Spectral (Bin) Microphysics Coupled with a Mesoscale Model (MM5). Part I: Model Description and First Results

Abstract

Considerable research investments have been made to improve the accuracy of forecasting precipitation systems in cloud-resolving, mesoscale atmospheric models. Yet, despite a significant improvement in model grid resolution and a decrease in initial condition uncertainty, the accurate prediction of precipitation amount and distribution still remains a difficult problem. Now, the development of a fast version of spectral (bin) microphysics (SBM Fast) offers significant potential for improving the description of precipitation-forming processes in mesoscale atmospheric models. The SBM Fast is based on solving a system of equations for size distribution functions for water drops and three types of ice crystals (plates, columns, and dendrites), as well as snowflakes, graupel, and hail/frozen drops. Ice processes are represented by three size distributions, instead of six in the original SBM code. The SBM uses first principles to simulate microphysical processes such as diffusional growth and collision. A budget for aerosols is used to obtain the spectrum of condensation nuclei, which is used to obtain the initial drop spectrum. Hence, SBM allows one to take into account aerosol effects on precipitation, and corresponding cloud effects on the atmospheric aerosol concentration and distribution. SBM Fast has been coupled with the three-dimensional fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), which allows SBM Fast to simulate microphysics within a realistic, time-varying mesoscale environment. This paper describes the first three-dimensional SBM mesoscale model and presents results using 1-km resolution to simulate initial development of a cloud system over Florida on 27 July 1991. The focus is on initial cloud development along the west coast, just prior to sea-breeze formation. The results indicate that the aerosol concentration had a very important impact on cloud dynamics, microphysics, and rainfall. Vertical cross sections of clouds obtained using SBM Fast are compared to those from a version of the “Reisner2” bulk-parameterization scheme that uses the Kessler autoconversion formula. The results show that this version of “Reisner2” produced vertically upright clouds that progressed very quickly from initial cloud formation to raindrop formation. In contrast, clouds obtained using SBM were relatively long lasting with greater production of stratiform clouds.

Uncertainty from the choice of microphysics scheme in convection-permitting models significantly exceeds aerosol effects

“…Many studies have shown that aerosol effects on precipitation depend on the large-scale environment and cloud type (e.g. Khain et al, 2004;Fan et al, 2007;Lynn et al, 2005a, b;Lynn and Khain, 2007;Seifert and Beheng, 2006a;Tao et al, 2007) for reasons related to differences in different cloud types between the timescale of increased sedimentation through aerosol loading and subsequent sublimation and evaporation timescales. Further, several studies of deep convection have found that the effects of aerosol on deep convection are much weaker than those of relative humidity (e.g.…”

“…This can lead to significant differences in the cloud and precipitation simulated by bin vs. bulk schemes. For example, bulk schemes have been shown to underestimate areas of weak and stratiform rain in an MCS compared to a bin scheme which performed better against observations (Lynn et al, 2005a, b). Li et al (2009a, b) showed that a one-moment bulk scheme was shown to be worse at partitioning rain into stratiform and convective components in a continental squall line compared to a bin scheme (although many studies have shown that two-moment schemes are a significant improvement on single-moment schemes; e.g.…”

Sensitivity of aerosol and cloud effects on radiation to cloud types: comparison between deep convective clouds and warm stratiform clouds over one-day period

“…Delayed autoconversion due to higher CDNC provides more abundant cloud liquid to be transported into unsaturated areas as the source of this increased evaporation. These feedbacks between dynamics and microphysics are described in more detail in Lee et al (2008a) and simulated in Khain et al (2003Khain et al ( , 2004Khain et al ( , 2005Khain et al ( , 2008 and Lynn et al (2005). Lee et al (2008b) indicated that increases in updrafts, leading to increases in condensation and deposition, were much larger in deep convective clouds than those in shallow clouds at high aerosol as shown in Fig.…”

“…Ramanathan et al (1989) indicated that radiaitve properties of deep convective clouds were different from those of stratiform clouds, regarding the modulation of outgoing longwave radiation. Also, recent studies showed aerosols could change microphysical and dynamical properties of deep convective clouds (Khain et al, 2003(Khain et al, , 2004(Khain et al, , 2008Lynn et al, 2005;Tao et al, 2007;Lee et al, 2008a). Lee et al (2008b) found that aerosol effects on cloud mass and precipitation were different for deep convective and shallow stratiform clouds.…”

The effects of mineral dust particles, aerosol regeneration and ice nucleation parameterizations on clouds and precipitation

“…Recent and similar efforts to couple a bin microphysics scheme into MM5 and WRF were presented by Lynn et al (2005), Lynn et al (2007), Khain and Lynn (2009) and Khain et al (2010), who utilized the spectral microphysics scheme of the Hebrew University of Jerusalem (HUJI-SBM) into MM5 and later in WRF. One feature that makes the Xue et al (2010) scheme the most adequate to study mineral dust and cloud interaction is their development and coupling of an aerosol recycling mechanism that prevents the diminishing of aerosol particles from the domain due to cloud and rain drops evaporation.…”