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Objectives of the Atmospheric

Dynamics Mission

The prim ary aim of the Earth Explorer

Atm ospheric D ynam ics M ission (AD M )

is to p rovide im proved analyses of the

global three-dim ensional w ind field, by

dem onstrating the ability to correct the

m ajor deficiency in w ind-profiling of

the G lob al C lim ate O bserving System

(G C O S) and the current G lobal

O bserving System (G O S). The A eolus-

A D M w ill provide the w ind-profile

m easurem ents required to ad vance

atm osp heric m od elling and analyses.

N ew insights into the atm osphere,

through the provision of w ind profiles,

are required, not only for clim ate

research, but also for num erical

w eather pred iction (N W P). The A eolus-

AD M w ill address one of the m ain

areas discussed under Them e 2Physical C lim ateof the ESA Living

Planet Program m e [ESA 1998;ESA

1999b]. A lthough there are several

w ays of m easuring w ind from a

satellite, the active D oppler w ind lidar

(D W L) is the only candidate, so far

identified, that can provide direct

observations of w ind profiles, and thus

has the potential to provide the

requisite data globally. In addition, the

D W L also has the potential to p rovide

ancillary inform ation about cloud topheights, vertical distribution of cloud,

aerosol properties, and w ind variability.

These w ould be b y-products.

The Need for Atmospheric Wind

Fields for Climate Studies and

Numerical Weather Prediction

(NWP)

C lim ate-change issues have received

substantial attention in recent yearsdue to the increasing aw areness that

hum an activities m ay substantially

m odify the future clim ate of the Earth.

The glob ally averag ed tem perature

has increased by ab out 0.6C over the

past hundred years and 1998 w as the

w arm est year recorded in the

instrum ental tem perature d ata

covering the last 150 years. These

facts, and other pieces of evidence,

suggest that an increase in the

greenhouse effect, due to hum anactivities, is starting to influence the

global clim ate system . A very

im portant question is, thus, to assess

how any future increase in

greenhouse g ases m ay affect this

system .

R eliab le instantaneous analyses and

longer term clim atologies of w inds areneeded to im prove our understanding

of atm ospheric d ynam ics and the

glob al atm ospheric transport and of

the cycling of energy, w ater, aerosols,

chem icals and other airborne

m aterials. H ow ever, im provem ents in

analysing global clim ate, its variability,

predictability and change, require

m easurem ents of w inds throug hout

the atm osp here.

The m ost effective tools available toansw er such q uestions are g lob al and

regional clim ate m odels, w hich to a

very large extent resem ble the

Earth Explorer Missions 12

The Atmospheric Dynamics Mission

P. Ingmann and J. Fuchs (1), J. Pailleux (2) and A. Stoffelen (3)

(1) ESA/ESTEC , P.O . Box 299, 2200 AG N oordw ijk, The N etherlands

(2) M to-France, 42 avenue C oriolis, 31057 Toulouse C edex, France

(3) Royal N etherlands M eteorological Institute (KN M I), PO Box 201, 3730 AE D e B ilt, The N etherlands

The quality of m odels used in clim ate research and num erical w eather prediction (N W P) relies m uch on the availability of

observations. M odels have im proved m uch over the last decade. Better param ersiation schem es are used in clim ate

m odels; advanced data assim ilation techniques are now being used for the analysis in N W P. H ow ever, conventional w ind

profile data lack coverage and uniform distribution over the globe. Thus, there is a need for w ind profilers in order to

im prove the G lobal (C lim ate) O bserving System .

In the context of the Earth Explorer C ore M issions, ESA is preparing a m ission aim ing at the observation of the atm ospheric

w ind profile, nam ely the Atm ospheric D ynam ics M ission (AD M ). Its m ain com ponent w ill be a D oppler w ind lidar..

Fig. 1: The intim ate links betw een num erical w eather prediction (N W P) and

clim ate studies.

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corresponding N W P m odels. A ll the

benefits of w ind data related to

circulation m odels used for clim ate

stud ies are also relevant to N W P

m odels, as b oth m odel types are

based on the sam e physical and

num erical principles. The intim acy of

the links betw een N W P and clim ate

studies is d epicted in Fig.1: there is a

continuous transition betw een N W P

and clim ate stud ies on a prog ressive

tim e scale. A ny im provem ent in N W P

w ill result in an im provem ent in clim ate

m odels. This w as d em onstrated

recently w ith the ER A -15 data set (i.e.the re-analysis of the w hole 15 year

period 1979 to 1994 carried out at

EC M W F and N C EP). A s an exam ple,

for the zonal w ind com ponent, a

com parison of the N C EP and EC M W F

m od els w as carried out, w hich

show ed m ajor differences in the

tropics and in the low er stratosphere

(Fig. 2).

