1994: Evolution of Cloud Structure over Storm Lifecycles

Global warming is a topic at the forefront of the international agenda and a problem of great interest for the community of Earth scientists around the world. The basic question is whether the Earth is warming from the gasses that man is putting into the atmosphere since the beginning of the industrial revolution. In its simplest form the answer is that gasses released by factories, car exhausts and other modern inventions block the Earth's heat from escaping into space, and since the quantities of those gasses (better known as greenhouse gasses) increase every year, today the planet should be in the middle of a warming trend.

Answering this question is more complicated when one considers those entities called feedbacks. Let us consider one such potential feedback that is relevant to this project. The Earth is covered to a great extent by clouds that tend to reflect the heat of the sun and therefore keep the planet cooler than it would be without them. Let's assume that as we add more greenhouse gasses into the atmosphere and we start heating the planet, that extra heat causes more water to evaporate from the oceans, creating more clouds and causing more sunlight to be reflected back to space. The final result of this process is that the original warming will decrease or even be eliminated. This would be a negative feedback as it works against the thing that caused it, i.e. the original warming.

Whether the Earth will warm from the increase in greenhouse gasses will depend on the action of such negative and positive feedbacks that will try to decrease or increase the original warming. It is widely accepted in the Earth Science community that clouds are the strongest potential source of such feedbacks. Therefore, if we want to predict whether and how much the planet will warm in the future, we must be able to understand how the clouds will respond to the warming. In order to do that, we must first understand the processes that make clouds in the atmosphere and, more important, the types of clouds that each process produces.

This brings us to the objectives of the "Cloud Structure Over Storm Lifecycles" project. In the area where we live, the primary cloud-makers are the so called midlatitude storms, better known as cold and warm fronts from the weather reports. Those storms tend to produce distinct cloud decks (clusters) that cover large areas and produce rain and snow_ We intend to study how the structures of clouds change as a storm evolves from the developing stage to the mature and finally to the dissipating stage of its lifecycle.

This study will allow us to isolate the different cloud-making processes in the life of a storm and to determine the types of clouds that each of these processes produces. The knowledge from this study will be incorporated into a computer model (general circulation model) that simulates the different processes in the Earth's atmosphere and that is used at NASA GISS to predict changes in the Earth's climate. If we are able to improve the representation of clouds in the model, we will be able to say with more confidence whether and by how much the Earth will warm in the next century.

Research Tasks and Scientific Questions

Become familiar with the three sets of data that will be used in this study. The first set is the International Satellite Cloud Climatology Project (ISCCP) dataset that includes observations of clouds and their properties from satellites that circle the Earth. The second set is the National Meteorological Center (NMC) dataset that includes observations of all the known weather parameters from weather stations around the globe. The third set is the data that comes from the General Circulation Model (GCM) that resides in the Institute. This is a model that uses mathematical formulas in order to simulate the different processes in the Earth's atmosphere. (1 week)

Examine one month of data from each dataset to identify the midlatitude storms that are present in each set. Use a computer tracking program to do the same job and compare the results of the manual and the computer analysis. (2 weeks)

Select a storm from the observations and one from the model with similar lifecycles. Examine the cloud structure in the two storms and compare the clouds of the model to those of the real world. Time permitting repeat the process for more storms. (5 weeks)

• Can we use the properties of clouds to identify midlatitude storms?
• How do clouds in storms differ from the rest of the clouds?
• How do clouds change as the storm evolves through its lifecycle?
• How different are the clouds in the different sections of a storm?
• What are the processes in the different sections that produce the clouds, and what cloud types does each process produce?
• Are the clouds produced by the model similar to the ones observed in the real world?
• How can we change the physics of the model to produce more realistic storm clouds?

Key Concepts

• Midlatitude storms: Thermodynamic instability, Pressure gradient
• Clouds: Condensation of water vapor Droplet growth by collision
• Observations: Solar and thermal radiation Reflection and absorption of radiation