1 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM ACECHEM Atmospheric Composition Explorer for Chemistry - Climate Interactions

2 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Contents 1. Scientific Rationale 2. Observational Requirements 3. Evaluation of Techniques 4. Mission Elements 5. Data Assimilation and Utilisation 6. Performance Assessment 7. Summary of Scientific Case 8. System Concept

3 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Scientific Imperative Human activities are changing atmospheric composition. Composition changes in the UTLS couple strongly with climate. The processes involved are complex and not well understood. Important constituents are highly variable in the UT, requiring that vertical and horizontal structure be captured on fine scales.  There is a need for high-fidelity, global observations of Upper Tropospheric / Lower Stratospheric (UTLS) composition.  The Atmospheric Composition Explorer for CHEMistry - climate interactions (ACECHEM) mission will meet this need and will greatly improve our understanding of these issues.

4 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Changing Composition & Impacts Important changes have been detected in trace gas concentrations and related variables during recent decades: –Increases in long-lived greenhouse gases (eg CO 2, CH 4, N 2 O). –Height-dependent changes in O 3, H 2 O and T. Decrease in stratospheric O 3 and concomitant increase in UVB. Changes to tropospheric O 3 vary from region to region. Stratospheric H 2 O increase Stratospheric cooling Natural phenomena such as the solar cycle and volcanoes (El Chichon, ‘82; Pinatubo, ’91) also perturb composition.

5 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Trends in O 3 and H 2 O profiles O 3 trend from European sondes at 50 o N

6 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale The Four Mission Objectives Objective 1: Role of the UTLS for Radiative Forcing and Feedback Objective 2: Role of Stratosphere-Troposphere Exchange in Atmospheric Composition and Climate Objective 3: Interaction of Stratospheric Chemistry and Climate Objective 4: Impact of Pollution on the Upper Troposphere

7 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Objectives 1 & 2 and specific questions Objective 1: Role of the UTLS for Radiative Forcing and Feedback –What are the spatial and temporal variabilities of greenhouse gases in the UTLS region and their radiative forcing or feedback? –What are the chemical and dynamical processes responsible for the variability of ozone and its precursors in the UTLS region? –What are the climatic and chemical effects associated with aerosols and cirrus in the UTLS region? Objective 2 : Role of Stratosphere-Troposphere Exchange in Atmospheric Composition and Climate –How are the stratosphere and the troposphere coupled dynamically and how do the processes involved differ with latitude? –What is the role of STE for the budgets and distributions of O 3, H 2 O and the greenhouse gases which are destroyed in the stratosphere? –What is the spatial and temporal variability of tropopause height and temperature and how does this variability impact on STE?

8 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Objectives 3 & 4 and specific questions Objective 3: Stratospheric Change and its Interactions with Stratospheric Chemistry and Climate –How will the ozone layer evolve in a cooler stratosphere where Cl begins to decrease and H 2 O continues to increase? –What are the mechanisms underlying future ozone changes? –How large is chemical ozone loss in the presence of substantial variations due to transport?. Objective 4: Impact of Pollution on the Upper Troposphere – What is the influence of pollutant export from industrial regions on the composition of the upper troposphere? –How does the atmosphere cleanse itself of greenhouse gases and aerosol precursors and how will its oxidizing capacity evolve in future? –What is the contribution of forest fires and other biomass burning events to the composition of the upper troposphere, particularly in tropical regions?

9 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Radiative Forcing & Feedback Tropospheric O 3 has contributed ~15-20% (0.35 Wm -2 ) to radiative forcing since pre-industrial times. Stratospheric O 3 depletion has contributed –0.15 Wm -2 to radiative forcing between 1979 and H 2 O contributes most to natural greenhouse effect and to variance in clear-sky outgoing longwave radiation. H 2 O feedback amplifies radiative forcing from other gases in GCM predictions and depends on vertical profile of H 2 O change. Sensitivity high at tropopause because temperatures are coldest  Vertical profiles of O 3 and H 2 O in the UT and LS are crucial to their radiative forcing and feedback, respectively.

