Unit: 6

Elements of weather and climate modeling

Weather and climate modeling involve using mathematical equations and computer simulations to represent the various physical processes that govern the Earth's atmosphere and oceans. There are several key elements that are typically included in weather and climate models:

The Dynamics Of The Atmosphere: This includes the equations that govern the motion of the air, such as the Navier-Stokes equations, which describe how air flows and interacts with the Earth's surface.

The Physics Of The Atmosphere: This includes the processes that govern the movement of heat, moisture, and other properties through the atmosphere, such as radiation, convection, and precipitation.

The Representation of the Earth's surface: This includes the topography, vegetation, and land-use patterns that influence the exchange of energy and water between the atmosphere and the surface.

The Representation Of The Oceans: This includes the circulation patterns, temperature, and salinity of the oceans, which play a key role in regulating the Earth's climate.

The Representation of the Earth's radiation budget: This includes the amount of energy that enters and leaves the Earth's system due to the sun, and how that energy is distributed and absorbed by the atmosphere, oceans, and surface.

The Representation of the Earth's carbon cycle: This includes the movement of carbon through the atmosphere, oceans, and land, and how it affects the Earth's climate.

The Initialization Of The Model: This includes the initial conditions, such as the temperature, pressure, and wind patterns, that are used to start the model's simulation.

The Boundary Conditions: This includes the information, such as the solar radiation, that is input into the model from outside the model's domain, such as the upper atmosphere.

The Model's Resolution: This includes the spatial and temporal resolution of the model, which determines how finely the model can represent the processes that are being simulated.

The model's validation: This includes comparing the model's output with observations to assess its accuracy and identify areas where it can be improved. 

Basic Equation and Dynamics of atmosphere : 

The basic equation that governs the dynamics of the atmosphere is the Navier-Stokes equation, which describes how air flows and interacts with the Earth's surface. The Navier-Stokes equation is a set of nonlinear partial differential equations that describe the conservation of mass, momentum, and energy in a fluid.

For the atmosphere, the Navier-Stokes equation can be simplified by assuming that the air is a compressible, isotropic fluid, with a small density variation. The simplified equation can be written as:

∂u/∂t + (u . grad)u = -1/ρ * grad p + ν * ∇^2 u

where u is the velocity field of the air, t is time, p is pressure, ρ is density, and ν is the kinematic viscosity of the air.

The first term on the left-hand side of the equation represents the time rate of change of the velocity field, while the second term represents the advection of the velocity field by the fluid. The right-hand side of the equation represents the balance between the pressure gradient and the viscous forces. The pressure gradient force acts to compress the fluid, while the viscous forces act to dissipate the energy of the fluid. 

This equation is a fundamental equation that describes the dynamics of the atmosphere. However, it's non-linearity and complexity makes it difficult to solve analytically, therefore it is typically solved using numerical methods, such as the finite-difference method, finite-volume method, or spectral method.

It should be noted that the Navier-Stokes equation is a simplified version of the full equations of atmospheric dynamics, which also includes the effects of rotation, rotation, and the Coriolis force that is caused by the Earth's rotation.

Climate variability and climate change : 

Climate variability refers to natural fluctuations in the Earth's climate that occur over a wide range of temporal and spatial scales. These fluctuations include phenomena such as El Niño-Southern Oscillation, the North Atlantic Oscillation, and the Arctic Oscillation, which can influence temperature, precipitation, and other climate variables on regional and global scales.

Climate change, on the other hand, refers to long-term changes in the Earth's climate that are primarily caused by human activities, such as the burning of fossil fuels, deforestation, and agriculture. These activities increase the concentration of greenhouse gases, such as carbon dioxide, in the atmosphere, which trap more heat from the sun, causing the Earth's surface to warm.

Climate change can exacerbate the effects of climate variability, making extreme weather events such as heat waves and droughts more severe. Climate change can also cause changes in precipitation patterns, sea level rise, and the acidification of the oceans.

It is important to note that climate variability and climate change are not mutually exclusive, and climate variability can influence the rate and magnitude of climate change. For example, natural variability in ocean currents and temperatures can influence the rate at which heat is absorbed by the oceans and the rate at which sea level is rising.

Understanding the difference between climate variability and climate change is important for making accurate predictions about future climate conditions and for developing effective strategies to mitigate and adapt to the impacts of climate change.

Elementary idea of Global climate models : 

Global climate models (GCMs) are computer-based simulations that are used to study the Earth's climate system. They are based on mathematical equations that represent the physical processes that govern the atmosphere, oceans, land surface, and ice. GCMs are used to simulate the past, present, and future climate and to project future climate change under different scenarios of greenhouse gas emissions.

The basic idea behind GCMs is to use mathematical equations to represent the various processes that govern the Earth's climate system, including the dynamics of the atmosphere, the physics of the atmosphere, the representation of the Earth's surface, the representation of the oceans, the representation of the Earth's radiation budget, and the representation of the Earth's carbon cycle.

GCMs divide the Earth's surface into a grid of cells, and the equations are used to calculate the state of the climate system within each cell. The calculations are performed using powerful supercomputers, and the model's output includes information such as temperature, precipitation, wind, and other climate variables.

The GCMs are constantly updated and improved to increase the accuracy of the simulations. The GCMs are also verified by comparing their output with observational data. GCMs are a powerful tool for understanding the Earth's climate system and for making predictions about future climate change.

