Forcing, Precipitation and Cloud Responses to Individual Forcing Agents by Tao Tang Earth and Ocean Sciences Duke University Date:______________________ Approved: ___________________________ Drew Shindell, Supervisor ___________________________ Prasad Kasibhatla ___________________________ M. Susan Lozier ___________________________ Apostolos Voulgarakis ___________________________ A. Brad Murray Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Earth and Ocean Sciences in the Graduate School of Duke University 2020 i v ABSTRACT Forcing, Precipitation and Cloud Responses to Individual Forcing Agents by Tao Tang Earth and Ocean Sciences Duke University Date:______________________ Approved: ___________________________ Drew Shindell, Supervisor ___________________________ Prasad Kasibhatla ___________________________ M. Susan Lozier ___________________________ Apostolos Voulgarakis ___________________________ A. Brad Murray An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Earth and Ocean Sciences in the Graduate School of Duke University 2020 Copyright by Tao Tang 2020 Abstract Previously, we usually analyze climate responses to all the climate drivers combined. However, the climate responses to individual climate drivers are far from well-known, as it is nearly impossible to separate the climate responses to individual climate drivers from the pure observational records. In this dissertation, I analyzed the responses of effective radiative forcing (ERF), precipitation and clouds to five individual climate drivers by using the model output from the Precipitation and Driver Response Model Inter-comparison Project (PDRMIP, consisting of five core experiments: CO2x2, CH4x3, Solar+2%, BCx10, and SO4x5). Firstly, I compared the ERF values estimated by six different methods and demonstrated that the values estimated using fixed sea- surface temperature and linear regression methods are fairly consistent for most climate drivers. For each individual driver, multi-model mean ERF values vary by 10-50% with different methods, and this difference may reach 70-100% for BC. Then, I analyzed the dynamical responses of precipitation in Mediterranean to well-mixed greenhouse gases (WMGHGs) and aerosols and found that precipitation in Mediterranean is more sensitive to BC forcing. When scaled to historical forcing level, WMGHG contributed roughly two-thirds to the Mediterranean drying during the past century and BC aerosol contributed the remaining one-third by causing a northward shift of the jet streams and storm tracks. Lastly, I explored the responses of shortwave cloud radiative effect (SWCRE) to CO2 and the two aerosol species and found that CO2 causes positive iv SWCRE changes over most of the Northern Hemisphere during boreal summer, and BC causes similar positive responses over North America, Europe and East China but negative SWCRE over India and tropical Africa. When normalized by global ERF, the change of SWCRE from BC forcing is roughly 3-5 times larger than that from CO2. SWCRE change is mainly due to cloud cover changes resulting from the changes in relative humidity, and to a lesser extent, changes in circulation and stability. The SWCRE response to sulfate aerosols, however, is negligible compared to that from CO2 and BC, because the radiation scattered by clouds under all-sky conditions will also be scattered by aerosols under clear-sky conditions. As SW is in effect only during daytime, positive (negative) SWCRE could amplify (dampen) daily maximum temperature (Tmax). Using a multi-linear regression model, I found that Tmax increases by 0.15 K and 0.13 K given unit increase in local SWCRE under the CO2 and BC experiments, respectively. When domain-averaged, SWCRE changes contributed to summer mean Tmax changes by 10-30% under CO2 forcing and by 30-50% under BC forcing, varying by regions, which can have important implications extreme climatic events and socio- economic activities. v Dedication This dissertation is dedicated to my family and beloved ones. vi Contents Abstract ......................................................................................................................................... iv Dedication ..................................................................................................................................... vi Contents ....................................................................................................................................... vii List of Tables ................................................................................................................................. ix List of Figures ................................................................................................................................ x Acknowledgements .................................................................................................................... xii 1 Introduction and Overview ...................................................................................................... 1 1.1 Basic Concept .................................................................................................................... 1 1.2 PDRMIP Background ....................................................................................................... 3 1.3 Motivations ........................................................................................................................ 5 2. Comparison of Effective Radiative Forcing Calculations using Multiple Methods, Drivers and Models .................................................................................................................... 11 2.1 Introduction ..................................................................................................................... 11 2.2 Data and Methods .......................................................................................................... 13 2.2.1 Model data .................................................................................................................. 13 2.2.2 Estimating ERF .......................................................................................................... 16 2.3 Results .............................................................................................................................. 20 2.4 Discussion and Summary .............................................................................................. 33 3. Dynamical Response of Mediterranean Precipitation to Greenhouse Gases and Aerosols ........................................................................................................................................ 42 3.1 Introduction ..................................................................................................................... 42 3.2 Data and Method ............................................................................................................ 44 vii 3.2.1 Data ............................................................................................................................. 44 3.2.2 Method ........................................................................................................................ 47 3.3 Results .............................................................................................................................. 48 3.4 Case Study ....................................................................................................................... 53 3.5 Discussions and Conclusion ......................................................................................... 61 4. Response of Shortwave Cloud Radiative Effect to Greenhouse Gases and Aerosols and its Impact on Daily Maximum Temperature .......................................................................... 67 4.1 Introduction ..................................................................................................................... 67 4.2 Data and Methods .......................................................................................................... 70 4.2.1 Data ............................................................................................................................. 70 4.2.2 Method ........................................................................................................................ 71 4.3 Results .............................................................................................................................. 73 4.3.1 SWCRE Change ......................................................................................................... 73 4.3.2 Mechanism of the Cloud Changes .......................................................................... 75 4.3.3 Fast and Slow Responses .......................................................................................... 80 4.3.4 SWCRE Response to Sulfate Aerosol ...................................................................... 81 4.3.5 Impact on Radiation and Tmax ............................................................................... 82 4.4 Discussion and Conclusion ........................................................................................... 87 5. Conclusion and Summary ..................................................................................................... 92 5.1 Key Findings ..................................................................................................................