1. Ground-based Remote Sensing an overview of sensors and applications
Types:
Passive
Visible (cameras, solar radiometers, spectrometers)
Infrared (total emission, spectrometers)
Microwave emission
Active
Lidar (Light [vis] Detection and Ranging)
Radar (Radio [waves] “)
Sodar (Sound “)
RASS (radar for winds plus T(z) from microwave
Orientation
Upward (usually nadir)
Horizontal (e.g., CODAR)

2. Multifilter Rotating Shadowband Radiometer (MFRSR)
Passive Vis
Multifilter Rotating Shadowband Radiometer (MFRSR)
Measures column ozone and water vapor
Instrument takes spectral measurements of direct normal, diffuse horizontal, and total horizontal solar irradiances
measurements taken every 20s (SGP site)
The MFRSR is an instrument that directly measures global and diffuse components of spectral solar irradiance. The direct normal component is calculated from the difference of the global and diffuse measurements. At the start of a measurement series, a global measurement is first taken. The shadowband is then rotated from the home position and stops in three positions before returning home. The first and third stops are just before and after shading the diffuser. At the second stop the diffuser is completely shaded. Measurements at the first and third stops are used to correct the error introduced by the shadowband shading a portion of the sky.
The Multi-Filter Rotating Shadowband Radiometer (MFRSR) measures both global and diffuse radiation in six narrow bands, approximately 10 nm wide, centered on 415, 500, 615, 673, 870, and 940 nm. The first four channels are in the visible and last two are in the near infrared part of the solar spectrum. These particular bands were selected to allow for the computation of optical depths for aerosols, water vapor and ozone. The MFRSR also has one silicon broadband detector for measuring total solar irradiance. The receiving surface is a small unprotected horizontal diffuser disk covering the aperture atop of a cylindrical, temperature-controlled enclosure. Within this enclosure selective waveband sampling is accomplished by interference filters, and photodetectors beneath the filters measure the signal strengths. These signals can be converted to an equivalent radiant energy flux by applying calibration factors or optical depth calculations can be made using the Langley slope method [1]; which circumvents the need for absolute calibrations. The ability to obtain both global and diffuse measurements is made possible by the rotating curved metal strip (the shadowband). While the band is at rest, below the receiving enclosure, the instrument measures downwelling global radiation. Periodically (four times per minute at most), the curved band swings over the top and shades the aperture, restricting the measured solar radiation to the diffuse component. Measurements are made every 15 seconds and one-minute averages are recorded. The MFRSR is heated to keep its components at a constant temperature, and to keep it free of snow and ice.
Data from MFRSR includes measurements of the diffuse and direct radiation components and derived values for total downwelling radiance. Total downwelling spectral and unfiltered solar radiances are measured while the radiometer is exposed to full light and the diffuse radiation component is measured while the instrument is shaded. The direct normal component is derived from the difference between the two measurements.

3. Passive Vis

4. Passive Vis
Direct normal radiance

5. Passive Vis

6. Passive Vis
Barrow, AK
Lamont, OK
Manus, Papua New Guinea

7. Passive Vis

8. Passive Vis

9. Net Radiometer help determine the total energy exchange
Passive Vis & IR
help determine the total energy exchange
provides measurements of shortwave (solar) and longwave (atmospheric or infrared) irradiances for downwelling and upwelling components.
Shortwave Irradiance
Three pyranometers are positioned to measure the hemispheric irradiance fields:
DS Unshaded, ventilated, and mounted in horizontal orientation US Inverted, unventilated and mounted 10m above the ground level DD Shaded, ventilated, and mounted on automatic solar tracker in horizontal orientation.
A single pyrheliometer is mounted on an automatic solar tracker and aligned to the sun’s disc.
The following relationships are expected from the SIRS measurement system for the various broadband shortwave irradiance elements.
where,
DS = Downwelling Hemispheric Shortwave (Global) Irradiance
DNI = Direct Normal Shortwave (Beam) Irradiance
Z = Solar Zenith Angle (sunrise/sunset = 90°)
DD = Downwelling Diffuse Shortwave (sky) Irradiance
US = Upwelling Diffuse Shortwave (reflected) Irradiance
rho= Surface Shortwave Albedo (typically 0.2 for vegetation, 0.8 for fresh snow)
ETR = Extraterrestrial (exo-atmospheric) Radiation on horizontal surface
ETRN = Extraterrestrial Radiation Normal (beam) to the sun (1366 ±5 Wm-2 [REF??]).
Longwave Irradiance
Elements of the pyrgeometer measurements are used to compute the infrared:
IR = Infrared Irradiance (W/sq m) Ttp = PIR thermopile voltage (mV) Tc = PIR case temperature (K) Td = PIR dome temperature (K) C1 = PIR Calibration Factor (W/sq m per mV)
C2 = PIR Dome Correction Factor (4.0 = default for all PIRs)
sigma= Stephan-Boltzman Constant = 5.67E-08 W/m2 K4.

10. Passive Vis & IR
Net Radiometers

11. Passive Vis & IR

12. Microwave Radiometer (MWR)
Passive Micro
provides time-series measurements of column-integrated water vapor and liquid water
Measures microwave radiation at 23.8 and 31.4 GHz
WV dominates the 23.8GHz channel
Cloud liquid in the atmosphere dominates the 31.4 GHz frequency
The instrument itself is essentially a sensitive microwave receiver. That is, it is tuned to measure the
microwave emissions of the vapor and liquid water molecules in the atmosphere at specific frequencies.
For a specific frequency, n, the amount of microwave radiation observed by a radiometer at the earth's
surface looking directly upward can be expressed as:
The first term represents the amount of cosmic (i.e. extraterrestrial) radiation entering at the top of the
atmosphere Ic that reaches the radiometer. The exponential decay factor accounts for attenuation of the
cosmic radiation by the intervening atmosphere; t is the optical thickness:
where r is the density [mass per volume] or [number per volume] and k is the extinction coefficient [area
per mass] or [area per number]. It is highly dependent on frequency. (Note that extinction is the sum of
absorption plus scattering; however, because scattering is negligible in the microwave region of the
electro-magnetic spectrum - except during heavy rain - k can be taken as the absorption coefficient alone.)
The physical significance of t is that it represents an “effective thickness” of the atmosphere for a
particular frequency: t will be large (and the attenuation e-t great) when either z, r or k is large. Put another
way, if r and k are large enough that a very small value of z will still cause e-t =1, then the region is said to
be “optically thick” - one cannot “see” very far into it. On the other hand, if r and k are sufficiently small,
a very large value of z will be required to produce e-t =1 and the region is said to be “optically thin” - one
can “see” a large distance at this frequency.
The second term in Eq. (1) represents the sum of the contributions from the atmosphere along the line-ofsight
(i.e., the path). B[T(z)] is the Planck function which describes the blackbody emission from the
molecules at height z (which are at a temperature T(z)). The product rk is the amount of blackbody
radiation that is emitted (i.e., not re-absorbed) by the molecules in the layer. The factor e-t accounts for the
attenuation by the atmosphere between the source molecules and the microwave radiometer antenna.
In the microwave region, the Planck function may be expressed as:
B(T) = 2KTc/l4 (3)
where K is Boltzmann's constant, c is the speed of light and l is the wavelength of the radiation. We can
rearrange this expression to define the equivalent blackbody brightness temperature: where Tc = 2.75 kelvins. To actually calculate TB, the atmosphere is divided into a number of layers N
which are considered isothermal:

13. Passive Micro

14. Passive Micro

15. Passive Micro

16. Passive Micro

17. MicroPulse Lidar (MPL)
ACTIVE Vis
optical remote sensing system designed
to determine the altitude of clouds overhead
Pulses of energy are transmitted into the atmosphere; the energy scattered back to the transceiver is collected and measured as a time-resolved signal
time delay between each outgoing transmitted pulse and the backscattered signal used to infer the distance to the scatterer
The principle is straightforward. A short pulse of laser light is transmitted from the telescope. As the
pulse travels along, part of it is scattered by molecules, water droplets, or other objects in the atmosphere.
The greater the number of scatterers, the greater the part scattered. A small portion of the scattered light
is scattered back, collected by the telescope, and detected.
The detected signal is stored in bins according to how long it has been since the pulse was transmitted,
which is directly related to how far away the backscatter occurred.
The collection of bins for each pulse is called a profile. A cloud would be evident as an increase or spike
in the back-scattered signal profile, since the water droplets that make up the cloud will produce a lot of
backscatter.
300 meter height resolution
Emits and recieves pulses quickly, meaning it can detect more clouds than lidar. Height range of up to 18km

18. ACTIVE Vis

19. ACTIVE Vis

20. ACTIVE Vis

21. ACTIVE Vis

22. Visibility measurements
ACTIVE Vis
Visibility meter FD-12 from Vaisala measures forward scattering of light

23. Vaisala Ceilometer (VCEIL)
ACTIVE Vis
ground-based, active, remote-sensing device designed to measure cloud-base height at up to three levels and potential backscatter signals by aerosols
transmits near-infrared pulses of light, and the receiver telescope detects the light scattered back by clouds and precipitation
Emits energetic pulse into atmosphere and detects backscatter from cloud droplets. Return signal sampled once every 100ns. Cloud is defined as the altitude at which visibility is reduced by 100 meters. Height resolution is 15 meters and can detect up to 3 cloud layers up to 7.6km.

24. ACTIVE Vis

25. ACTIVE Vis

26. ACTIVE Vis
Several things in this slide. In the first row is a picture of a Belfort Laser Ceilometer and example data (from April 15, 1998). Second row is a picture of the Micropulse Lidar and example data (from April 15, 1998). The big thing to note here is the difference in resolution. The third row is a picture of myself on board the Ka'imi Moana launching a BBSS and example data from a similar system at ARM for April 15, 1998.

