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The CloudSat Mission. CEE: 6900 -Environmental Application of Remote Sensing Abel Tadesse Woldemichael. Overview Clouds: are not just white things that break up the monotony of the sky, Actually are the fundamental stages of cycle of water in the atmosphere,

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the cloudsat mission

The CloudSat Mission

CEE: 6900-Environmental Application of Remote Sensing

Abel Tadesse Woldemichael



    • Clouds:
      • are not just white things that break up the monotony of the sky,
      • Actually are the fundamental stages of cycle of water in the atmosphere,
      • also play a crucial role in influencing our environment,
      • Even a small change in their abundance or distribution can alter the climate more than the anticipated changes in greenhouse gases,
    • How much do we know about clouds?
      • Not Much!
      • Our current global perspective about clouds is derived from spectral radiances measured by sensors on satellites,

These satellites have produced comprehensive pictures of global cloud cover,

  • They also depict how clouds either reflect or hold in radiant heat energy from the sun,
  • But so far we do not understand how that energy is distributed throughout the atmosphere,
  • what we need is a tool like RADAR that can actually see into clouds,
  • Hence, the birth of the CloudSat mission

This heat energy is what drives the planet’s climate and weather

The NASA CloudSat mission uses radar in a unique way to discover more about the interior of clouds and hence resolving much of the unknowns about clouds.


Mission Objectives:

    • Why CloudSat?
      • It has a number of important goals in its mission, including:
        • improving weather prediction,
        • help mitigate natural hazards,
        • aid water resource management,
        • clarify climatic processes, and
        • develop critical spaceborne technologies.
      • Furthermore,
        • It is designed to clarify the relationship between clouds and climate,
        • It contributes to the better understanding of cloud-climate feedback problem,
        • Also furnish data needed to evaluate and improve the way clouds are parameterized in global models,

Results of CloudSat mission can help the world’s weather forecasters answer the following questions:

    • How much water and ice is the cloud expected to contain?
    • How much of that water is likely to turn into precipitation?
    • What fraction of the globe’s cloud cover produces precipitation that reaches the ground?
    • Quantitatively evaluate the representation of clouds and cloud processes in global atmospheric circulation models, leading to improvements in both weather forecasting and climate prediction;
    • Quantitatively evaluate the relationship between the vertical profiles of cloud liquid water and ice content and the radiative heating by clouds.

CloudSat Operation:

    • Launch History, site and vehicle:
      • History:
        • CloudSat was selected as NASA Earth System Pathfinder (NASA-ESSP) satellite mission in 1999,
        • CloudSat was launched on April 28, 2006,
        • its primary mission is scheduled to continue for 22 months,
        • Since 2006, CloudSat has flown the first satellite-based millimeter-wavelength cloud radar (—a radar that is more than 1000 times more sensitive than existing weather radars.)
      • Launch Site:
        • Together with CALIPSO (another ESSP mission satellite), was launched from space Launch Complex 2W at Vendenberg Air Force Base, California .

the Earth System Science Pathfinder Program sponsored missions are designed to address unique, specific, highly focused scientific issues, and to provide measurements required to support Earth science research


Launch Vehicle:

    • CloudSat was launched from a two stage Delta launch vehicle (a vehicle that has a success rate of 98%) with a dual payload attachment fitting (DPAF).
    • Delta II payload Capability ranges from 2.7 to 5.8 metric tons,
    • With its payload, the vehicle stood 39meters.

The A-Train Concept:

    • The satellite will fly in orbit around Earth in a tight formation with the CALIPSO satellite, which carries a backscattering lidar,
    • In turn, the two satellites will follow behind the Aqua satellite in a looser formation,
    • As a group, the satellites have been referred to as the A -Train,
    • The combination of data from the CloudSat radar with coincident measurements from CALIPSO and Aqua provides a rich source of information that can be used to assess the role of clouds in both weather and climate.