10 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Sensitivity of T s to O 3 & H 2 O O 3 H 2 O

11 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Stratosphere-Troposphere Exchange STE occurs on a range of scales and is highly variable in space and time, as is tropopause height. –Tropics: upward transport of tropospheric air –Extra-tropics: downward transport of stratospheric air Global budgets of eg O 3 in UT and H 2 O in LS depend on STE. Changing atmospheric composition affects the thermal and dynamical structure of the UTLS, and hence STE.  Feedbacks on composition and climate via STE are possible.  Observations of trace gases and temperature needed at high vertical and horizontal resolution to quantify STE at global scale.

12 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Mean Meridional Circulation

13 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Stratosph. Chemistry - Climate Interaction Stratosphere expected to continue to cool during coming decades Further increase in stratospheric H 2 O would cause additional cooling Change in thermal structure will also modify circulation patterns:  distribution of O 3 and its radiative heating/cooling will change  feedback on temperature. Change to stratospheric O 3 will also affect penetration of solar UV into the troposphere.  Stratospheric O 3 is a medium for chemistry-climate interaction  Global observations needed at end of the decade to study processes of interaction between O 3 and climate in a cooler stratosphere.

14 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Arctic winter/spring chemical O 3 loss HALOE O 3 Predicted O 3

15 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale O 3 loss in a colder stratosphere

16 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Pollution in the Upper Troposphere Chemical composition of UT affected by: –Convective transport of pollutants up from boundary layer –Local sources (O 3 from CO, CH 4 & NMHCs in presence of NO x ; NO x from lightning) NO x reservoir in UT is HNO 3 (PAN). Many gases emitted from biosphere and human activities (eg CH 4 and HCFCs) are removed by OH. [OH] controlled by H 2 O, O 3 (locally and in stratosphere), CH 4 and CO (CH 3 COCH 3 & HCHO).  Tropospheric OH is a medium for chemistry - climate interaction.  Global, height-resolved measurements of key trace gases needed in the UT to: –distinguish local chemistry from pollutant transport –constrain the OH distribution in models.

17 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Scientific Rationale Model Distributions of CO in UT CO (ppbv) at 200hPa in September Pollution from N.AmericaPlume from Biomass Burning in Brazil

18 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Observational Requirements Mission objectives span tropical - polar latitudes  Measurements with fully global coverage required. Mission must cover: two quasi-biennial oscillations; cold & warm Arctic winters; El Niño/La Niña.  Mission duration of 5 years or longer  Satellite observations essential for global perspective on UTLS. For each objective: height-resolved observations required of a suite of trace gases and other geophysical variables For each variable: height-range, precision, vertical resolution, horizontal & temporal sampling must be considered in parallel.  High-fidelity measurements required of a number of variables

19 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Observational Requirements Quantitative Requirements Left / right = target / threshold

20 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Evaluation of Techniques Limb-Sounding and Cirrus Impact Passive techniques capable of measuring full suite of target gases over 5-years. Vertical resolution and sensitivity requirements in UT and LS stringent but attainable by limb-sounding. Tropospheric limb-sounding possible only in mm-wave, mid-IR and near-IR “windows”. Cloud climatology for UT limb-viewing at 1  m from SAGE-II: opaque: <10% in mid & high latitudes; ~15% in tropics sub-visual: ~20% in mid & high latitudes; ~40% in tropics Cirrus extinction comparable to 1  m in mid-IR, but orders of magnitude smaller in mm-wave.  Limb-mm affected by only a fraction of opaque clouds and not affected by sub-visual clouds (or by aerosol or PSCs).

21 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Evaluation of Techniques SAGE-II Cloud Climatology

22 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Evaluation of Techniques Cirrus Extinction vs Wavelength

23 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Mission Elements Dedicated Platform for UT & LS Mission to sound the UT & LS will comprise three limb-sounders on a dedicated platform: MASTER - Spectrometer with three mm-wave bands to target: –H 2 O, O 3 and CO in the UT –H 2 O, O 3, CO, HNO 3 and N 2 O in the LS and two sub-mm bands to target: –ClO, BrO and HCl in the LS AMIPAS - Spectrometer with three mid-IR bands to target: –T, H 2 O, O 3, HNO 3 and organic species in the UT –T, H 2 O, O 3, HNO 3, CH 4, N 2 O, NO 2, N 2 O 5, ClONO 2 & (H)CFCs in LS LCI - Imager with mid-IR and near-IR channels for: –cirrus/PSC and aerosol detection in the UT & LS at higher spatial resolution than the spectrometers.

