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Series I: Global Environmental Change, Vol. 35
Springer Berlin Heidelberg New York Barcelona Budapest Hong Kong London Milan Paris Santa Clara Singapore Tokyo
Clouds, Chemistry and Climate
Edited by
Paul J. Crutzen Max-Planck-Institute for Chemistry Postfach 3060 0-55020 Mainz, Germany
Veerabhadran Ramanathan Center for Clouds, Chemistry, and Climate Scripps Institution of Oceanography La Jolla, CA 92093-0239, USA
Springer Published in cooperation with NATO Scientific Affairs Division
Proceedings of the NATO Advanced Research Workshop "Clouds, Chemistry and Climate", held at Ringberg, Germany, March 21-25,1994
Cataloging-in-Publication Data applied for
Die Deutsche Bibliothek - CIP-Einheitsaufnahme
Clouds, chemistry and climate: (proceedings of the NATO Advanced Research Workshop "Clouds, Chemistry and Climate", held at Ringberg, Germany, March 21 - 25, 1994J / ed. by Paul J. Crutzen ; VeerabhadranRamanathan. - Berlin; Heidelberg; New York; Barcelona; Budapest; Hong Kong London; Milan; Paris; Santa Clara; Singapore; Tokyo: Springer, 1995
(NATO AS! series: Ser. I, Global environmental change; Vol. 35) Additional material to this book can be downloaded from http://extra.springer.c
ISBN·13:978-3-642-64672-0 e- ISBN·13:978-3-642-61051-6 DOl: 10.1007/978-3-642-61051-6
NE: Crutzen, Paul J. [Hrsg.]; Advanced Research Workshop Clouds, Chemistry and Climate <1994, Ringberg, Kreuth>; NATO: NATO AS! series / I
ISBN-13 :978-3-642-64672-0
This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations am liable for prosecution under the German Copyright Law.
© Springer-Verlag Berlin Heidelberg 1996 Softcover reprint of the hardcover 1st edition 1996
Typesetting: Camera ready by authors/editors Printed on acid-free paper SPIN: 10466185 31/3137 - 5 43210
FOREWORD
The objective of this NATO Advanced Research Workshop was to discuss our current understanding of the role of clouds in climate and chemistry. The range of topics discussed during the workshop included: modeling of clouds in GCMs; observations of the cloud micro physical properties; the water vapor cycle; troposphere-stratosphere exchange; role of in-cloud transport in tropospheric ozone; regulation of current and paleo climate by clouds; and anthropogenic sulfate aerosols and modification of cloud properties. The essence of the discussions is captured in the accompanying summary by the rapporteurs and the chapters by some of the speakers.
The underlying message is that significant progress has been made, resulting in exciting new developments in our perception of the role of clouds in the global system .
.. The tropical convective-cirrus cloud systems emerge as a major influence on the climate system. Micro physical processes, such as the rate of precipitation and re evaporation of ice particles, seem to regulate the large scale vertical distribution of water vapor which is the dominant greenhouse gas. Water vapor data collected during the Central Equatorial Pacific Experiment (CEPEX), document the large moistening effect of deep convection on scales of thousands of kilometers.
A major chemical finding in the same region was the observation of extremely low ozone volume mixing ratios of less than 10-8 in the entire troposphere of the central equatorial Pacific extending over a distance of about 2000 km. This finding establishes the Pacific as a major chemical sink region for tropospheric ozone.
It is well known that ozone can be destroyed efficiently in the marine boundary layer by the reactions
O('D) + O2 A. ~ 325 nm 20H
However, is is not yet clear whether these reactions alone are sufficient to explain the large ozone depletion (Kley et aI., 1995) or if additional chemical ozone loss may take place through aqueous -or heterogeneous- phase processes in water -or ice- clouds, respectively (Crutzen,1995). The very low ozone mixing ratios, in conjunction with the high albedo of deep convective clouds, may cause suppressed hydroxyl concentrations. If so, the transport of sulfur- and halogen- containing organics to the upper troposphere and, perhaps, even to the lower stratosphere could be more efficient than hitherto thought which might influence sulfate particulate (CCN) and ozone depletion reactions, e.g. via iodin catalysis (Solomon et aI., 1995).