    • CloudSat uses advanced radar to “slice” through clouds, (Active Sensor scenario)
    • It uses millimeter wave radar that operate at wavelengths of approximately 3 to 8 mm (or frequency of 94 or 35 GHz)

CloudSat Operations

  • Cloud Profiling Radar (CPR)
    • The CloudSat payload is a 94GHz CPR [developed jointly by NASA's Jet Propulsion Laboratory (JPL) and the Canadian Space Agency (CSA)],
    • Why 94GHz Radar Frequency (=3.1 mm wavelength)?
      • It was explained by NASA as a tradeoff between:
        • Sensitivity
        • Antennae Gain,
        • Atmospheric Transmission,
        • Radar Transmitting efficiency.
      • Sensitivity and antenna gain increase with frequency while atmospheric transmission and transmitter efficiency decrease with frequency.
      • 94GHz was found to be a Good Compromise
Competing Factors

Conflicting factors

  • High Vertical Resolution
  • Resolving Atmospheric attenuation,
  • and hence improving Sensitivity of the radar receiver,
    • Radar Technology
  • Launch constraint
    • (both affecting antennae size and transmitter power
Other effects that come in to play with selecting a 94GHz radar frequency are:Matching the competing and conflicting factors:

Radar Intensity is measured by a reflectivity

factor (Z)

                • Z [mm^6/m^3]
    • Where:
      • ni = No. of particles per unit volume,
      • Di = Diameter of particles
  • Also Z is expressed in dBZ:

This is to account for very large and very small numbers


What does dBZ stand for?

    • Literally:
      • dB= “decibel” ( unit used to express differences in relative power or intensity)
      • Z= Reflectivity factor (amount of transmitted energy that is reflected back to the radar receiver)
    • In general:
      • The higher the dB value the larger the object detected (Ex: Large rain drops),
      • Values of dBZ<15 usually are indication of very light precipitation that evaporates before reaching the ground.
      • From this stand point: original requirements on CPR were: sensitivity defined by a minimum detectable reflectivity factor of -30 dBZ
  • (this is due to the fact that clouds are weak scatterers of microwave radiation)

Other CPR Properties

  • Radar sampling takes place at 625KHz:
    • Burst rate = 0.16s/burst
    • PRF = 4300
    • For this we can compute:
      • (4300 pulse/sec)(016 s/burst) = 688pulse/burst
    • The CloudSat antennae has a diameter of 1.85m
    • It will provide an instantaneous footprint of approximately 1.4km (=Cross Track HorizontalSpatial Resolution)
  • TERMS:
  • burst rate: interval to create a CloudSat “ra y” (also called Profile)
  • PRF = Pulse Repetition frequency
  • Footprint: an area covered by a satellite

The CPR instrument will be flown in a sun-synchronous orbitat an 89o inclination angle, and a nominal altitude of 705 km. (720km?)

  • This orbit character will produce an along track velocity of 7km/s
  • Using this velocity, and the sample rate of 0.16 sec/profile, we can approximate that a CPR profile will be generated every 1.1 km along track.





Each profile will have 125 vertical bins (slices, representing), and each bin will be approximately 240m thick. ( Vertical Spatial resolution


A CloudSat Data “Granule” is defined as one orbit (which is equal to earth's circumference, 40,022km),

Vertical Resolution


CloudSat Data Products:

    • CloudSat's standard data products include:
      • calibrated cloud-profiling radar reflectivity data, as well as
      • cloud geometric profile,
      • cloud classification,
      • cloud optical depth by layer,
      • cloud liquid water content,
      • cloud ice water content,
      • atmospheric radiative fluxes and heating rates,
      • cloud geometrical profile with lidar input from CALIPSO, and
      • cloud classification with lidar input from CALIPSO

This heating exerts a dominant influence on the large-scale circulation of the atmosphere as well as on deep convective cloud systems.

  • Major Areas of Application
  • Model-to-model variation of prediction of climate warming,
    • Occurring as a result of the inadequate prediction of cloud properties and the different way models specify vertical climate distribution,
    • the vertical distribution and overlap of cloud layers directly determine both the magnitude and vertical profile of radiative heating, (Graeme S.L)
    • CloudSat has got its application in slicing through the cloud and finding out the radiative heating rate,

Cloud Radiative Heating (K/Day) for various thickness of clouds:

For example, high cloud layers heat the tropical atmosphere by more than 80 W m−2 (relative to clear skies)

12 W/m2

45 W/m2

3 W/m2