Evolution of tropical cirrus clouds. Clare Allen. Supervisor: Dr Steven Dobbie. Co-supervisor: Dr Alan Blyth. RSG: Dr Ken Carslaw.

Importance of cirrus clouds: Cirrus are high altitude clouds consisting mainly of ice. Globally cirrus covers approximately 20% of the Earth’s surface (Luo et al 2002). Cirrus clouds have a significant role in regulating the radiation balance of the earth-atmosphere system.

Tropical cirrus clouds. Majority of tropical cirrus forms as result of anvil outflow from the convective processes within cumulonimbus towers. The convective core dissipates leaving a remaining background cirrus cloud.

Example of tropical cirrus anvil.

Ice nucleation mechanisms. Homogeneous ice nucleation: Spontaneous freezing of supercooled water or solution droplets. Temperatures < -35oC and RH > 100%. Heterogeneous ice nucleation: Freezing of supercooled water or solution droplet aided by a foreign body known as an ice nuclei. Temperatures between 0oC and -35oC.

Aim: “To understand the influence of dust aerosol of the evolution of microphysical and radiative properties of tropical cirrus clouds.”

Dust altering anvil properties:

CRYSTAL-FACE location: Southern Florida was chosen as deep convection frequently occur in July in the surrounding waters. The aircraft based at Key-West Naval Station. Ground based instruments were based on the southwest coast of Florida.

Flight plans:

Model used: Large Eddy Model Using anelastic approach, allowing reference states of pressure, temperature and density to vary with height. Using double moment scheme to represent IN: Mass Number

Current work: Integrating the influence of gravity waves in the cirrus anvil region. Increase cirrus cloud lifetime?

Gravity waves: Large scale upper tropospheric wave. Initialized by the convective cloud. Starr et al (2004). IR GOES-8 satellite image 23/07/02

Preliminary results.

Future work: