NOAA Teacher at Sea
Onboard R/V Roger Revelle
November 11-25, 2003
Mission: Ocean Observation
Geographical Area: Chilean Coast
Date: November 22, 2003
Data from the Bridge
1. 221600Z Nov 03
2. Position: LAT: 20-00.0’S, LONG: 083-44.8’W
3. Course: 090-T
4. Speed: 12.6 Kts
5. Distance: 102.7 NM
6. Steaming Time: 8H 06M
7. Station Time: 15H 54M
8. Fuel: 2583 GAL
9. Sky: OvrCst
10. Wind: 140-T, 14 Kts
11. Sea: 140-T, 2-3 Ft
12. Swell: 130-T, 3-4 Ft
13. Barometer: 1015.9 mb
14. Temperature: Air: 20.0 C, Sea 19.4 C
15. Equipment Status: NORMAL
16. Comments: Deployment of surface drifter array #4 in progress.
Science and Technology Log
NOAA Climate Studies of Stratocumulus Clouds and the Air-Sea Interaction in Subtropical Cloud Belts. Today we are still underway and I am going to talk about another science group that is onboard and how their research is related to the Stratus Project. We are presently located along the coast of Northern Chile and I just finished interviewing scientist Chris Fairall with NOAA’s Environmental Technology Laboratory in Boulder, Colorado. A group of 4 ETL scientists are participating in a study of oceanography and meteorology in a region of the ocean that is known for its persistent stratus clouds.
The Woods Hole Oceanographic Institution (WHOI) has maintained a climate monitoring buoy at this location for the last 3 years. Each year they come out to take out the old buoy and replace it with a brand new one with fresh batteries and new sensors. A year in the marine environment takes a toll on the toughest instruments. This is a special buoy which is festooned with atmospheric sensors to measure air-sea fluxes and with a long chain of subsurface instruments to measure ocean currents, temperature and salinity. If you go to the WHOI website ( http://uop.whoi.edu/stratus) you can read about this project and see the data from the buoy. The data are transmitted via satellite everyday. WHOI removed the old buoy on Nov 17 and put in a new one on Nov 19.
Why are these clouds so important? Because the earth’s climate is driven by energy from the sun and clouds dominate how much solar energy reaches the surface. On average, almost 40% of the sun’s energy is reflected back into space and half of that is reflected by clouds. In the cloudy regions more than 60% of the sun’s energy can be reflected by clouds. The surface temperature of the ocean is a result in a near balance between solar heating and cooling by evaporation and cooling by infrared (IR) radiation from the water surface into the sky. The global circulation of the atmosphere and ocean are driven by region differences in this net heat input, so clouds have a direct effect on the winds and currents. Cloud effects on the ocean surface energy balance are very tricky because clouds affect both the solar flux (i.e., by reflecting energy back into space) and the IR flux. It might surprise you, but the sky is ‘warmer’ when there are low clouds present than when the sky is clear. Think about those cold clear nights in the winter and note the ‘cold’ often appears with ‘clear’. More specifically, the IR radiation coming down from the sky is higher when clouds are present than when skies are clear. In the tropics and sub-tropics, the solar reflection cooling effect of the clouds is much stronger than their compensating IR warming effect. Thus, these stratus clouds play an important role in keeping the subtropical oceans cool.
The region we are studying is one of 5 stratus regions around the globe (west coast of U.S.. west coast of S. America, west coast of S. Africa, west coast of N. Africa/Europe, and the west coast of Australia) that occupy vast expanses of ocean. Both of the pictures I attached to this log show the stratocumulus clouds in this region. Each of these cloud types has about the same area-average liquid water content but, because of the horizontal distribution, vastly different radiative properties. The physical processes that lead to these different forms are one of the objective of the ETL studies.
Clouds are formed through various related mechanisms; most involve cooling air to below its dew point temperature so droplets condense ( i.e., clouds are suspensions of liquid water drops with typical sizes of about 10 micrometers radius). Convective clouds are associated with cooling in strong updrafts; fog and many mid-atmospheric clouds form when an atmospheric layer cools by IR radiation. The stratus clouds we are studying are quite different. The key elements are a strong atmospheric cap that traps ocean moisture in a fairly thin ( about 1 km high) boundary layer over the surface. The stratus clouds occupy the top of the trapped layer from just below the cap to down the altitude ( cloud base height) where temperature and dew point just meet. Below that, the relative humidity is less than 100%. The ‘cap’ on the atmosphere boundary layer is warm/dry air descending in subtropical regions, particularly on the western boundaries of continents. This descending air is actually driven by deep convection in the tropics. To meteo- nerds this is an amusing paradox – cool stratus clouds off Chile and California are essentially caused by thunderstorms near the Equator.
Clouds are a pain to study because they are so inaccessible. To get into clouds with sensors you need a really tall tower, a tall building or an aircraft. Most of these are hard to come by 500 miles from land. Thus, most climate studies of clouds rely on remote sensing methods using satellites and surface based sensors.
ETL has deployed a suite of remote sensors on the R/V Revelle to study clouds from the bottom. The showcase sensors are a special high frequency cloud radar and a 2-frequency microwave radiometer system (this system is the attached picture of the large, white van). This is the 6th time such sensors have ever been deployed from ships and only the second time to a stratocumulus region. The first time was to this same spot in 2001; see the web site: http://www.etl.noaa.gov/programs/2001/epic for information on that cruise.
The radar has a wavelength of 8mm, which is so small that it is sensitive enough to receive detectable signals from scattering cloud droplets. With this device the ETL group can determine profiles of cloud properties ( such as size of the droplets) through the entire cloud. The microwave radiometer uses the emissions from the atmosphere at 2 frequencies ( 21 and 31 GHz, or wavelengths of 14 and 9mm) to determine cloud base height and, most importantly, we also measure IR and solar radiative energy reaching the surface. Instead of just looking at the cloud, they collect megabytes of data every minute. The beauty of this set up is that they can simultaneously measure the effect the clouds have on the surface energy budget of the ocean and the cloud properties ( liquid water content, thickness, soiled versus broken, number of cloud droplets per unit volume) that go with the radiative effects. The ETL group are only out here a few weeks each year, but their detailed measurements provide vital information to interpret long-term continuous time series measured by the buoy or inferred from satellite overpasses.
We are surveying for a location for the PMEL Tsunami buoy and the weather is beautiful. Due to our heading we have lost internet connections periodically. The food on the REVELLE is really amazing; last night we had steak and King crab for dinner and a group of the crew and science party met in the lounge to watch a movie. Card games and cribbage are popular in the dining room and some of us just sit outside and enjoy the sunsets. I’m going to sleep early as I have the late watch.