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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

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The cloudsat mission

The CloudSat Mission

CEE: 6900-Environmental Application of Remote Sensing

Abel Tadesse Woldemichael

The cloudsat mission

  • 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,

      • 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,

The cloudsat mission

  • 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.

The cloudsat mission

  • 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,

The cloudsat mission

  • 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.

The cloudsat mission

  • 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

The cloudsat mission

  • 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 cloudsat mission

  • 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.

The cloudsat mission

  • Operations:

    • 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)

The cloudsat mission

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

The cloudsat mission

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:

    The cloudsat mission

    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

  • The cloudsat mission

    • 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)

    The cloudsat mission

    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 cloudsat mission

    • 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.




    The cloudsat mission

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

    The cloudsat mission

    FIGURE: Instantaneous footprint when satellite travels one sample period or 0.16 sec

    1.1km apart

    The cloudsat mission

    FIGURE: effect of “sliding” the instantaneous footprint along track for one sample period.

    The cloudsat mission

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

    Vertical Resolution

    The cloudsat mission

    • 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

    The cloudsat mission

    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

    The cloudsat mission


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