This reflects m ajor deficiencies in

m od el param eterisations, w hich needto be resolved.

The different types of w ind

observations currently available and

constituting the G lobal O bserving

System (G O S), are d ocum ented in full

detail in [ESA 1996]. They can be

classified in the follow ing w ay:

Surface d ata the synoptic reports

from land stations and ships, data

from m oored and drifting buoys as

w ell as scatterom eter w ind s from

satellites (such as ER S). A ll these

are single level data and do not

provide any inform ation on

atm ospheric profiles.

Single-level upper-air data m ainlyaircraft reports and cloud m otion

w inds derived from geostationary

satellite im ag ery. M ore and m ore

aircraft observations (w ind and

tem perature) are b eing m ade

during ascent and d escent phases,

thus tending to b ecom e m ulti-

level. Their m ain deficiency is the

poor geographic data coverage,

especially over the oceans and in

the S outhern H em isphere.

M ulti-level upper-air data radiosondes are the only current

observing system providing vertical

profiles of the w ind field, but they

are confined m ainly to the

continents in the N orthern

H em isphere (Fig. 3).

C om plem entary observations are

provided by the sound er instrum ents

on board low -Earth orbiting satellites.

Satellite sounders p rovide global

coverag e w ith rad iance data, but

these can only be used indirectly for

the definition of the m ass field

(tem perature and hum idity). From

these observations, synoptic scale

w ind fields at higher latitudes can be

derived. This does not w ork at sm allerscales or at low latitudes.

The W orld M eteorolog ical

O rganisation (W M O ) states in its

recent evaluation of user

requirem ents and satellite capabilities

that, for global m eteorological

analyses, m easurem ent of w ind

profiles rem ains m ost challenging and

m ost im portant [W M O, 1998]. The

W M O recognises the p rim e need for

w ind-profile d ata [W M O, 1998] andhas d efined a set of w ind- profile

m easurem ent requirem ents [W M O,

1996]. The W M O (e.g. [W M O, 1996])

ADM13

Fig. 2: C om parison of N C EP (N ational C entre for Environm ental prediction) and ERA (EC M W F R e-Analysis) derived zonal

w inds. M ajor differences are found m ainly in the tropics and in the low er stratosphere (courtesy K N M I)

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attaches great im portance to w ind -

profile m easurem ents. The realisation

of these req uirem ents w ould rep resent

a m ajor step forw ard in im proving the

quality of atm ospheric flow analyses.

There is a clear requirem ent for a

high-resolution system observing

atm osp heric w ind profiles directly.

The Observational Requirements of

the Atmospheric Dynamics Mission

Existing system s cannot m eet the

requirem ents for better w ind profiles.

In order to m eet the needs of N W P,

clim ate and atm osp heric research

ob jectives, an ob serving system has

to be set up that provides three-

dim ensional w ind data over the glob e.This m eans that it is essential to

devote significant effort to the

developm ent of space-based

system s.

For a m ission intended to dem onstrate

the feasibility of a full-scale sp ace-

borne w ind ob serving system to

im prove glob al atm osp heric analyses,

the req uirem ents on data quality and

vertical resolution are very stringent

and difficult to achieve. The horizontaldensity of observations is of low er

priority. The derivation of the coverage

specification is supported by w eather-

forecast-im pact experim ents, w hich

included the inputs from the

conventional w ind-profile netw ork, that

provides poor geographic coverage

and is irregular in tim e but

nevertheless is of key im portance. The

use of variational assim ilation

techniques im plies that only line-of-

sight (LO S) w inds are required; vector

w ind s are not necessary. Table 1

specifies the principal param eters for

w ind-profile observations that have

been extracted from the previously

m entioned W M O requirem ents and

capab ilities docum ents.

The Technical Concept of theAtmospheric Dynamics Mission

The m easurem ent concep t is

illustrated in Fig. 4: the heart of the

m ission is a space-borne D oppler

w ind lidar w hich em its ultraviolet

pulses tow ards the Earth. These are

reflected by the atm osp here and by

Earth Explorer Missions 14

O bservational R eq uirem ents

PB L Troposph. Stratosph.

Vertical D om ain [km ] 0-2 2-16 16-20

Vertical R esolution [km ] 0.5 1.0 2.0

H orizontal D om ain global

N u m b er of P rofiles [hour-1] 100

Profile Separation [km ] > 200

Tem poral Sam pling [hour] 12

Accuracy (C om ponent) [m s-1] 2 2-3 3

H orizontal Integration [km ] 50

Tim eliness [hour] 3

Leng th of O bservational

D ata Set [yr] 3

Table 1: O bservational requirem ents for the Atm ospheric D ynam ics M ission

(PBL = planetary boundary layer).

