Impact of Cloud Microphysics on the Development of Trailing Stratiform Precipitation in a Simulated Squall Line: Comparison of One- and Two-Moment Schemes Reporter: Prudence Chien Reference: Morrison, H., G. Thompson, V. Tatarskii, 2009: Impact of cloud microphysics on the development of trailing stratiform precipitation in a simulated squall line: comparison of one- and two-moment schemes. Mon. Wea. Rev., 137, 991–1007.

Outline Keywords Motivation Experimental design Results Summary & conclusions

Keywords Squall line A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms. The squall line is a type of mesoscale convective system distinguished from other types by a larger length-to-width ratio. Conceptual model of a squall line

(Biggerstaff & Houze 1991) Collision Coalescence Aggregation Deposition Melting layer Evaporation

Keywords N 0 jump N 0r decreases rapidly during the transition from convective to widespread stratiform rain. Drop Size Distribution (DSD) N 0 : intercept Λ: slope μ: shape parameter D: particle diameter ( 吳子瑜 2008)

Keywords Two-moment scheme (double-moment scheme) – One-moment scheme (single-moment scheme) – Predicts mass mixing ratio(q) and number concentration(N) – 5 hydrometer species: cloud droplets, cloud ice, snow, rain, and graupel

Motivation In general, models cannot catch squall-line features very successfully, the transition zone and the trailing stratiform region in particular. Limitation: – Bulk schemes assume an underlying shape for the hydrometeor size distribution, and predict one or more bulk quantities of the distribution. – Bin microphysics schemes not feasible for most application. Compare the Impact of one- and two-moment scheme on the evolution of an idealized 2-D squall line.

Experimental design WRF V2.2 2D idealized squall-line case Horizontal domain: 600 km, Δx = 1000m Vertical domain: 20 km, Δz = 250m Simulated period: 7h, Δt =5s Environmental temperature and moisture profile: Weisman and Klemp (1982, 1984) Thermal bubble: θmax= 3K at 1.5km height Specified N 0r = 10 7 m -4 in 1-M

Results Storm Morphology Precipitation and Cold Pool Mesoscale Dynamics Convective Updrafts

2-M 1-M q x (thin solid line) Θ’ = -2K (thick solid line) Wind vector (arrows) Much narrower and weaker region of trailing stratiform precipitation in 1-M Radar reflectivities in 1-M are lower than 2-M in the stratiform region Large rain evaporation rate between the melting layer and the surface in the stratiform region in 1-M 1) Storm morphology CV SF CVSF

2-M1-M Surface rainfall rate 2-M : Larger rain rate in stratiform region Slightly smaller rain rate in convective region 2) Precipitation and the cold pool

Rain mixing ratioRain evaporation rate Larger rain evaporation rate in 1-M leads to smaller rain mixing ratio than 2-M below the melting layer. CVSF CVSF CVSF CVSF

2-M warmer than 1-M (shaded gray) Rain intercept parameter (N 0r )Difference in perturbation potential temperature Larger value of N 0r leads to larger evaporation rate. Largest values of N 0r occur in the convective region, and steadily decrease through the trailing stratiform region. The increased evaporation rate in the stratiform region leads to a broader and generally colder cold pool in 1-M than 2-M. CV SF CV SF

2-M > 1-M (shaded gray) Difference in vertical velocity Difference in buoyancy The mesoscale updraft at midlevels of the stratiform region is stronger in 2-M than 1-M. Main factor: (1)Latent heating rate (2)Rearward horizontal fluxes of condensate and buoyancy 3) Mesoscale dynamics CV SF CV SF

Latent heating rate due to vapor deposition growth of ice is greater in 2-M than 1-M in the stratiform region. Difference in latent heating rate CV SF

Ice mixing ratio CV SF CV SF

Difference in front-to-rear flux of buoyancy Difference in front-to-rear flux of condensate 2-M > 1-M (shaded gray) Much larger flux of buoyancy at midlevels from the convective to the stratiform region in 2-M than in 1-M. Much smaller amounts of ice condensate in the convective region in 2-M than 1-M. => Rearward flux of condensate from the convective region tends to be much smaller in 2-M than in 1-M. CVSFCVSF

Sensitivity simulation of evaporation rate Calculate rain evaporation rate by limiting N 0r ’s maximum=10 7 m -4 in convective region (2-M*) Rain evaporation are reduced in convective region, leading to reduced latent cooling and increased mean convective updraft intensity. 2-M > 2-M* (gray shaded) Difference in vertical velocity 4) Convective updrafts CV SF

Sensitivity simulation of evaporation rate 2-M > 2-M* (gray shaded) Difference in front-to-rear flux of buoyancy Difference in front-to-rear flux of condensate Reduced rearward flux of positively buoyant air to stratiform region Increased rearward flux of condensate CV SF

1-M sensitivity test Reduced rain evaporation rate Produce much more trailing stratiform precipitation The leading edge of convective precipitation is poorly defind. Cold pool is much weaker than in 2-M Smaller rearward buoyancy and condensate flux at midlevels into the stratiform region Reflectivities are weak above the melting layer compared to 2-M 1-M* ( N 0r =2*10 6 m -4 ) 1-M ( N 0r =10 7 m -4 ) 2-M CV SF

1-M sensitivity test 1-M* ( N 0r =2*10 8 m -4 ) Increased rain evaporation rate Very little surface rainfall in the stratiform region Increased rearward flux of positively buoyancy into the stratiform region Reflectivities are weak above the melting layer compared to 2-M 1-M ( N 0r =10 7 m -4 ) 2-M => The importance of capturing the variability of N 0r between the straitiform and convective region. Two-moment schemes allow a more rigorous treatment of N 0r CVSF

Summary & conclusion 2-M scheme produced a much more widespread and prominent region of stratiform precipitation, relative to 1- M scheme. Key factor: reduced rain evaporation rate in the stratiform region in 2-M – Larger mean raindrop size in 2-M than in 1-M in the stratiform region directly contributed to the larger radar reflectivity. Secondary factor: increased rain evaporation rates in the convective region at midlevels – Reduction in the intensity of the convective updrafts. – An increased flux of positively buoyant air at midlevels from the convective to stratiform region. – An increased intensity of the mesoscale updraft.

Difference in the rain evaporation rate between 1-M and 2-M were the result of differences in the rain size distribution parameters. – Larger N 0r in the convective region were associated with significant collision-coalescence; in contrast, rain in the stratiform region was primarily produced by melting of snow. – The variability of N 0r is consistent with surface disdrometer measurements. There are a sharp decrease in N 0r (N 0 jump) between convective and stratiform region.

Rain evaporation plays a key role in the development of the trailing stratiform region. Rain evaporation rate is sensitive to the number concentration in the 2-M scheme. A scheme with two-moments predicted for rain only was able to reproduce the stratiform region features using the full 2-M scheme.

Thanks for your attention. Questions?

Gamma function:

Rain evaporation rate

Positive vertical velocity (gray shaded) 2-M vertical velocity