VI
Data collected during CEPEX demonstrate that the radiative energy converging into the
convective-cirrus systems is a significant source for the diabatic heating of the upper tro
posphere. GCM studies reveal that this cirrus heating contributes significantly to the
tropical Walker and Hadley circulation.
Until recently, we were unable to measure the micro physical characteristics of these cir
rus clouds. Breakthroughs in instruments (CVI instrument) and availability of high alti
tude aircraft (ER-2) are enabling us to have a glimpse of this important issue. Preliminary
data seem to show the existence of numerous small crystals (size ::;20 J.1m), which have to
be included in models. But the CEPEX data also show the preponderance of large crystals
(2:250 J.1m), which have also been ignored in models.
The CEPEX data demonstrated that the reduction of solar radiation at the sea surface by
the convective-cirrus clouds far exceeded their greenhouse effect at the surface, thus ex
erting a large net radiative cooling at the sea surface over the warm tropical oceans. It is
still a matter of considerable debate whether this convective-cirrus cooling effect is the
dominant regulator of the surface temperatures in the western Pacific warm pool or com
peting processes are in effect. The discussion in this volume, not only captures some of
the essentials of this debate, but also reveals the primitive state of our understanding of
this fundamental issue in climate and climate change. Tropical temperature changes dur
ing the last glacial period and warm epochs should offer clues regarding the stability of
tropical climate. Studies of the GRIP and GISP2 ice cores demonstrate that large and
rapid climate changes occurred during the last ice age and during the transition towards
the present Holocene. Such changes are best documented in the North Atlantic and we
need to document the past changes in the tropical oceans .
.. The concept of stratospheric-tropospheric exchange has evolved over the last few years.
This pertains particularly to extra tropical wave disturbancies as drivers of the "overworld"
meridional circulation with upward transport in the tropics and compensating down
ward transport at extratropicallatitudes (see review by Holton and Lelieveld at this work
shop). However, much more needs to be learned about the transport in and to the lowest
parts of the stratosphere in the tropopause region. Observations during CEPEX in the
tropics have shown record low H 20 volume mixing ratios of about 1 ppm which clearly
indicates the importance of local, small scale, exchange processes for the trace gas compo
sition of the lower stratosphere.
Recent observations by the Upper Atmospheric Research Satellite have been of particular
significance for an understanding of stratospheric dynamics. With regard to the transfer
of ozone from the stratosphere into the troposphere at extra tropical latitudes, the correla
tions between NOy and 03' characteristic of air masses with origin in the lower" overworld"
(Murphy et al.,1993), combined with the NOy production rate by the reaction N 20 + 0(10)
-72 NO establish the role of 0 3 and NOy as conservative tracers in the lowest few kilome
ters of the stratosphere. This does not, however, applies to the late winter and early spring
VII
periods at high latitudes (especially over the Antarctic continent), probably implying a gradual decrease in the downward flow of ozone to the troposphere during the past two decades with some potential repercussions for tropospheric chemistry .
.. The role of anthropogenic sulfates in climate is clearly an important issue for future climate changes. Scattering of solar radiation by sulfates enhance the clear sky albedo (the direct effect) and can also enhance the number of cloud drops, which in tum can enhance the cloudy sky albedo (the indirect effect). Together, the direct and indirect effect of sulfates during the last century may be as large as the combined greenhouse effect. But, unlike the sound observational basis for the increases in concentration of greenhouse gases, the sulfate effect awaits observational confirmation. The direct effect depends critically on the rate of formation of new particles through gas to particle conversion of the emitted S02' The indirect effect depends on the increase in cloud droplet number concentration in
marine cloud systems. Recognizing the need for observational verification, inter national field campaigns have been initiated in the north Atlantic. But, the Far East and Asian countries are projected to be the major emitters of 502 in the next century, and the role of sulfates in the tropical setting should be investigated. In our discussions, the equatorial Indian ocean emerged as a potential site for a field campaign to test the sulfate hypothesis. The equatorial Indian ocean, during the spring season, is down wind of continental air from the Indian sub continent with very little precipitation along the way. Furthermore, the continental air ultimately converges into the inter tropical convergence zone, which provides a contrast between the polluted air from the north to the pristine air from the southern Indian ocean.
