Modelling the diurnal cycle* of winds and clouds
Dr Gilles Dominique Bellon, Senior Lecturer, Physics
Cloud forming above a tropical island
A day on a tropical island
Over a tropical island, the temperature rises after sunrise and become warmer than the temperature over the ocean around. This causes a sea breeze that brings humidity over the coast, where clouds develop and precipitation starts around midday. Afterwards, the precipitating clouds propagate inland until the early night. Then the island cools, a land breeze develops, and clouds form off the coast and propagate towards the open ocean. Of course, the canonical diurnal cycle of winds and clouds is modulated by many factors: the location of the island and the season, the geography of the island (shape, size, topography), and the state of the atmosphere at the regional, “synoptic” scale (dominant winds, humidity) all have an influence on this diurnal cycle. In return, archipelagos can have a strong impact on the regional climate through the interaction of scales between the local diurnal processes and the regional mechanisms, which set the long-term climate.
Our project investigates mechanisms of the meteorological diurnal cycle over islands and the factors that affect them as well as how groups of islands can impact the regional climate. To do so, we analyse high-resolution satellite estimates, and we perform simulations of the climate over fictional, idealized islands using a cloud-resolving model, a high-resolution model of the atmosphere based on the anelastic equations (i.e., simplified version of the Navier-Stokes equations, in which air density is considered to almost follow a reference vertical profile) which explicitly models the large-cloud processes.
Both the analysis of observations and the modelling work require large computer resources. The largest computations are performed on the NeSI High Performance Computing facility, and some post-treatment and smaller analysis run on a Research Virtual Machine that the Centre for eResearch (CeR) set up for our group. CeR also provided much needed storage space. The storage on Seafiles allowed us to share our data with oversea partners. For outreach purpose, CeR Visualisation Specialist Nick Young also helped to create a video on YouTube (see below) of a simulation of the diurnal formation of clouds over a circular island.
A two-day cloud-resolving simulation of the atmosphere around a circular tropical island of 70 km in diameter. Colors at the surface show the temperature of surface air while grey and black show surface precipitation rate. White show clouds and blue and purple show rain falling from these clouds.
*Note: Definition of Diurnal cycle from Wikipedia: A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth round its own axis. In climatology, the diurnal cycle is one of the most basic forms of climate patterns. The most familiar such pattern is the diurnal temperature variation. Such a cycle may be approximately sinusoidal, or include components of a truncated sinusoid (due to the sun’s rising and setting) and thermal relaxation (Newton cooling) at night.
