REMOTE SENSING IN THE BOUNDARY LAYER

Weather Doppler Radars (WSR-88Ds)

• Doppler weather radars (radio detection and ranging) send out pulses of electromagnetic radiation.  
  • This radiation scatters off of precipitation particles in the atmosphere
  • some of this energy comes back to the radar
    • the intensity of this back scattered radiation is related to the intensity of the precipitation
    • the radar also measures the Doppler shift of this radiation - the Doppler shift is related to echo movement
• Weather Doppler radars are most often used for monitoring precipitation events.

• However, during the National Weather Service re modernization process in the 80-90's, a new national network of Doppler radars was installed
• These radars, the Weather Survellience Radar -88D (WRS-88D, represented a huge advance in our ability to monitor the evolving weather situation.  Why?
• Well, one reason is that they can monitor boundary layer phenomena and their evolution.
• Let's look at a couple of examples:

Here is an example of what you are acustomed to looking at:

Yet, these radars are capable of monitoring the boundary layer:

More specifically, the 88-Ds can detect "clear-air echoes", or echoes that are apparent even when there is no precipitation around:

• Notice the presence of "thin lines" of enhanced reflectivity in the above image.
• Q:  What is the source of the clear-air echoes and how are these "thin lines" created?  
• A:  Most likely, the source of clear-air echoes is bugs and insects!!  
• Clear-air returns are in fact, temperature dependent, adding further support to the idea that they are created by bugs and insects:

• Another scattering source producing clear-air echoes is likely refractive index gradients (density variations created by temperature and moisture fluctuations).
• However, most folks in the radar community believe that the primary source of clear-air echoes comes from bugs and insects.
• None the less, these sensitive radars allow us to monitor boundary layer phenomena in great detail.

915 Mhz Profilers w/ RASS 

• Wind Profilers are similar to Doppler radars such as the WSR-88D in that they send out pulses of radiation and measure the intensity of backscattered radiation and it's Doppler shifted frequency.
• Wind profilers continuously point upward.
• They transmit radiation at a much longer wavelength than the 88-D.
  • As a result, these radars are sensitive to refractive index gradients.  Hence, these radars are able to retrieve vertical wind profiles in clear air conditions.
• The 915 Mhz profilers are often referred to as boundary layer profilers.  They collect data:
  • from 100m above the ground to 3-5 km AGL
  • every 60-400 meters
• So, you can see that the data resolution is great enough to show the detailed evolution of the boundary layer winds
• Note however, that there is generally no data from the surface to 100 m AGL, hence observations of the surface layer will not be made.
• Lets take a look at an example:

• Q:  What's going on in this image????

RASS

• The Radio Acoustic Sounding System, or RASS, is often collocated with profilers.  They collect vertical profiles of virtual potential temperature by measuring changes in sound wave propagation (more on this in Remote Sensing).

Here is an example of both winds and RASS collected by a profiler/RASS system.

• So, what's going on in this data set?
• Here is a great page created by the Forecast Systems Laboratory where you can get real-time 915 Mhz profiler and RASS data in real time. 

Sodars 

• Sodars (Sonic detection and ranging) operate in a similar manner as a wind profiler
• They send out acoustic waves and measure the associated Doppler shift to provide detailed wind information.
• Sodars collect data in only the lowest 100-200 meters
• However, it is high resolution.
• Therefore, it is ideally suited for studying the surface layer.
• Check out the data example below:

Lidars

• OK, so:
  • Weather Doppler radars are most sensitive to precipitation-size particles along with bugs and insects
  • 915 Mhz profilers rely on refractive index gradients having spatial scales of 1/2 l.
  • RASS and SODARS transmit and detect sound waves and their associated Doppler shift
• What else can possibly be done to remotely sense the boundary layer?????
• Well, quite a bit actually.
• Lidars (Light detection and ranging) transmit radiation at very small wavelengths.
  • We're talking about 1-2 microns here.
• As a result, lidars are very sensitive to aerosols in the atmosphere.  Again, you get back-scattered power and Doppler-shifted frequency information from these systems. 
  • Recall that aerosols are tiny suspended particles, liquid or solid, that are floating around the the atmosphere.
  • Since the earth's surface is a major source of aerosol, there should be many of them in the boundary layer.
• The data resolution with lidars is amazing.  Very high-resolution (spatially) data can be collected.
• The two primary limitations of lidar data are:
  • how far out they can collect data
  • the lidar radiation is easily attenuated.  A cloud can easily block most of the beams radiation.
• Regardless, it is quite easy to point a lidar vertically, turn it on, and watch the boundary layer evolve.
• Check out the boundary layer evolution below.

1246	1346	1446	1546	1646
1746	1846	1946	2046	2146

Amazing!!!

Integrated Remote Sensing Systems:

NCAR/EOL Integrated Sounding System (ISS) 915 MHz wind profiler (with RASS) Radiosonde sounding system 10 m meteorological tower solar radiation sensors incoming LW and SW outgoing LW and SW diffuse SW Can also request a Sodar
UAH Mobile Integrated Profiling System 915 MHz profiler Ceilometer 12 Channel Radiometer Sodar Electric Field Mill Meteorological Instruments

Photogrammetry

• Last, but not least, one can use a camera as a remote sensing tool.
• Knowing the locations of cloud positions is often crucial when studying the boundary layer and boundary layer phenomena.
• Using the technique of photogrammetry, it is possible to superimpose an azimuth/elevation angle grid on a photograph:

• This is analogous to putting a latitude/longitude grid on a satellite picture.
• Once this is done, you can then determine the positions of distinct clouds in your photo.  More on this in Remote.