric Science

Articles Discussed Thermodynamic regulation of ocean warming by cirrus clouds deduced from observations of the 1987 El Niño. V. Ramanthan & W. Collins. Nature Vol. 351 May The effect of environmental conditions on tropical deep convective systems observed from TRMM satellite. Bing Lin, Bruce A. Wielicki, Patrick Minnis, Lin Chambers, Kuan-Man Xu, and Yongxiang Hu. Journal of Climate Vol. 19 November 2006.

Thunderstorms, Get Excited! Australian Storm Australian Storm

Acronyms DCS=Deep Convective System SST=Sea Surface Temperature

How to Make a Thunderstorm 3 Key Conditions… 1. Moisture 2. Instability 3. Lifting Mechanism

Key Features of Thunderstorms  Updrafts (Mesoscale and Convective)  Downdrafts (Mesoscale and Convective)  The storm reaches 12-18km at it’s peak  When the downdrafts cut off the updrafts, the storm will start to die.

The ITCZ  The Northeast trades and the Southeast trades come together near the equator, which leads to surface convergence and lifting air.  ITCZ video ITCZ video

Thunderstorm Facts Men are 6 times more likely to be struck by lightning then women. 9 out of 10 people who are struck by lightning survive. One lightning bolt has enough electricity to service 200,000 homes. The Greeks fenced off spots that had been struck by lightning so that man would not trod on ground touched by Gods. Lightning has been detected on Jupiter (also Venus) and while these strikes are less frequent then on the Earth, lightning on Jupiter can be up to 1,000 times as powerful as lightning on Earth.

Thermostat Hypothesis Ramanthan and Collins 1991  1987 El Niño Event  Data based off of ERBE measurements (Earth Radiation Budget Experiment)  Thermostat Hypothesis: The presence of deep convective storms over the Oceans act as a thermostat, limiting the sea surface temperatures to 305K.

Variables S c : incoming solar radiation C s : solar radiation reflected by clouds. C 1 : longwave radiation trapped by clouds G a : Longwave radiation trapped by the atmosphere

Experiment 1  Greenhouse effect G=G a +C 1 increases sharply when SST>300K  SST increase  atmospheric moisture increase  G a increase  C 1 and C s also increase with temperature.

Experiment 2 Clouds always reflect more solar radiation with increasing SST  Data compares1987 el niño to 1985 normal conditions  Central and Eastern Pacific (region of max warming) MonthYearSST (K)dC s (W/m 2 ) April February May July October

Math! From these graphs we get:  dC s =-0.92 (dC 1 +dG a )  dC s =1.2(dC 1 )  dG a /dT=6.8Wm -2 So… dC s /dT=-27Wm -2 and dC 1 /dT=23Wm -2  Therefore: reflection of SW radiation is increasing more rapidly with temperature than the trapping of longwave radiation.

Summary of Ramanthan and Collins…(more math)  303K < SST max < 305K  In 1987 and 1983 El Niño events, SST did not exceed 305K  Theory: The sun heats the Ocean, making the BL more unstable. This creates DCSs, which block more solar radiation from reaching the ocean surface. This acts like a thermostat, capping the SST between 303 and 305K.

Article 2 Lin et al. Data (2006) The Study  Statistical analysis of DCS and the environment around DCS  Latitude: 30°N-30°S  January  August 1998  TRMM (Tropical Rainfall Measuring Mission) 1. CERES (Clouds and the Earth’s Radiant Energy System scanner) 2. VIRS (Visible and Infrared Sensor 3. TMI (the TRMM microwave imager)  Other data: ECMWF

What counts as DCS?  Looked for CPCCs (cold precipitating cloud clusters)  CPCCs: 1. At least one precipitating pixel 2. Thermal IR brightness temperature T bIR < 245K 3. Cloud top height above 6km  DCS: a CPCC with at least one rainfall cell with T bIR <218K

70,000 storms in 8 months!

Cloud Coverage

Precipitation

Back to the ITCZ  Study defines areas of convergence as SST > 299K

Relating this to the thermostat Hypothesis…

Summary  During DCS conditions less LW radiation escapes at the top of the atmosphere, and more SW radiation is reflected than under normal environmental conditions  As SST increases…  Convergence increases  DCS increases  Areal coverage of DCS increases  Precipitation increases

Do they agree with Ramanthan and Collins? Yes, as SST increases there are more DCSs, with a greater areal coverage and more precipitation. They agreed that this will create a negative feedback, but not to the extent that Ramanthan and Collins suggested. When they did the calculations they came up with a less severe negative feedback. (Ramanthan and Collins suggested a net radiative feedback of -4Wm -2 K -1 to -6Wm -2 K -1, Lin et al suggested -1.75Wm -2 K -1

Who cares about clouds and the SST  Climatologists do! The Ocean temperature is an integral part of climate change  Cloud feedbacks are an important factor in Earth’s energy balance, yet there is no general agreement on their climate feedbacks (especially for tropical deep convective systems)

My Opinions Can’t rely on this hypothesis to counteract global warming More long range studies should be done, especially since Ramanthan and Collins, and Lin et al. found different magnitudes.

Happy Valentines Day

Sources  Fun Facts:    Background Knowledge  ent.htm ent.htm