WWRP 2018 - 1 Peer Review Report on Global Precipitation Enhancement Activities Authors: Andrea I. Flossmann, Michael Manton, Ali Abshaev, Roelof Bruintjes, Masataka Murakami, Thara Prabhakaran and Zhanyu Yao Reviewers: Zev Levin, Steven Siems and 2 anonymous reviewers EDITORIAL NOTE METEOTERM, the WMO terminology database, may be consulted at: http://www.wmo.int/pages/prog/lsp/meteoterm_wmo_en.html. Acronyms may also be found at: http://www.wmo.int/pages/themes/acronyms/index_en.html. © World Meteorological Organization, 2018 The right of publication in print, electronic and any other form and in any language is reserved by WMO. Short extracts from WMO publications may be reproduced without authorization, provided that the complete source is clearly indicated. 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The findings, interpretations and conclusions expressed in WMO publications with named authors are those of the authors alone and do not necessarily reflect those of WMO or its Members. This publication has been issued without formal editing. CONTENTS EXECUTIVE SUMMARY .................................................................................................... iii 1. INTRODUCTION ................................................................................................... 1 2. NATURAL CLOUD SYSTEMS AND THEIR VARIABILITY ................................................ 5 2.1 General introduction ........................................................................................................... 5 2.2 Formation of clouds and the role of aerosol particles ............................................................... 5 2.2.1 Formation of drops on cloud condensation nuclei (CCN) ................................................ 7 2.2.2 Formation of ice crystals on ice nucleating particles (INP) ............................................. 7 2.3 Cloud microphysics ............................................................................................................ 10 2.3.1 Growth of cloud particles .......................................................................................... 10 2.3.2 Electrical processes in cloud ...................................................................................... 12 2.4 Cloud systems of interest in weather modification ................................................................. 13 2.4.1 Introduction ........................................................................................................... 13 2.4.2 Wintertime orographic cloud systems ......................................................................... 13 2.4.3 Convective cloud systems ......................................................................................... 14 2.5 Conclusions ....................................................................................................................... 16 3. POTENTIAL FOR PRECIPITATION ENHANCEMENT ...................................................... 17 3.1 Introduction ...................................................................................................................... 17 3.2 Winter orographic clouds .................................................................................................... 17 3.2.1 Physical basis of seeding winter orographic clouds ....................................................... 17 3.2.2 Seeding strategies ................................................................................................... 20 3.2.3 Snow chemistry ...................................................................................................... 21 3.3 Convective and stratiform cloud systems .............................................................................. 22 3.3.1 Physical basis of seeding convective cloud systems ...................................................... 24 3.3.1.1 Hygroscopic seeding ................................................................................... 24 3.3.1.2 Glaciogenic seeding .................................................................................... 25 3.3.2 Seeding strategies ................................................................................................... 25 3.3.2.1 Seeding methods ....................................................................................... 25 3.3.2.2 Seeding materials ...................................................................................... 26 3.3.2.3 Transport and dispersion ............................................................................. 28 3.3.2.4 Tracer releases .......................................................................................... 29 3.4 Redistribution and “negative enhancement” of precipitation .................................................... 30 3.5 Alternative technologies ...................................................................................................... 32 3.5.1 Ionization ............................................................................................................... 32 3.5.2 Electric fields and modification of electricity ................................................................ 33 3.5.3 Laser technologies ................................................................................................... 33 3.5.4 Acoustic waves ....................................................................................................... 34 3.6 Conclusions and recommendations ....................................................................................... 34 4. OBSERVATIONS OF AEROSOL, CLOUDS AND PRECIPITATION ..................................... 36 4.1 Introduction ...................................................................................................................... 36 4.2 Observations of precipitation ............................................................................................... 36 4.2.1 Ground-based rain and snow observations .................................................................. 36 4.2.2 Ground-based radar observations .............................................................................. 37 4.2.3 Space-borne observations ........................................................................................ 38 4.3 Observation of the synoptic environment .............................................................................. 39 4.4 Observations of cloud dynamics ........................................................................................... 40 4.5 Aerosol and cloud microphysics measurements ...................................................................... 41 4.5.1 Microphysics ........................................................................................................... 41 4.5.2 Supercooled liquid water (SLW) ................................................................................ 42 4.5.3 Satellite observations .............................................................................................. 42 4.5.4 Aerosol particles ..................................................................................................... 43 4.5.5 Ice nucleating particles (INP) .................................................................................... 43 4.5.6 Snow physics .......................................................................................................... 44 4.5.7 Emerging technology ............................................................................................... 44 4.6 Laboratory measurements .................................................................................................. 44 4.7 Conclusions ...................................................................................................................... 45 5. MODELLING OF NATURAL CLOUDS AND SEEDED CLOUDS ......................................... 47 5.1 Introduction ...................................................................................................................... 47 5.2 Numerical models used in weather modification ..................................................................... 47 5.2.1 Types of model frameworks ...................................................................................... 47 5.2.2 Cloud microphysics .................................................................................................. 49 5.2.3 Seeding schemes .................................................................................................... 50 5.3 Hygroscopic seeding ........................................................................................................... 51 5.3.1 Model studies using simplified