Sounders vs. Imagers
What is an atmospheric sounder?
- designed for vertical information rather than image information
- spectral measurements are required
 
 

Image from Advanced Infrared Sounder project at JPL

Examples

- Advanced Infrared sounder (AIRS)
- Advanced Microwave Sounding Unit (AMSU)

National Weather Service Goals with AIRS:
- Temperature accuracy of 1 degree Celsius in layers 1 km thick
- Humidity accuracy of 20% in layers 2 km thick in the troposphere
    (lower atmosphere)

AIRS has 2400 spectral bands in the ranges 0.4 to 1.0 micrometers and
3.7 to 15 micrometers.  How can spectral measurements be exploited to give soundings?
- CO₂ absorbs at 4.2 and 15 micrometers
- H₂O absorbs at 6.3 micrometers
_{- The strength of the absorption is dependent on temperature}
- Invert the data using weighting functions to derive vertical profiles

AIRS channels:

 
 

Advanced Microwave Sounding Unit
    Channels
    Resolution
    Weighting Functions
    Microwave Spectrum
 

Tropical Rainfall Measuring Mission (TRMM)  NASA and NASDA
Sensors:  Rain radar,microwave radiometer, and VIR like GOES

How does the microwave radiometer detect rain?
1) Water appears colder than its actual temperature, rain appears to be its actual temperature.
2) A high frequency channel is strongly affected by ice particle scattering (high clouds).

Why measure tropical rainfall?
1) Precipitation releases concentrated heat.
2) The heat released by precipitation is high in the atmosphere and has a great affect on large scale circulation.

Images

Houston TX thunderstorms, radar

 
 
 

Hurricane Mitch, passive microwave

 

Global microwave imager data --> warm=dark, cold = light, probable rain and snow = yellow and white,
                               over the ocean--> light blue = low water vapor, dark blue = high water vapor

 

Total Ozone Mapping Spectrometer (TOMS, a sounder with 4 UV channels)
Why measure ozone?

Link to AMSU temperature and link to pressure altitude calculator
 
 

The ozone hole: less than 220 Dobson units, but what is a Dobson unit?

NOTE: This copyrighted graphic, is based on a page developed by Owen Garrett for the
Centre forAtmospheric Science at Cambridge University, UK.
Take their Multimedia Ozone Hole Tour
 
 

South Pole December 1997 ozone hole

South Pole December 1998 ozone hole


 
 

Link to Web class on stratospheric ozone from Goddard Space Flight Center
 
 
 

Lidar applications

Schematic from NOAA Environmental Technology Laboratory

 

Cloud Lidar System: Green and NIR laser from the ER-2
notes: Pulsed laser, gated detection for range information, the backscattered laser signal is detected, good up to an
            optical thickness of 3-4, information on cloud position, water phase, particle size, and density.

Clouds in Alaska

 

Wave structure in cirrus clouds, Tropical Western Pacific

 
 

Differential Absorption Lidar (DIAL) basics (from NOAA Environmental Technology Laboratory)
water vapor technique at IR wavelengths: the backscattered signal depends on the strength of the absorption band
                                                                              at the laser wavelength.

ozone technique for UV wavelengths: because the ozone absorption is broad there is uncertainty in the aerosol scattering
                                                                      at the widely separated on and off wavelengths.  The difference in the aerosol can
                                                                      be extrapolated from the third wavelength.  Another technique is to use two wavelengths
                                                                      (308 and 351 nm) and observe the nitrogen molecule Raman scatter which is shifted to longer
                                                                      longer wavelengths where the affect of aerosol scatter is lessened.  The nitrogen Raman
                                                                       scatter from the 308 nm laser is partially absorbed by ozone while the scatter from the 351 nm
                                                                       laser is not.

 

Ozone Lidar Specifications:

   Ultraviolet Wavelengths
      277, 292, 313, 319, 360 nm
   Pulse energy
      5 - 15 mJ / wavelength
   Pulse rate
      20 Hz
   Scan
      Fixed down-looking from an airborne platform
   Resolution
      Ozone: 90 m
      Aerosol Backscatter: 15 m
   Minimum range
      0.7 km
   Maximum range
      3.5 km
   Accuracy
      Less than 10 ppbv typical
   Other
      Platform: Casa 212 cargo aircraft
      Laser Type: Krypton Fluoride with Raman shifting in H2 and D2
      Receiver: 50 cm diameter Newtonian telescope (down-looking)
      Detectors: Hamamatsu R2055 and R2076 PMT's, gated
      A-to-D converters: DSP 12-bit 5 MS/s (ozone), 10 MS/s