The Role of Post Cold Frontal Cumulus Clouds in an Extratropical Cyclone Case Study Amanda M. Sheffield and Susan C. van den Heever Colorado State University Dynamics and Predictability of Middle Latitude Weather Systems and their Higher and Lower Latitude Interactions World Weather Open Science Conference August 18, 2014 Funding for this work supported by NASA Grant NNX13AQ33G.
Introduction Extratropical cyclones (ETCs) are important in the global precipitation and radiation budget (eg Boutle et al. 2011; Schemm & Wernli 2014; Martin et al., and others) Past research has focused on the role of latent heating in these cloud systems (Posselt and Martin 2004) and differences in characteristics between the northern hemisphere and southern hemisphere (Naud et al. 2012) Recent work has studied impacts of aerosols on these systems (Igel et al. 2013; Thompson et al. 2014; Wang et al. 2014)
Introduction Aerosol Impacts IPCC AR4
Introduction Aerosol Impacts Igel et al. (2013) found local aerosol induced changes to precipitation formation in the warm frontal region but very little change to the total precipitation buffering Today we will focus on the cold frontal region Process Buffering
Post Cold Frontal (PCF) Clouds Observational Examples Model Example Naud et al. 2006, 2013; Boutle et al. 2011
Goals Examine the relative role of: 1. cloud nucleating aerosols 2. sea surface temperature 3. extratropical cyclone (ETC) lifecycle on post cold frontal (PCF) cumulus cloud through cloud resolving model (CRM) simulations of an extratropical cyclone (ETC) case study
Winter Case Study: 13 January 2010 Region of several sources of aerosol, including asian dust.
3 Grid Cloud Resolving Model Setup CSU Regional Atmospheric Modeling System (RAMS) Two moment, bin emulating microphysics(saleeby & Cotton 2004; Saleeby and van den Heever 2013) Initialize with ECMWF Reanalysis Data Δx/Δy= 25 km; 5 km; 1 km 60 hour simulation (grid 3 in latter 24 hours) Surface model with climatological sea surface temperatures Aerosol (vary cloud nucleating aerosol); SST CLEAN: CCN = 100 cm 3 (constant vertical profile) (CONTROL) POLLUTED: CCN = 1000 cm 3 (constant vertical profile) CLEAN, LOW SST: Decreased SST by 2K CLEAN, HIGH SST: Increased SST by +2K
Extratropical Cyclone Development: Grid 1 Cloud Fraction (%) CONTROL
Extratropical Cyclone Development: Grid 2 CONTROL Shading: Surface Potential Temperature (K) Solid: Pressure (mb)
Extratropical Cyclone Development: Grid 3 CONTROL Shading: Surface Potential Temperature (K) Solid: Vertically Integrated Condensate (mm)
Extratropical Cyclone Development: Grid 3 CONTROL Shading: Surface Potential Temperature (K) Solid: Vertically Integrated Condensate (mm) Two Regimes: Boundary layer cloud below the inversion layer Transition to deeper convection
Regime 1: Environmentally Suppressed Regime 2: Environmentally Active Dry Cold Front Moist CONTROL Simulation: Red Blue: Relative Humidity (%) Solid Contours: Potential Temperature (K) White Blue: Total Condensate (g kg 1 ) > > Post cold frontal averages where total condensate mixing ratio > 0.1 gkg 1
ETC Accumulated Precipitation Accumulated Precipitation (mm) Aerosol Impacts: Overall extratropical cyclone response to aerosols as a whole has little to no precipitation change Accumulated Precipitation (mm) 5 10 15 20 25 30 35 Hour SST Impacts: Overall extratropical cyclone response to lower (higher) SST change is a decrease (increase) in the amount of precipitation 5 10 15 20 25 30 35 Hour
Aerosol Impacts on the PCF Region Regime 1: Dry and Stable Cloud Fraction (%) Similar cloud fraction Increased cloud mass Decreased rain mass (not shown) Mixed response in precipitation Clean Polluted Average Precipitation Rate Under PCF Clouds (mm hr 1 ) Regime 1 Regime 2 Cloud Mixing Ratio (g kg 1 ) 12 15 18 21 24 27 30 Hour
Aerosol Impacts on the PCF Region Regime 2: Moist &Less Stable Cloud Fraction (%) Clean Polluted Overall cloud fraction slightly increases Increased cloud mass Increased ice mass Enhanced latent heat release, convective invigoration Increase in precipitation Average Precipitation Rate Under PCF Clouds (mm hr 1 ) Regime 1 Regime 2 Cloud Mixing Ratio (g kg 1 ) Ice Mixing Ratio (g kg 1 ) 12 15 18 21 24 27 30 Hour
SST Impacts on the PCF Region (Relative Humidity (%) & Potential Temperature (K)) Active Suppressed 2K Control +2K
Low SST Impacts on the PCF Region Regime 1: Dry and Stable Cloud Fraction (%) Clean Clean, Low SST Increased amount of shallow boundary layer clouds Similar cloud mass Reduced Precipitation Average Precipitation Rate Under PCF Clouds (mm hr 1 ) Regime 1 Regime 2 Cloud Mixing Ratio (g kg 1 ) 12 15 18 21 24 27 30 Hour
High SST Impacts on the PCF Region Transition to deeper convection faster than the environmental changes Clean Clean, Low SST Clean, High SST Suppressed Active 4 hours 8 hours 12 hours Cloud Fraction (%)
SST Impacts on the PCF Region Regime 2: Moist &Less Stable Cloud Fraction (%) Clean Clean, Low SST Clean, High SST Low SST (High SST): Decreased (Increased) cloud fraction Similar cloud and reduced (increased) ice mass Reduced (Increased) precipitation Cloud Mixing Ratio (g kg 1 ) Ice Mixing Ratio (g kg 1 )
Aerosol versus SST Clean Polluted (dashed) Clean, Low SST Clean, High SST SST alters the PCF region precipitation characteristics more than aerosol impacts Results linked to ETC lifecycle 12 15 18 21 24 27 Hour
Conclusions Simulated an oceanic extratropical cyclone (ETC) from winter January 2010 with a cloud resolving model, including a high resolution finer grid. Environmental characteristics are shown to be a primary control of Post Cold Frontal (PCF) convection type: Regime 1: Dry, stable atmosphere Regime 2: Moist, less stable atmosphere Shift from shallow to moderately sized cumulus clouds With increased aerosol: Regime 1: Stable Regime Boundary Layer Cumulus Similar cloud fraction Increased cloud mixing ratio Mixed signal in precipitation Regime 2: Deep Convective Response Increased cloud fraction Increased ice mass Convective invigoration Enhancement of Precipitation
Conclusions With decreased (increased) SST: Decreased (increased) initial moistening (i.e. suppressed phase) and weaker (stronger) environmental moistening (i.e. active phase) shown to be an important control of PCF cloud. Aerosol impacts influence the PCF precipitation while not varying the overall ETC precipitation, while SST is a stronger control. Future Work: Quantitative assessment of the relative role of environment versus aerosol Quantitative assessment of these impacts on the water vapor budget