It is important to note that GCMs are complex models with many assumptions, simplifications, and parameterizations. Therefore, the results of GCMs should be considered as a projection of the most likely outcome, rather than a definite prediction.

Comparison of various IPCC reports : 

The Intergovernmental Panel on Climate Change (IPCC) is a scientific body established by the United Nations that assesses the science related to climate change. The IPCC produces periodic assessment reports that provide a comprehensive overview of the current state of knowledge on climate change. The assessment reports are widely considered to be the most comprehensive and authoritative source of information on climate change.

The IPCC has produced five assessment reports so far, each one building on the previous one. Here is a brief comparison of the main findings of each report:

The First Assessment Report (FAR), published in 1990, was the first comprehensive assessment of the science of climate change. The FAR concluded that the Earth's climate was warming and that human activities, particularly the burning of fossil fuels, were the primary cause of the warming.

The Second Assessment Report (SAR), published in 1995, reinforced the conclusions of the FAR and provided new evidence that the warming was caused by human activities. The SAR also introduced the concept of "climate sensitivity", which is a measure of how much the Earth's surface will warm in response to a doubling of carbon dioxide concentrations in the atmosphere.

The Third Assessment Report (TAR), published in 2001, provided more robust evidence that the Earth's climate was warming and that human activities were the primary cause of the warming. The TAR also introduced the concept of "climate scenarios", which are used to project future climate change under different scenarios of greenhouse gas emissions.

The Fourth Assessment Report (AR4), published in 2007, reinforced the conclusions of the TAR and provided new evidence that the warming was causing changes in precipitation patterns, sea level rise, and the acidification of the oceans. The AR4 also introduced the concept of "climate projections", which are used to make more detailed predictions about future climate change.

The Fifth Assessment Report (AR5), published in 2013-2014, reinforced the conclusions of the AR4 and provided new evidence that the warming was causing changes in precipitation patterns, sea level rise, and the acidification of the oceans. The AR5 also introduced the concept of "climate models", which are used to make more detailed predictions about future climate change.

The Sixth Assessment Report (AR6) will be published in 2022-2023 and is expected to provide more robust evidence of the impacts of climate change, including the physical and biological systems, human and socio-economic systems. It is also expected to provide more detailed projections of future climate change and new insights on mitigation and adaptation options.

It should be noted that the IPCC assessment reports are based on the peer-reviewed scientific literature, and the findings are agreed upon by a large number of scientists and governments worldwide, making them widely accepted as the most comprehensive and authoritative source of information on climate change.

Important Findings Of IPCC AR5 : 

The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) was published between 2013 and 2014, and it is considered as one of the most comprehensive and authoritative sources of information on climate change. Here are some of the key findings of the AR5:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and oceans have warmed, the amounts of snow and ice have diminished, and sea level has risen.

It is extremely likely (95-100% probability) that human activities, particularly the burning of fossil fuels, are the dominant cause of the observed warming since the mid-20th century. The increase in greenhouse gases such as carbon dioxide, methane and nitrous oxide is the primary driver of the warming.

Global warming is projected to continue, and it is likely to reach 1.5°C (likely range: 1.0°C to 2.0°C) above pre-industrial levels by around 2040, even if greenhouse gas emissions are reduced. Further warming of 2°C or more above pre-industrial levels is projected to occur during the 21st century.

Climate change will lead to a wide range of impacts including more frequent and severe heatwaves, extreme precipitation, sea level rise, and increased frequency and intensity of some extreme weather events. These impacts will disproportionately affect vulnerable communities, including low-income and coastal populations.

Limiting warming to 2°C or below, as agreed in the Paris Agreement, will require rapid, far-reaching and unprecedented changes in all aspects of society, including energy, transportation, buildings, agriculture, and land use.

Climate change will also have impacts on biodiversity and ecosystems, including the loss of many species, the alteration of ecosystem functions, and the displacement of many species from their natural habitats.

The ocean is warming, becoming more acidic and losing oxygen, with impacts on marine life and ecosystems. The ocean also plays a crucial role in absorbing excess carbon dioxide from the atmosphere, which has led to ocean acidification.

Mitigation options exist to limit greenhouse gas emissions and are consistent with sustainable development. A portfolio of technologies and measures, including renewable energy, energy efficiency, and carbon capture and storage, can be used to reduce emissions.

Adaptation options are available to reduce the vulnerability of human and natural systems to the impacts of climate change. These options include early warning systems, green infrastructure, and the development of climate-resilient crops.

Overall, the IPCC AR5 provides a comprehensive overview of the current state of knowledge on climate change and highlights the urgent need for action to mitigate and adapt to its impacts.

Impacts of climate change – Global and India

Climate change has a wide range of impacts on the global and Indian environment, economy and society. Some of the key impacts include:

Global impacts:

  • Rising sea levels, which can lead to coastal flooding and erosion
  • Increased frequency and intensity of heatwaves, droughts and extreme weather events
  • Changes in precipitation patterns, leading to water scarcity and flooding in some areas
  • Loss of biodiversity and damage to ecosystems, such as coral reefs and forests
  • Disruption of agricultural production and food security

Indian impacts:

  • Increased frequency and severity of heatwaves, droughts, and floods. 
  • Reduced agricultural productivity, particularly in the semi-arid and arid regions. 
  • Increased water scarcity, particularly in the northern and western regions. 
  • Negative impact on the health and livelihoods of millions of people, particularly those in rural and coastal areas. 
  • Damage to important coastal and marine ecosystems, including mangroves and coral reefs.