24 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Mission Elements Formation Flight with MetOp for LT Observations of the lower troposphere (LT) are also required: Nadir-sounders can see to the surface in atmospheric windows. Suite of nadir-sounders to be deployed on MetOp. GOME-2: O 3 profiles; NO 2, H 2 CO and SO 2 columns in polluted air IASI: Tropospheric T and H 2 O profiles; CO and CH 4 columns AVHRR-3: Images of cloud, column aerosol & surface properties  Combining with co-located, limb observations of UT will allow LT to be discriminated.  ACECHEM platform will fly in formation to synchronise with MetOp’s nadir observations.

25 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Data Assimilation & Utilisation Data assimilation provides an objective means to: –interpolate, integrate diverse observations and assess data quality –derive the distribution of species which interact chemically with observed species It will play a key role for ACECHEM through integration of: 1mm-wave and mid-IR limb-sounders 2limb-sounders with MetOp nadir-sounders, and with other satellites 3satellites with airborne and ground-based sensors 4observations with dynamical information at finer scales from models 4Unmeasured trace gases (eg OH) will be derived through their interaction with measured species by chemical data assimilation The large research community will use data in diverse ways: –Process studies (the four objectives) –Constraints for global and regional chemical transport models –Trace gas climatologies (>5 yrs) for evaluation of models –Improve GCM dynamics and radiation, and hence their predictions

26 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Data Assimilation & Usage Assimilation of GOME O 3 columns Ozone “Mini-Hole” 30 th Nov’99

27 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Retrieval Simulations The ability to fulfil research objectives can be gauged by comparing simulated retrievals with quantitative requirements. ACECHEM sensors (as per System Concept) have been simulated extensively with realistic errors. Simulations for MetOp and missions preceding ACECHEM performed on common basis.

28 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Simulations & Requirements: O 3

29 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Simulations & Requirements: H 2 O

30 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Simulations & Requirements: CO

31 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Simulations & Requirements: HNO 3

32 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Simulations & Requirements: N 2 O

33 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment Simulations & Requirements: ClO

34 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment MASTER 2-D H 2 O simulations

35 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment MASTER 2-D H 2 O simulations

36 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment MASTER 2-D H 2 O simulations

37 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Performance Assessment H 2 O & T retrieval in NWP frame

38 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Summary of Science Case There is a compelling science case for the: Atmospheric Composition Explorer for Chemistry - Climate Interactions ACECHEM will: 1Meet the stringent, quantitative requirements for the four mission objectives 2Sound UT composition with unprecedented fidelity 3Add value to MetOp in data assimilation for NWP 4Underpin environmental policy and contribute to GMES

39 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Summary of Scientific Case Lower Stratosphere Innovation ACECHEM will uniquely allow the processes which control stratospheric O 3 at the global scale to be examined in 2010: –O 3 and other key variables will be observed with higher fidelity and/or improved geographical coverage than preceding missions.  This innovation will greatly improve our understanding of stratospheric composition - climate interaction.

40 Earth Explorers, User Consultation Meeting, October 2001 ACECHEM Summary of Science Case Upper Troposphere Innovation ACECHEM is the first space mission optimised specifically to sound the composition of the upper troposphere: –O 3 and H 2 O will be observed in the UT with unprecedented vertical resolution.  Unique, global perspective on their roles in climate and on the dynamical processes which govern their distributions, notably STE. –HNO 3, CO and organic species will be observed in the UT simultaneously with O 3 and H 2 O, with unprecedented resolution  Differentiate UT chemistry from upward transport of pollutants and downward transport of O 3 and NO y, and constrain OH in models.  These innovations will greatly improve our understanding of the interactions between UT composition and climate.