References D. KIey, H.G.J. Smit, H. Vamel, S. Oltmans, H. Grassl, V. Ramanathan and P.J. Crutzen: Record Low Ozone Observatons in the Convective Regions of the Pacific, Science (submitted)
P.J. Crutzen: Ozone in the Troposphere, in "Composition, Chemistry and Climate of the Atmosphere" (H. Singh, Editor), pp. 349-393, Van Nostrand, 1995.
S. Solomon, RR Garcia and A.R. Ravishankara: On the Role of Iodine in Ozone Depletion,J. Geophys. Res. 99 (D10),20491, 1994.
D.M. Murphy et al.: Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere, J. Geophys. Res., 98,8751, 1993.
The Editors
ACKNOWLEDGMENT
The NATO Advanced Research Workshop on Clouds, Chemistry and Climate was supported by the NATO-ARW930752 grant and the NSF-ATM8920119 grant to the Center for Clouds, Chemistry and Climate.
Pre-press production of this manuscript was supported by the Vetlesen Foundation Grant to the Scripps Institution of Oceanography.
This is publication #144 of the Center for Clouds, Chemistry and Climate, a National Science Foundation Science and Technology Center.
TABLE OF CONTENTS
Chapter 1: NATO ARW Session Summaries ............................................................ 1
1. Radiative Convective Interaction in the Tropics (T.E. Nordeng) ........................ 1 2. Role of Cirrus Clouds (5. Bormann and A. Heymsfield) ...................................... 2 3. Paleoclimate (5. E. Schwartz) .... ...................................................................... 4 4. Modeling and Parameterization (K.E. Emanuel and ]. Lelieveld) ..... ................... 7 5. Primary Results from CEPEX (W. Collins) ........................................................ 8 6. Stratosphere-Troposphere Exchange (P. Rasch and Th. Peter) ........................... 11 7. Interaction of Aerosols, Clouds and Radiation (M. Baker and J. Heintzenberg). 14
Chapter 2: Microphysical and Dynamical Control of Tropospheric Water Vapor (K.A. Emanuel and RT. Pierrehumbert) ............................................................... 17
Chapter 3: Interactions of Radiation and Microphysics (J. Heinzentberg, Y. Fouquart, A. Heymsfield, J. Strom, G. Brogniez) ............................................................ 29
Chapter 4: Lifetimes of Ice Crystals in the Upper Troposphere and Stratosphere (Th. Peter and M. Baker) .......................................................................... 57
Chapter 5: Abrupt Climatic Changes: A Global Perspective from Ice Cores (J. Jouzel, C. Hammer, C. Lorius, S. Johnsen, P. Grootes, M. Stievenard, J. White) ................ 83
Chapter 6: GCM Studies and Parameterization (E. Roeckner and H. Le Treut) ...... 109
1. Review (E. Roeckner and H. Le Treut) ............................................................ 109 2. Cloud vertical overlapping and cloud inhomogeneities: Their impact
on model validation (H. Le Treut, G. Seze, W. Yu, M. Doutriaux) .............. 118 3. Cloud sensitivity experiments with the ECHAM model (E. Roeckner
and U. Lohmann) ...................................................................................... 122
Chapter 7: The Central Equatorial Pacific Experiment (W. D. Collins, V. Ramanathan, P. J. Crutzen, A. Heymsfield, J. P. Kuettner, D. Kley, RL. Grossman) ....... 135
Some Remarks on Mechanisms for the Regulation of Tropical Sea Surface Temperature (R. Pierrehumbert) .. .............................................. 153
Chapter 8: Stratosphere-Troposphere Exchange and Its Role in the Budget of Tropospheric Ozone (T. Holton and J. Lelieveld) ................................... 173
Chapter 9: Enhanced Shortwave Cloud Radiative Forcing due to Anthropogenic Aerosols (S.E. Schwartz and A. Slingo) ......................................... 191
Chapter 10: Satellite Observations of Upper Tropospheric Aerosols (G. S. Kent) .................................................................................................................. 237
List of Participants ....................................................................................................... 261
