Drop Size Distribution Broadening Mechanisms in a Bin Microphysics Eulerian Model

Author(s)

Pardo, Lianet Hernandez ^[1] ; Morrison, Hugh ^{[2, 3]} ; Machado, Luiz A. T. ^{[4, 1]} ; Harrington, Jerry Y. ^[5] ; Lebo, Zachary J. ^[6]

Affiliation

^[1] Inst Nacl Pesquisas Espaciais, Ctr Preview Tempo & Estudos Climat, Sao Paulo - Brazil

^[2] Natl Ctr Atmospher Res, Mesoscale & Microscale Meteorol Lab, POB 3000, Boulder, CO 80307 - USA

^[3] Univ New South Wales, Australian Res Council, Ctr Excellence Climate Syst Sci, Sydney, NSW - Australia

^[4] Max Planck Inst Chem, Multiphase Chem Dept, Mainz - Germany

^[5] Penn State Univ, Dept Meteorol & Atmospher Sci, University Pk, PA 16802 - USA

^[6] Univ Wyoming, Dept Atmospher Sci, Laramie, WY 82071 - USA

• Document type: Journal article
• Source: JOURNAL OF THE ATMOSPHERIC SCIENCES; v. 77, n. 9, p. 3249-3273, SEP 2020.
• Web of Science Citations: 0

Abstract

In this study, processes that broaden drop size distributions (DSDs) in Eulerian models with two-moment bin microphysics are analyzed. Numerous tests are performed to isolate the effects of different physical mechanisms that broaden DSDs in two- and three-dimensional Weather Research and Forecasting Model simulations of an idealized ice-free cumulus cloud. Sensitivity of these effects to modifying horizontal and vertical model grid spacings is also examined. As expected, collision-coalescence is a key process broadening the modeled DSDs. In-cloud droplet activation also contributes substantially to DSD broadening, whereas evaporation has only a minor effect and sedimentation has little effect. Cloud dilution (mixing of cloud-free and cloudy air) also broadens the DSDs considerably, whether or not it is accompanied by evaporation. This mechanism involves the reduction of droplet concentration from dilution along the cloud's lateral edges, leading to locally high supersaturation and enhanced drop growth when this air is subsequently lifted in the updraft. DSD broadening ensues when the DSDs are mixed with those from the cloud core. Decreasing the horizontal and vertical model grid spacings from 100 to 30mhas limited impact on the DSDs. However, when these physical broadening mechanisms (in-cloud activation, collision-coalescence, dilution, etc.) are turned off, there is a reduction of DSD width by up to;20%-50% when the vertical grid spacing is decreased from 100 to 30 m, consistent with effects of artificial broadening from vertical numerical diffusion. Nonetheless, this artificial numerical broadening appears to be relatively unimportant overall for DSD broadening when physically based broadening mechanisms in the model are included for this cumulus case. (AU)

• FAPESP's process: 19/06988-4 - On the reasons for the DSD broadening in bin-microphysics large eddy simulations
• Grantee: Lianet Hernández Pardo
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• FAPESP's process: 16/24562-6 - Combining observations and numerical modeling to improve the description of thunderstorm microphysical processes
• Grantee: Lianet Hernández Pardo
• Support Opportunities: Scholarships in Brazil - Doctorate