Lecture 6

• Observational network

• Direct measurements (in situ= in place)

• Indirect measurements, remote sensing

• Application of satellite observations to study the tropical atmosphere

ASOS (automated surface observing system)

• Cloud height• Visibility• Precipitation• Pressure• Temperature• Dew point• Wind direction and speed• Rainfall accumulation

The station model for weather obs.

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

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

Reflection -- Refraction

Index of refraction:Speed of light inVacuum divided by speed of light in substance

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).

Scattering (cont’ued)

• Rayleigh scattering: same amount of energy scattered both forward and backward

• Geometric scattering: more energy scattered forward

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

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

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.

Bottom appears grey– little light

Top appears white because of scattering

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

Satellite observations – GEO, LEOGeostationary Earth Orbit

Low Earth Orbit

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.

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).

Visible image

Infrared image

Visible/IR satellite images of clouds

Water vapor channel

Upper to middle troposphere

Other satellite measurements (active)

• Surface vector wind (scatterometer), example: QuikScat

• Atmospheric composition, Aura

• Surface characteristic of land, Terra

• Ocean properties, Aqua

• Rainfall, TRMM

The global satellite observation system: geostationary (35800 km), polar orbiting (850 km), R&D (orbit between certain latitudes at a few 100 km)

Satellites are the primary means of global-scale obs, grouped by orbit

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

IR water vapor from GOES

Visible

Combine measurements from many satellites

POES soundings

Water vapor and cloud track winds

(each color a different satellite)

Vertical structure (soundings)

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

Low level wind

Soundings from GPS radio occultation

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

CloudSat, CALIPSO, Aqua pass almost at the same time

CloudSat profile

GOES image

3D structure

TS Ernesto

Aqua IR image

CloudSat profile (reflectivity)

Vertical structure of hurricanes from CloudSat. 3D structure by combining with OLR(Gordon)

With the radar can distinguish between cirrus and deep convection

Scatterometry from space: surface wind (2D)

QuikScat has proven incredibly useful for tropical meteorology as well as for oceanography

Satellite images of the shallow ITCZ (20 Sept 2000)

VS

IR