Energy Balance Models With Three Phases Of Water Feedback
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Simple one-dimensional heat balance equations have been used to understand climate concepts since the 1960s, when a class of models was developed known as energy balance models (EBMs). EBMs use the growth or loss of polar surface ice as a climatic feedback, giving rise to surprisingly complex non-linear behaviours. One aspect of EBMs that has been relatively poorly examined is the effects of feedbacks caused by the other two phases of water in Earth’s climate other than ice: water clouds and water vapour. Cloud and water vapour play a critical role in the energy balance of Earth’s climate, and yet are some of the least well understood elements of the global climate system. This thesis explores the behaviour and interrelationships of climatic feedbacks caused by water in all three phases as it exists in the climate: surface ice caps, water vapour, and liquid water clouds. A two-layered EBM was modiﬁed with parameterizations of water vapour and liquid water clouds in order to conduct experiments. Three variants of the model were produced, each with progressively more water feedbacks than the last: a 1 phase model (with only surface ice feedback), a 2 phase model (with surface ice and water vapour) and a 3 phase model (with surface ice, water vapour, and cloud). The models were found to give generally realistic results, but with an underestimation of water vapour density, which in turn reduced the generated cloud fraction in the 3 phase model. Thus, the impacts of these extra feedbacks were likely to be underestimated in the analysis in general. The sensitivity of the model to several prognostic variables was studied by observing the changes in the model to a range of each variable. The 3 phase model was less sensitive to changes to the solar constant, S0, which measures incoming solar radiation, than the 1 phase model. This was probably caused by cloud reﬂecting and absorbing some radiation from the sun that would have otherwise reached the surface, changing the ratio of atmospheric heat transport to surface heat transport from 2.4953 for the 1 phase model to 2.0626 for the 3 phase model. Changing surface and ice albedo values resulted in changes in the model’s stability. The model was found to be insensitive to changes in surface humidity that drives the amount of water vapour the system has available, due to underestimation of water vapour in the model. The stability of the model was examined, and the 1 phase model was found to respond faster to changes in S0 than the 3 phase model. The model was tested for hysteresis, which was conﬁrmed for all three model variants. The 1 phase model showed less stability then the 3 phase model as S0 was increased, but both models were similarly stable as S0 was decreased.