27. Doppler Radar
ACTIVE Radio
Radio-wave radiation scatters off of precipitation particles, some returns to antenna
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

28. Doppler Radar
ACTIVE Radio

29. Millimeter-Wavelength Cloud Radar
ACTIVE Radio
Determines cloud boundaries, radar reflectivity and vertical velocity
transmitts a pulse of millimeter-wave energy from its transmitter through the antenna.
energy propogates through the atmosphere until it hits objects that reflect some of the energy back to the MMCR (clouds, precipitation, insects, spider webs, etc.)
received signal is split into two channels, termed I and Q (for in-phase and quadrature).
Any radar’s sensitivity is proportional to the transmit power, the square of the antenna gain, and the square of the radar's wavelength. The sensitivity is inversely proportional to the square of the range from the radar to the target.
The MMCR works by transmitting a pulse of millimeter-wave energy from its transmitter through the
antenna. The energy propogates through the atmosphere until it hits objects that reflect some of the
energy back to the MMCR. These objects can be clouds, precipitation, insects, spider webs, man-made
objects, etc. The same antenna is used to receive the return signal. The received signal is split into two
channels, termed I and Q (for in-phase and quadrature). A digital signal processor processes these signals
and provides power, Doppler velocity, and spectral width. The power measurement is processed by
knowing the MMCR's calibration coefficient to provide the radar reflectivity.
Looking at the meteorlogical radar range equation gives insight as to how the MMCR works and what
parameters affect its sensitivity. Any radar’s sensitivity is proportional to the transmit power, the square
of the antenna gain, and the square of the radar's wavelength. The sensitivity is inversely proportional to
the square of the range from the radar to the target.

30. ACTIVE Radio
This is a radar image from the SHEBA program showing a fairly solid layer of liquid cloud that was present for several hours on 18 May It is a case study day for something Joe is working on intercomparing radiative transfer codes in the arctic.

31. SODAR Measures profile of wind
ACTIVE Sound
Measures profile of wind
Transmits a short pulse of sound which is refracted by the small scale turbulence in the atmosphere.
radial velocity of the air can be determined by measuring the Doppler shift of the sound being refracted from the turbulence.
The range of the turbulence is determined from the delay between the transmission of the acoustic pulse, and the reception of the refracted signal
A SODAR (SOund Detection And Ranging) system is an instrument for the measurement of wind velocity, remotely, from the ground. It operates by transmiting a short pulse of sound which is refracted by the small scale turbulence in the atmosphere. This turbulence is transported by the wind, and the radial velocity of the air can be determined by measuring the Doppler shift of the sound being refracted from the turbulence. The range of the turbulence is determined from the delay between the transmission of the acoustic pulse, and the reception of the refracted signal. By repeating this process in three different directions, each direction having a large component being orthogonal to the other two directions, the three dimensional wind field can be calculated.
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 meters
However, it is high resolution.
Therefore, it is ideally suited for studying the surface layer.

32. ACTIVE Sound
SODAR

33. ACTIVE Sound

34. Radio Wind Profiler (RWP)
ACTIVE Radio & micro
Measures wind and virtual temp profiles
transmits electromagnetic energy into the atmosphere and measures the strength and frequency of backscattered energy
Radio Acoustic Sounding System (RASS) at PAM site
The radar wind profiler/RASS (RWP) measures wind profiles from (nominally) .1 km to 5 km and virtual temperature profiles from .1 km to 1.5 km.
Virtual temperatures are recovered by transmitting an acoustic signal vertically and measuring the electromagnetic energy scattered from the acoustic wavefront. The propagation speed of the acoustic wave is proportional to the square root of the virtual temperature.
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
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 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
Data acquired 11 – 12 times an hour…hourly averages produce vertical profiles of average wind speed and direction between 0.1 and 5km above the surface, with a resolution of 10 meters.

35. ACTIVE Radio & micro

36. ACTIVE Radio & micro

37. ACTIVE Radio & micro
This is a picture of the RASS (915 MHz profiler), I'm not sure from where. But the data on the RHS is from the ARM program RASS for April 15, There are several things of note - the variability of wind speed with height, the variability as a function of time and data dropout. This wind speed is used to get the MFRSR chord lengths.

38. CODAR – Coastal Ocean Radar (much more on this later in semester)
ACTIVE Radio, horizontal
CODAR – Coastal Ocean Radar
(much more on this later in semester)

39. What are the main advantages and disadvantages of ground-based remote sensing over space-based?
Background is homogeneous
High temporal resolution
Instrument can be fixed if it malfunctions
Power is less of a problem
Cost much lower
Disadvantages
Limited area of data coverage
Instruments must be located on land or ship
Vandalism, environmental effects