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2019 | Buch

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Models for Tropical Climate Dynamics

Waves, Clouds, and Precipitation

verfasst von: Boualem Khouider

Verlag: Springer International Publishing

Buchreihe : Mathematics of Planet Earth

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik" , Springer Professional "Wirtschaft"

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Über dieses Buch

This book is a survey of the research work done by the author over the last 15 years, in collaboration with various eminent mathematicians and climate scientists on the subject of tropical convection and convectively coupled waves. In the areas of climate modelling and climate change science, tropical dynamics and tropical rainfall are among the biggest uncertainties of future projections. This not only puts at risk billions of human beings who populate the tropical continents but it is also of central importance for climate predictions on the global scale. This book aims to introduce the non-expert readers in mathematics and theoretical physics to this fascinating topic in order to attract interest into this difficult and exciting research area. The general thyme revolves around the use of new deterministic and stochastic multi-cloud models for tropical convection and convectively coupled waves. It draws modelling ideas from various areas of mathematics and physics and used in conjunction with state-of-the-art satellite and in-situ observations and detailed numerical simulations. After a review of preliminary material on tropical dynamics and moist thermodynamics, including recent discoveries based on satellite observations as well as Markov chains, the book immerses the reader into the area of models for convection and tropical waves. It begins with basic concepts of linear stability analysis and ends with the use of these models to improve the state-of-the-art global climate models. The book also contains a fair amount of exercises that makes it suitable as a textbook complement on the subject.

Inhaltsverzeichnis

Frontmatter

Part I

Frontmatter

Chapter 1. The Governing Equations and Dry Dynamics

Abstract

The equations of motion that govern atmospheric (and also oceanic) flows are based on the theory of fluid mechanics comprising the Euler and/or Navier Stokes equations which model conservation of mass, momentum, and energy [27, 11] of a Newtonian fluid such as air and water. The so-known hydrostatic primitive equations are derived from these basic laws of physics after some major simplifications or approximations taking into account the particular topology of planetary flows [61, 216, 166].

Boualem Khouider

Chapter 2. Moisture and Moist Thermodynamics

Abstract

The most popular state variables that are used to define a thermodynamic system are the pressure, the temperature, and the composition. For our purpose, the atmospheric composition is divided into two main constituents: dry air, comprising nitrogen, oxygen, carbon dioxide, etc., and water in its various states or phases including water vapour (referred to herein as moisture), liquid water in the form of suspended cloud or rain droplets, ice crystals, and snowflakes.

Boualem Khouider

Chapter 3. Observations of Tropical Climate Dynamics and Convectively Coupled Waves

Abstract

The first evidence of equatorially trapped waves in observational records appeared in 1966 in the work of Yanai and Maruyama [294], at the same time as the theoretical work of Matsuno [187]. Yanai and Maruyama [294] found signals of wave-like motion with strong cross equatorial wind in US Navy stratospheric wind data (which were apparently used to monitor nuclear activity during the cold war) when they were looking for evidence of eddy momentum transport as a plausible energy source for the quasi-biannual oscillation (QBO) in the equatorial stratosphere [188]. These waves correspond to the mixed Rossby-gravity waves from the Matsuno theory, which are also sometimes called Yanai-Maruyama or simply Yanai waves. Two years later Wallace and Kousky [271] (see also [83]) published their work on the discovery of Kelvin waves in the tropical stratosphere, which unlike those identified earlier by Yanai and Maruyama they are characterized by dominating zonal winds in phase with pressure perturbations. They were also motivated by the search for an energy source for the QBO in the form of wave eddy momentum.

Boualem Khouider

Chapter 4. Introduction to Stochastic Processes, Markov Chains, and Monte Carlo Simulation

Abstract

A major part of this book discusses the use of stochastic models for atmospheric convection, namely through the use of a stochastic model for CIN and a stochastic multicloud model which pertains to tracking the statistics of clouds of various types. These models rely on the theory of stochastic processes and Markov chains in particular. Here, we provide a brief introduction to this topic to help the reader better appreciate and comprehend the cloud models. The expert reader can skip this chapter. It is intended to readers with no or rather very little background in probability theory and stochastic processes. Nonetheless, we assume that the reader is familiar with the basic notions of probability distributions and random variables.

Boualem Khouider

The Deterministic Multicloud Model

Frontmatter

Chapter 5. Simple Models for Moist Gravity Waves

Abstract

This chapter introduces the reader to basic simple ideas for convectively coupled wave models mostly in order to put the multicloud model, which will be discussed in Chapter 6, in the context of preceding theories. It explores some of the ideas that have been proposed in the literature and their main characteristics and pitfalls using simplistic models.

Boualem Khouider

Chapter 6. The Multicloud Model with Congestus Preconditioning

Abstract

As we saw from the previous chapter, among all the theories mentioned so far, for convectively coupled waves, the stratiform instability seems to be the most promising one. However, it suffers from a major shortcoming that the linear instability is not self-sustained without the use of WISHE, which itself is deemed unphysical as demonstrated by the nonlinear simulation presented there. There is no observational evidence of sustained easterlies in the tropical atmosphere to force eastward moving (Kelvin and MJO) waves that are ubiquitous in the tropical atmosphere, through WISHE [248, 126]. While the stratiform instability is very appealing and appears to be physically sound [184, 175], something fundamental seems to be missing. The answer to this hard question is rooted from the observations led by Lin and Johnson [156, 157] Johnson et al. [99], using data from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). These authors discovered that unlike the common belief at that time, the dynamics of organized tropical convection involves three cloud types besides shallow cumulus clouds that are omnipresent. In addition to deep and stratiform clouds, congestus clouds that do not penetrate above the freezing level (4–5 km) play a major dynamical roles.

