Prof Christian Jakob (Monash University).
Climate models have significant errors in precipitation globally, but in particular in the tropics. Most models overestimate annual mean tropical precipitation, with the largest errors occurring over the warm tropical oceans of the Western Pacific, Indian Ocean and Atlantic. In addition to the mean errors, there are significant shortcomings in the rainfall variability in space and time. Given its immediate impact on tropical rainfall, most of the errors are usually attributed to shortcomings in the representation of deep tropical convection, and most attempts at their remedy focus on changes to the processes involved in it. We show that in observations the tropics, defined as the latitude belt from 30 degrees north to 30 degrees south, are very close to radiative convective-equilibrium every day. This indicates that to first order tropical precipitation and radiative cooling are in balance even on short (daily) timescales. We show that this balance is a result of small regions of precipitation with very inefficient cooling – the deep tropics – and large regions of efficient cooling but little precipitation – the subtropics. We show that models obey this balance. This opens the intriguing possibility that mean rainfall errors in the deep tropics are the result of errors in radiative cooling and that the model’s convection is simply a slave to the radiative cooling away from the rainy regions. We investigate this hypothesis using several models run in a variety of configurations, ranging from Aquaplanet experiments with the German ICON model, to AMIP simulations with a range of models with and without cumulus parametrization. The modelsremes produce a wide range of tropical rainfall values and all of them are directly related to the tropical radiative cooling. We investigate the key regions of radiative cooling changes as the model rainfall changes and highlight potential mechanisms involved. We conclude that it is just as likely that climate model rainfall errors in the tropics are the result of errors in radiative cooling due to the misrepresentation of tropical cirrus and/or subtropical boundary layer clouds as they are the result of errors in the cumulus parameterisation.
Brief Biography: Professor Christian Jakob was awarded his PhD in Meteorology from the Ludwig Maximilians University, Munich, in 2001. As a research and senior research scientist for the European Centre for Medium-Range Weather Forecasts from 1993 to 2001, he worked on the development and evaluation of the model representation of clouds, convection and precipitation. From 2002 to 2007 he was Senior and Principal Research Scientist of the Australian Bureau of Meteorology and since 2007 he has been a professor at Monash University. He currently is the Chair of Climate Modelling at Monash’s School of Earth, Atmosphere and Environment. Prof Jakob’s experience and current interests are in the development and evaluation of the processes crucial to the energy and water cycles in global atmospheric models. Internationally, he is engaged in many scientific and collaborative activities. He is a past Co-chair of the World Climate Research Programme’s (WCRP) Modelling Advisory Council. He led the prestigious Working Group on Numerical Experimentation from 2008 to 2012 and was the first university-based researcher to be appointed in that position. He was Chair of the WCRP’s Global Energy and Water Cycle Experiment (GEWEX) Modelling and Prediction Panel from 2007 to 2010. As recognition of his prominent position in the climate science field, Prof Jakob was a lead author for the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report, Working Group 1. In 2016, his research was recognised by the Ascent Award of the American Geophysical Union’s Atmospheric Sciences Section and he was elected a Fellow of the Australian Meteorological and Oceanographic Society (AMOS) in 2018. He was awarded the AMOS Morton Medal in 2019.