Fig. 3: The radiosonde netw ork radiosonde/pilot ascents containing w ind profile inform ation that w ere available for the 6-

hour tim e w indow centred around 12 U TC on 28 April 1999. W ind profile inform ation is generally m issing over all ocean

areas.

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M ean O rbit Param eters

O rbit Type Sun-synchronous orbit

at 18:00 LTAN

M ean A ltitude 408 km

Inclination 96.99

the Earths surface. These back-

scattered signals are p rocessed to

provide quantitative inform ation on

vertical w ind profiles. The lidar

exploits the D oppler shifts induced by

the w ind on the received signal back-

scattered from aerosols or m olecules.

In addition, inform ation can be

derived ab out other param eters

(e.g. cloud cover or aerosol content).

Electrom agnetic radiation in the

ultraviolet is heavily attenuated by

cloud. A com plete profile can thusonly be derived in a cloud-free or

partly cloud-free atm osphere, through

gap s betw een clouds. H ow ever, it has

been dem onstrated that in about 50 %

of the cloudy cases it is still possible

to retrieve w ind profiles w ith high

quality [ESA 1999a]. If a scene is

overcast, w ind profiles can be derived

for the layers above the clouds.

Furtherm ore, sim ulations have show n

that cloud-free observations alone

w ould still have a significant im pact.

The m ission requires the m easurem ent

of horizontal w ind velocity

com ponents from the low er part of thetroposphere to the low er part of the

stratosphere (up to 20 km altitude). A s

the ob servation of a sing le com ponent

of the horizontal w ind velocity has

been show n to be adequate to m eet

the m ission ob jectives of A eolus-A D M ,

this has been b aselined to ease

instrum ent design.

For the satellite, a sun-synchronous

daw n-dusk orbit has been selected .

This p rovides quasi-glob al coverag eand corresp onds to a low er cloud

coverage w hile, in turn, it w ill also

im prove p erform ance. It also

facilitates som e asp ects of the satellite

design.

The LO S of the instrum ent w ill be 35

off nadir, to ensure optim al instrum ent

perform ance. and 90 across the flight

direction to avoid a contribution from

the satellite velocity to the D oppler-

frequency shift. To dim inish

background radiation effects, the LO S

of the instrum ent w ill point in the anti-

Sun direction. The equator crossing

tim e has b een chosen to p rovide lowcloud-cover conditions. The orbit w ill

provide a slightly b etter coverage over

the N orth Pole than over the South

Pole. A baseline altitude of 400 km

has been selected . The orbit

param eters are sum m arised in Table 2.

The instrum ents orientation is

depicted in Fig. 5. The sam pling

schem e foresees one m easurem ent

being m ade every 200 km and an

integration length of 50 km .

The req uired instrum ental accuracy

for any horizontal LO S w ind

ADM15

Fig. 4: D oppler W ind Lidar principle: the lidar em its a laser pulse tow ards the atm osphere, then collects, sam ples, and

retrieves the frequency of the back-scattered signal. The received signal frequency is D oppler-shifted from that em itted by

the laser, due to the spacecraft, Earth, and w ind velocities. The lidar m easures the w ind projection along the laser line-of-

sight (LO S), using a slant angle relative to nadir.

Table 2. M ain param eters of selected

baseline orbit.

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sea) echo w ill be d etected tocalibrate distance (height).

In order to transform raw data into

w ind m easurem ents, processing has

to be perform ed for both the aerosol

and the m olecular channels. This

requires taking into account

background radiances and the

spectrom eter spectral response, as

w ell as calibration data. Finally, using

satellite sensor data, each w ind

m easurem ent for each altitud e layerm ust be located in an E arth reference

fram e.

A schem atic flow diag ram for the

data processing, calibration andvalidation, and inform ation

dissem ination for the space-borne

D oppler w ind lidar is show n in Fig. 6.

The processing of D W L data in a

realistic atm osphere has been

sim ulated in order to verify

perform ance in b oth clear and cloudy

areas. C ontrary to passive satellite

sensors, w hich have relatively large

footprints, an active D W L, firing m any

shots per second , exhibits good

potential for extracting usefulinform ation on the atm osphere in

partly cloudy cases. This w as m ad e

clear in the LITE back-scatter lidar

com ponent has been translated from

the m ission accuracy req uirem ent of

2-3 m s-1 for each w ind com ponent

(see Table 1). Stringent requirem ents

on w ind accuracy and the large

vertical dom ain (up to 20 km ) lead to

the consideration of an instrum ent

concep t w hich relies on m olecular

backscatter at high altitude (w here

background aerosols becom e rare)

and on aerosol backscatter at low er

altitude. Fig. 5 also show s the

baseline m easurem ent profile.