Boualem Khouider

Chapter 7. Convectively Coupled Equatorial Waves in the Multicloud Model

Abstract

As we saw in the previous chapter, the multicloud model exhibits a new kind of instability of moist gravity waves that are self-sustained, in the sense that these waves amplify naturally during nonlinear simulation without the need of an artifact such as WISHE; this wasn’t the case for the stratiform instability in Chapter 5. We also demonstrated that the multicloud instability is a combination of the stratiform instability and congestus preconditioning through low-level convergence. The latter mechanism should not be confused with congestus moistening through detrainment which may not be effective in sustaining and organizing the propagation of convectively coupled equatorial waves (CCEWs) but can indeed be important during the initiation phase [40, 268, 82, 100]. To demonstrate that the moist gravity waves seen in the previous chapter are indeed the analogs of CCEWs as observed in nature (c.f. Chapter 3), here we consider the effect of rotation and the background wind shear in the multicloud model (MCM) and show that under this combined effect, the MCM is able to capture the observed spectrum of CCEWs.

Boualem Khouider

Chapter 8. Convective Momentum Transport and Upscale Interactions in the MJO

Abstract

As already discussed, the tropical atmosphere harbours a scale hierarchy of convective wave disturbances that are often embedded in each other like Russian dolls, evolving on a wide spectrum of scales ranging from the individual cloud cell diameter to the size of planetary scale disturbances such as the Madden-Julian oscillation, see Figure 8.1a. A particularly interesting issue is to understand the way these multiscale convective systems interact with each other across temporal and spatial scales. There is enough evidence from both observations and numerical simulations that momentum transport plays a genuine role in these interactions [147, 194, 145, 287, 177, 138, 113]; the smaller embedded waves provide turbulent fluxes for the envelopes, while the larger scale envelope waves provide a background advecting wind for the smaller scale waves.

Boualem Khouider

Chapter 9. Implementation of the Multicloud Model in an Aquaplanet Global Climate Model

Abstract

Global climate and numerical weather prediction models (GCMs and NWPMs) simulate the atmospheric large-scale dynamical processes by solving the hydrostatic or non-hydrostatic primitive equations on a fixed grid with a horizontal mesh size of 25 km to 200 km. Atmospheric processes occurring at smaller scales that cannot be represented on those coarse resolutions are either neglected or represented via subgrid models known as parameterizations [246, 132]. As already demonstrated in the previous chapters convective clouds have a major impact on the atmospheric dynamics on synoptic and planetary scales that cannot be ignored. Convectively coupled waves and convective motions in general make the bulk of the atmospheric circulation in the tropical latitudes and account for the majority of precipitation which falls in this part of the globe. Among those, the Madden-Julian oscillation (MJO) [164, 163], in particular, constitutes the major source of atmospheric variability on the intra-seasonal and planetary scales and interacts with important global weather an climate patterns [300, 302].

Boualem Khouider

The Stochastic Multicloud Model: SMCM

Frontmatter

Chapter 10. Stochastic Birth and Death Models for Clouds

Abstract

As already been stressed out, atmospheric convection is the process through which warm and moist air parcels rise from the surface, condense liquid water, and form cumulus clouds. In the tropics, moist convection constitutes the main energy source for both local and large-scale circulations. Precipitation patterns in the tropics are organized into cloud clusters and superclusters on a wide range of scales; they range from the convective cell (the cumulus cloud) of 1 to 10 km, to planetary scale waves with oscillation periods of 40 to 60 days. Due to the complex interactions between the local processes of convection and the large-scale waves, climate models fail to properly capture tropical circulation patterns and their effect on the global circulation. In a climate model, the governing equations are discretized on a coarse mesh of roughly 100 km to 200 km and the effects of processes that are not resolved on such grids are represented by a parameterization also called as subgrid model. According to the last report of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the interactions of clouds and the climate system is one of the major challenges in climate research.

Boualem Khouider

Chapter 11. Implementation of the SMCM in a Global Climate Model

Abstract

Here, we discuss the implementation of the stochastic multicloud model (SMCM) presented in the previous chapter in comprehensive climate models. First, we look at the case of an aquaplanet setting using the HOMME atmosphere only, dynamical core used in Chapter 9 and then consider, in Chapter 12, the case of a more involved state-of-the-art ocean-atmosphere coupled model, used in actual-operational climate predictions, namely, the National Centers for Environmental Predictions (NCEP)’s Climate Forecasting System (CFS). As portrayed by the results presented below important improvements in terms of the simulation of the MJO and convective coupled waves as well as the monsoon variability are achieved through the addition of stochasticity to the HOMME-MCM aquaplanet simulations presented in Chapter 9. Moreover, the implementation of the SMCM in CFS resulted in unprecedented improvements in terms of the simulation of the tropical modes of atmospheric variability on synoptic and intra-seasonal scales in coarse resolution GCMs.

Boualem Khouider

Chapter 12. SMCM in CFS: Improving the Tropical Modes of Variability

Abstract

The failure of the traditional cumulus parameterizations to adequately represent organized convective systems and tropical climate variability such as the Madden-Julian oscillation, convectively coupled waves, and monsoon weather and climate [e.g. 151, 95, 97] led the climate modelling community to think outside the box [224].

Boualem Khouider

Backmatter

Titel: Models for Tropical Climate Dynamics
verfasst von: Boualem Khouider
Verlag: Springer International Publishing
Electronic ISBN: 978-3-030-17775-1
Print ISBN: 978-3-030-17774-4
DOI: https://doi.org/10.1007/978-3-030-17775-1