Observational Network & Satellite Observation in Tropical Atmosphere

1. Lecture 6 • Observational network • Direct measurements (in situ= in place) • Indirect measurements, remote sensing • Application of satellite observations to study the tropical atmosphere

4. ASOS (automated surface observing system) • Cloud height • Visibility • Precipitation • Pressure • Temperature • Dew point • Wind direction and speed • Rainfall accumulation

5. The station model for weather obs.

6. Observations, data assimilation • The “point” observations are fed into a comprehensive numerical weather prediction model that “adjusts” the observations to correct for errors, fill in gaps etc to produce gridded fields that are dynamically consistent. • Analysis products are observations that have been “fed” through a model to check for errors, adjust for consistency etc

7. Direct measurements of upper-air variables • Radiosondes are instrument packages carried aloft by helium filled balloons • measure vertical profiles of temperature, humidity, pressure. Velocity may be inferred by tracking; in that case called rawinsonde • Rawinsonde measurements are made at weather stations worldwide, at least twice per day

8. Reflection -- Refraction Index of refraction: Speed of light in Vacuum divided by speed of light in substance

9. Scattering • Radiation can be absorbed, reflected or transmitted. Also, scattered = when light changes direction after interacting with a particle. • Rayleigh scattering: when the particles are small compared to the wavelength (e.g., molecules) • Geometric (Mie) scattering: when particles are large (e.g., drops).

10. Scattering (cont’ued) • Rayleigh scattering: same amount of energy scattered both forward and backward • Geometric scattering: more energy scattered forward

11. Indirect methods of observing the atmosphere • Passive sensors: measure radiation emitted by the Earth System or by the sun • Active sensors emit radiation into the atmosphere and then measure the returning radiation. • How does radiation interact with the atmosphere. Reflection/refraction

12. Note the atm window, the visible channel (both “see” the surface), water vapor channel measures total column water vapor (sees the atm higher up)

13. Multiple scattering Repeated scattering of light. Causes whitish light because light of all colors is scattered toward the eye. Both high and low clouds in sunshine look white due to scattering.

14. Bottom appears grey– little light Top appears white because of scattering

15. Multiple scattering and climate change Human activities have lead to increased amount of aerosols. Lead to different clouds (smaller and more numerous droplets) increased scattering. Also, increased concentration of aerosols increases scattering. • Cooling effect • It may be hiding some of the warming due increased greenhouse gases

16. Satellite observations – GEO, LEOGeostationary Earth OrbitLow Earth Orbit

17. Satellite observations • GEO: GOES E & W series of satellites. Always above the same point over the equator. Continuous view of the tropics and midlatitudes. High latitudes not seen. • LEO flies from pole to pole. Flies over tropics and midlatitudes twice per day.

18. Passive measurements (radiometers) • Visible channel, near 0.6 micron • IR channel, in atm window 10-12 micron • Water vapor channel (6.5—6.9) Visible detects albedo (of surface or cloud) IR detects emission temperature of object radiating (either surface or cloud top) Wvc detects the total column of H2O (g).

19. Visible image

20. Infrared image

21. Visible/IR satellite images of clouds

22. Water vapor channel Upper to middle troposphere

23. Other satellite measurements (active) • Surface vector wind (scatterometer), example: QuikScat • Atmospheric composition, Aura • Surface characteristic of land, Terra • Ocean properties, Aqua • Rainfall, TRMM

25. Satellites are the primary means of global-scale obs, grouped by orbit The global satellite observation system: geostationary (35800 km), polar orbiting (850 km), R&D (orbit between certain latitudes at a few 100 km)

26. Satellite based precipitation radar: Tropical Rainfall Measurement Mission (TRMM) launched in 1997. With a 250 km swath it can only observe each location once or twice per day. Provides precip measurements where most of it falls (tropics). GEWEX

27. IR water vapor from GOES

28. Visible

29. Combine measurements from many satellites Vertical structure (soundings) POES soundings (each color a different satellite) Water vapor and cloud track winds

31. Satellite derived mid-upper level wind (track water vapor features in upper troposphere and cloud elements in lower troposphere). Limitation: height determ

32. Low level wind

33. Soundings from GPS radio occultation

34. Observing tropical clouds and rain • The A train consists of a number of satellites that follow each other in succession so that they are approximately viewing the same scene at the same time • Polar orbiters • CloudSat is the first mm wavelength cloud radar in space • more than 1000 time more sensitive than weather radars • Collects data about the vertical structure of clouds, including liquid water and ice and how clouds affect solar & terrestrial radiation • Particle concentrations, cloud liquid water, precip rate

35. CloudSat, CALIPSO, Aqua pass almost at the same time

36. 3D structure CloudSat profile GOES image TS Ernesto

37. Vertical structure of hurricanes from CloudSat. 3D structure by combining with OLR (Gordon) Aqua IR image CloudSat profile (reflectivity)

38. With the radar can distinguish between cirrus and deep convection

39. Scatterometry from space: surface wind (2D) QuikScat has proven incredibly useful for tropical meteorology as well as for oceanography

40. Satellite images of the shallow ITCZ (20 Sept 2000) VS IR