The d esign features of the selected

baseline concep t are show n in [M gie

and Readings, this issue, Fig. 4]. The

satellite is a conventional boxstructure w ith a central cone upon

w hich the instrum ent is m ounted via

the three isostatic m ounts (bipods).

Since the satellite is flow n in a daw n-

dusk orbit, the y-side alw ays faces

aw ay from the Sun, and the solar

arrays can be w ing s, one forw ard and

one aft of the bus (w ith reference to

the flight direction), fixed in

orientation.

Data ProcessingThe instrum ent w ill transm it raw data

to the ground, w hich consists of the

accum ulated spectra from the M ie

receiver and the flux intensities from

the R ayleigh receiver. These data are

nom inally provided every 3.5 km

along-track. The instrum ent has the

potential to provide data every 1 km

horizontally for dedicated areas (when

required) and for each altitude bin

(-1 km (for calibration) to 16.5 km

height for the M ie channel, 0.5 km to

26.5 km for the R ayleigh channel). Thevertical resolution is configurable

betw een 500 m and 2 km .

Prior to integrating data over 50 km

along-track, som e signal processing

w ill be carried out to segregate the

sam ples from clear air from those

affected by cloud, to control the

processing in case of variable

conditions, e.g., due to cloud. A s

such, besides w ind profiles ab ove

clouds and optically thick cloudlayers, even full w ind profiles of useful

quality m ay be obtained in m any

cases of scattered cloud. G round (or

Earth Explorer Missions 16

Fig. 5: Baseline Aeolus-AD M m easurem ent geom etry the baseline

m easurem ent profile depicting the m apping of atm ospheric heights to layers

m easured by the detector. The vertical as w ell as the horizontal values can be

program m ed, thus providing good flexibility.

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European S pace A gency, 1999b : The

ESA Living PlanetProg ram m e, EO Q ,

N 63, pp 1-20

W orld M eteorolog ical O rganisation,

1996: G uide to M eteorolog ical

Instrum ents and M ethods of

O bservation, 6th edition, W M O -N o.8,

Secretariat of the W orld

M eteorolog ical O rganisation, G eneva,

Sw itzerland.

W orld M eteorolog ical O rganisation,

1998: Prelim inary S tatem ent of

G uidance R egarding H ow W ell

Satellite C ap ab ilities M eet W M O U ser

R equirem ents in Several A pplication

A reas. W M O Satellite R ep orts SAT-21.

W M O /TD No

913.

ADM17

Fig. 6: Schem atic flow diagram for the data processing, validation and

calibration of the inform ation from a space-borne D W L (L = processing level).

m ission on the Space S huttle. The

sim ulated D W L m easurem ents have

also been assim ilated in a state-of-

the-art atm ospheric data assim ilation

system , in a so-called O bserving

System Sim ulation Experim ent

(O SSE). Im proved analyses and

forecasts w ere dem onstrated in the

N orthern H em isphere through the

addition of A eolus-A D M D W L

m easurem ents.

Context

The A tm ospheric D ynam ics E arth

Exp lorer C ore M ission w ill for the first

tim e provide direct observations, on a

global scale, of atm ospheric w ind

profiles in clear or partly cloudy air

over the dep th of the atm osphere, a

notable deficiency of currentob serving system s. These data w ill

find w ide application in advancing the

perform ance of num erical m odels

used in clim ate research and w eather

forecasting, as these are suffering

increasingly from the lack of such

data. R eliab le m easurem ents of the

tropospheric, three-dim ensional, w ind

field are of the utm ost im portance for

N W P, seasonal-to-interannual

forecasting and for studying

atm osp heric dynam ics, energeticsand the w ater, chem ical and aerosol

cycles associated w ith the state of the

global clim ate and its future evolution.

W ith these data it w ill also be possible

to increase und erstanding of

atm ospheric processes occurring in

tropical regions to the point w here it

w ill be possible to take proper

account of them in clim ate m od els. A t

present, in tropical regions little

inform ation ab out atm ospheric

dynam ics can be inferred from the

existing G O S. The proposed concep t

also m eets the req uirem ents of grow th

potential, w hich is relevant in view of a

future operational m ission.

M ore detailed inform ation on the

scientific context and the m ission

im plem entation can b e found in

[ESA 1999a].

ReferencesEuropean Space A gency, 1996:

A tm ospheric D ynam ics M ission, ESA

SP-1196 (4).

European Sp ace A gency, 1998: The

Science and R esearch E lem ents of

ESAs Living Planet Prog ram m e, ESA

SP -1227, 105p p.

European Space A gency, 1999a:

A tm ospheric D ynam ics M ission,

R eport for M ission Selection, ESASP-1233(4) also on the internet at

< http://w w w.estec.esa.nl/m ag/doc/ad

m .pdf>