The CloudSat The CloudSat MissionMission

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:◦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.

◦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)

CloudSat OperationsCloud 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

• Other effects that come in to play with selecting a 94GHz radar frequency are:

Matching the competing and conflicting factors:

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

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 Horizontal Spatial 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 orbit at 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.

polarSun-syn.

equator

Each profile will have 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 “sliding” the instantaneous footprint along track for one sample period.

Vertical Resoluti

on

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

◦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

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)

45 W/m

2 3 W/m

2

12 W/m

2

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