Influence of Three Dimensional Thermal Radiation on Cloud Droplet Growth
Final Report Abstract
The underlying question of this work is if and how 1D and 3D thermal radiative heating and cooling rates at cumulus clouds can modify cloud droplet growth. To answer this question, a two-step approach was chosen. In a first idealized study, the pure effect of thermal radiation on microphysics is investigated, neglecting any dynamical feedbacks that could occur in a more complex framework. For this, 2.7 million parcel trajectories were recorded in an LES framework in a shallow cumulus cloud field. Different variables such as position, wind speed, heating and cooling rates and liquid water properties were recorded along the trajectories. These trajectories served as input for an offline parcel model which includes a bin-microphysical scheme. With this microphysical scheme, size-resolved simulations of droplets are possible. It could be shown that single ’lucky parcels’ exists along which radiative heating and cooling can affect cloud droplet growth. The time that a parcels spends in a certain cooling area, the magnitude of the cooling, the size of the droplets at the time of cooling and the dynamical forcing contribute to the droplet growth with different magnitude. The ideal situation where the radiative effects become largest would be droplets of the size of about 10µm or more, a cooling period of more than 5 minutes in a cooling of 20 K/d or more and moderate vertical velocities (around 1 m/s). If one or more of these factors occur, radiative cooling can enhance droplet growth. Radiation does not seem to activate additional droplet growth. However, it was also shown that the main effect of the radiation is an increase in the mass in rain size bins which results in an increase in rain water mass up to 15% for 3D thermal radiation. In a side-project, it was investigated if cloud integrated radiative heating and cooling of a stratocumulus cloud can be used to estimate cloud base updraft speeds and thus be used as a proxy for microphysical processes. Updraft speeds at cloud base affects supersaturation and therefore the activation of cloud condensation nuclei, which in turn affects cloud properties and precipitation formation. It was shown before in an empirical study that a correlation between cloud integrated heating and cooling and cloud base updraft speeds exists. Based on LES simulations and offline radiative transfer simulations this work aims for a process based understanding and recommendations for applicability. It could be shown that the cloud base vertical velocity is correlated with cloud integrated radiative heating and cooling exist. However, it was also shown that this relationship is only valid during daytime. At nighttime, the deepening of the stratocumulus cloud field changes radiative heating and cooling rates in a way that the correlation breaks down. It was shown with offline radiative transfer simulations that this change in heating and cooling rates at night results from the change in the cloud properties and not from the change in the background temperature and water vapor profiles. In ongoing cooperation, a more complex study on the cloud microphysics-radiation effects in a fully coupled LES framework is planned, where the aim is to investigate the combined effect of radiation and dynamics on the cloud field and cloud microphysics.
Publications
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Effects of 3-D thermal radiation on the development of a shallow cumulus cloud field, Atmos. Chem. Phys., 17, 5477-5500
Klinger, C., Mayer, B., Jakub, F., Zinner, T., Park, S.-B., and Gentine, P.
