CBL - Campus del Baix Llobregat

Projecte llegit

Títol: Cloud formation in the atmospheric boundary layer

Director/a: MELLADO GONZALEZ, JUAN PEDRO

Departament: FIS

Títol: Cloud formation in the atmospheric boundary layer

Data inici oferta: 02-02-2020 Data finalització oferta: 02-10-2020

Estudis d'assignació del projecte:

MU AEROSPACE S&T 15

Tipus: Individual

Lloc de realització: EETAC

Segon director/a (UPC): PINO GONZÁLEZ, DAVID

Paraules clau:
Aviation Weather, Cloud Physics, Turbulence

Descripció del contingut i pla d'activitats:
Poor visibility due to clouds is one of the main causes for aircraft accidents, particularly during take-off and landing. The representation of clouds in weather and climate models, however, remains a challenge. Clouds are multi-physics, multi-scale problems, and current computational capabilities do not allow to simulate all the details that are necessary for an accurate prediction of its formation, evolution and desiccation. We still need to rely on simplified models, and these models need improvement. This motivates this master's thesis. This thesis focuses on shallow clouds forming at the top of the atmospheric boundary layer, in particular, the effect of turbulent mixing on the supersaturation field. Supersaturation is the difference between the water vapor and the saturation water vapor, and therefore it determines the ability of clouds to form and desiccate. The aim of the project is to characterize turbulent properties of the water vapor at the top of the atmospheric boundary layer. More specifically, to obtain the variance and skewness of the water vapor as a function of surface fluxes, and lapse rates and wind velocity in the free atmosphere. The analysis would involve post-processing using python of data from simulations already run. Based on semi-empirical theories, we will derive relationships that are applicable to arbitrary environmental conditions. The student will learn about meteorology, in particular, about the atmospheric boundary layer, about atmospheric turbulence, and about atmospheric thermodynamics.

Overview (resum en anglès):
The representation of cloud formation in weather and climate models remains a challenge, since it involves very complex phenomena such as turbulent mixing, which is key for the ability of clouds to form and dissipate. Thus, we study temperature and specic humidity statistics in the quasy-steady entrainment regime of convective boundary layers that grow into linearly stratified free atmospheres, using dimensional analysis and data from direct numerical simulations. Thereby, we characterise the saturation field and the cloud formation probability. First, we provide a zero-order bulk model describing the temperature based on already existing models for buoyancy and specic humidity. We show that, in the entrainment zone, the wind-shear effects are important enough that a model that does not consider at least a mean wind is not good enough to predict accurately cloud formation in that boundary-layer region. Second, we parameterize the variances and skewnesses for arbitrary combinations of surface fluxes and free-atmosphere lapse-rates of buoyancy and specific humidity, in the mixed layer and in the entrainment zone separately. We find large turbulent fluctuations in the entrainment zone and a considerable well-mixed mixed layer, in accordance with previous studies. We also encounter different signs on the skewness of both variables. The specic humidity has a negative skewness in the mixed layer and a positive one in the entrainment zone, as shown in preceding works, while the temperature skewness is positive in the mixed layer and becomes negative in the entrainment zone. Finally, we combine these first three statistical moments to explore the cloud formation probability in terms of the aforementioned meteorological parameters and the surface background values of temperature and moisture. We perform the analysis using a scheme based on a joint-normal probability density function. We illustrate that a mean-value model is suitable enough to describe cloud formation in the mixed layer, while in the entrainment zone it is mandatory to represent more accurately the effect of turbulent fluctuations.