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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


  • 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,


  • 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: global cloud cover,

    • 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 global cloud cover,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: global cloud cover,

    • 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: global cloud cover,

    • 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: global cloud cover,

    • 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.


  • Operations global cloud cover,:

    • 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 global cloud cover,

  • 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 global cloud cover,

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 global cloud cover,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 global cloud cover,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 global cloud cover,

    • 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 global cloud cover,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.

    equator

    polar

    Sun-syn.


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


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

    1.1km apart


    FIGURE: effect of sample period or 0.16 sec“sliding” the instantaneous footprint along track for one sample period.


    Vertical Resolution


    • CloudSat Data Products: is equal to earth's circumference, 40,022km),

      • 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


    THANK YOU circulation of the atmosphere as well as on deep convective cloud systems.


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