Relative influence of meteorological conditions and aerosols on the lifetime of mesoscale convective systems

Abstract

Using collocated measurements from geostationary and polar-orbital satellites over tropical continents, we provide a large-scale statistical assessment of the relative influence of aerosols and meteorological conditions on the lifetime of mesoscale convective systems (MCSs). Our results show that MCSs' lifetime increases by 3-24 h when vertical wind shear (VWS) and convective available potential energy (CAPE) are moderate to high and ambient aerosol optical depth (AOD) increases by 1 SD (1σ). However, this influence is not as strong as that of CAPE, relative humidity, and VWS, which increase MCSs' lifetime by 3-30 h, 3-27 h, and 3-30 h per 1σ of these variables and explain up to 36%, 45%, and 34%, respectively, of the variance of the MCSs' lifetime. AOD explains up to 24% of the total variance of MCSs' lifetime during the decay phase. This result is physically consistent with that of the variation of the MCSs' ice water content (IWC) with aerosols, which accounts for 35% and 27% of the total variance of the IWC in convective cores and anvil, respectively, during the decay phase. The effect of aerosols on MCSs' lifetime varies between different continents. AOD appears to explain up to 20-22% of the total variance of MCSs' lifetime over equatorial South America compared with 8% over equatorial Africa. Aerosols over the Indian Ocean can explain 20% of total variance of MCSs' lifetime over South Asia because such MCSs form and develop over the ocean. These regional differences of aerosol impacts may be linked to different meteorological conditions. mesoscale convective systems | aerosols | meteorological parameters T he hypothesis that aerosols may delay precipitation and increase cloud lifetime of shallow marine clouds (1) has motivated many researchers to study the aerosol indirect effect on convective clouds; however, the influence of aerosols on enhancing cloud lifetime has remained under debate. Mesoscale convective systems (MCSs) are deep convective clouds that cover several hundred kilometers. Previous studies have shown that aerosols affect deep convection, in particular that aerosols increase the number of smaller size cloud condensation nuclei (CCN) (2), which weaken coagulation and coalescence that form rain droplets, and consequently delay warm rainfall (3, 4). These processes allow more cloud droplets to rise above the freezing level and increase latent heat released due to glaciation (5), resulting in stronger updraft speed, enhanced cloud ice content (6), larger anvil size (5), and higher cloud top height (7). The top of the troposphere warms owing to the aerosol-induced changes in convective anvils (8). Although these aerosol effects have been seen in observations from field campaigns (9, 10), they have been undetectable on large spatial and multiyear scales. Rosenfeld et al. (11) have attributed this lack of detectability on the large scale of aerosol invigoration of convection to its variation with meteorological conditions and to the lack of knowledge of the relative humidity (RH) outside the clo...

On the role of aerosols, humidity, and vertical wind shear in the transition of shallow-to-deep convection at the Green Ocean Amazon 2014/5 site

“…This permits investigation of the thermodynamic effect of BL humidity (between surface and 950 hPa), free tropospheric relative humidity (between 850 and 400 hPa), low-level VWS, deep tropospheric VWS, and CCN concentrations. Low-level VWS is defined as the difference of the mean wind speed (zonal, since meridional wind difference is smaller) between the two 100 mbar thick layers centering at 937 and 737 hPa (Weisman and Rotunno, 2000); the deep level VWS is the difference between the layers centering at the 887 hPa and 287 hPa pressure levels (Chakraborty et al, 2016;Petersen et al, 2006). We calculate VWS by subtracting the mean wind speed of the top layer from that of the bottom layer.…”

“…For example, VWS influences the rainfall and total condensation within developing convection (Weisman and Rotunno, 2004), slantwise ascent of the parcel (Moncrieff, 1978), storm rotation, maintenance, vorticity, updraft speed (Weisman and Rotunno, 2000), and lifetime (Chakraborty et al, 2016). Though detailed microphysical properties are not considered in our simple plume calculations, it is worth noting that a recent study by Wu et al (2017) found that lower tropospheric wind shear promotes the droplet collision and growth inside the shallow clouds by the production of turbulent kinetic energy.…”

“…Additionally, vertical wind shear (VWS) is known to influence deep convective clouds by influencing the slantwise ascent of the moisture (Moncrieff, 1978), separating the updraft and downdraft regions. In a recent study, it was shown that deep tropospheric VWS (DVWS) has a significant impact on the lifetime of mesoscale convective systems (Chakraborty et al, 2016) and can regulate the anvil's formation (Koren et al, 2010;Weisman and Rotunno, 2004;Petersen et al, 2006;Kilroy et al, 2014;Harrison, 1992) as well as the updraft speed of the parcels (Weisman and Rotunno, 2004). However, low-level VWS (LVWS) can influence the rainfall and total condensation within developing convection (Weisman and Rotunno, 2004).…”

“…In addition, aerosols can delay the formation of precipitation size hydrometeors, invigorating strong convection, while suppressing shallower and weaker convection (Rosenfeld et al, 2008;Koren et al, 2008;Lin et al, 2006;Andreae et al, 2004). Low-to-moderate aerosols enhance convective strength and such an influence depends on humidity (Chakraborty et al, 2016). Furthermore, satellite data analyses have suggested that during the dry-to-wet transition season over the Amazon, biomass burning aerosols can increase warm clouds through their indirect effect under higher relative humidity (RH) and moderate aerosol loading, whereas under lower RH and heavy aerosol loading conditions, biomass burning aerosols tend to decrease clouds (e.g., Yu et al, 2006).…”

On the role of aerosols, humidity, and vertical wind shear in the transition of shallow to deep convection at the Green Ocean Amazon 2014/5 site

“…However, many previous studies 34 illustrate difficulties in representing the shallow-deep evolution in models (Del Genio and Wu,35 2010; Waite and Khouider, 2010 influencing the slantwise ascent of the moisture (Moncrieff, 1978), separating the updraft and 48 downdraft regions. In a recent study, it was shown that deep tropospheric VWS (DVWS) has a 49 significant impact on the lifetime of mesoscale convective systems (Chakraborty et al, 2016) 50 and can regulate the anvil's formation (Koren et al, 2010;Weisman and Rotunno, 51 2004; Petersen et al, 2006;Kilroy et al, 2014;Harrison, 1992) as well as the updraft speed of the 52 parcels (Weisman and Rotunno, 2004). On the other hand, low level VWS (LVWS) can influence 53 the rainfall and total condensation within developing convection (Weisman and Rotunno,54 2004).…”