Explicit Forecasts of Winter Precipitation Using an Improved Bulk Microphysics

Scheme.Part : Description and Sensitivity AnalysisⅠ

GREGORY THOMPSON, ROY M. RASMUSSEN,

AND KEVIN MANNING

February 2004 Monthly Weather Review 519~542

1.Introduction

• Explicit bin-resolving cloud models(Rasmussen et al 2002, hereafter RG)

• Bulk microphysics parameterization• 利用 Reisner et al.(hereafter RRB) 作為修正基準• 目的：改善顯示 (explicit) 的即時過冷水滴預報，藉以防止飛機積冰 (aircraft icing) 的現象發生

Note : 利用 bulk model 來替代 explicit model ，節省計算時間

2.Description of the bulk microphysics

parameterization

• Initiation of cloud ice• Autoconversion • Treatment of graupel• Intercept parameter of the snow size distribution • Intercept parameter of the rain size distribution

Initiation of cloud ice

Fletcher 1962

Meyers 1992

Cooper 1986

Note : 在最近的研究指出 ice 的形成 Si >5% ， T<-5℃

Autoconversion

Kessler 1969

Berry 1968

(Berry and Reinhardt 1974)

υ is a gamma distribution parameter

Qr is rain content,Db=0.3, Nb is cloud droplet concentration, Qc is the cloud water content

Treatment of graupel

改進冰雹形成中兩個最大量：

Rutledge and Hobbs 1983

1983 Hobbsand Rutledge

)/(101.0

)/(105.0

,1/

3

3

kgkgq

kgkgq

PSACWPSDEP

s

c

Intercept parameter of snow size

• 因為固定的截斷參數 (N0,S) 使得過多的雲水被消耗• 發展一個跟混和比和 single moment 相關的關係式 (Sw

ann 1998)• 平均粒子直徑跟溫度一起增加，且 N0,S和 λ 減小時，溫度也會增加 (Houze et al. 1979)

Houze et al 1979

灰粗線 /虛線是利用 Houze et al.1979

細實線 /虛線是 Reisner et al.1998

Intercept parameter of rain size distribution

• Droplet size drizzle drops rain ( 雨滴的形成過程 )

• 在大部分的 single moment scheme 中，都是假設雨水突然由雲水形成而且呈現 Marshall-Pamler size distibution and intercept parameter(8106(m-4))

• 考慮 drizzle to rain

小雨滴

大雨滴

粗實線是利用上頁公式畫出來的終端速度和雨混和比圖，虛線是利用 N0,r= 8106(m- 4) 做出來的圖

3.Approach • Mesoscale model (MM5) • 2-D idealized configuration• 120points, spacing 10km

• 39σp level, flow is 15ms-1

• Bell-shaped mountain 1km high with 100km half-width

敏感度實驗的設定

4.Sensitivity tests

CONTROL experiment

b.Bin model

Initial cloud ice 是利用 Cooper(1986)

c.Ice initial:Meyers and Fletcher experiment

• Fletcher experiment 減少了 cloud ice 在鋒值的數量

• Meyers experiment 增加了 cloud ice 的數量

d.Autoconversion:Kessler1, Kessler5, B

erry, and BandR

Kessler1(qco=0.110-3)

Kessler5(qco=0.5 10-3)

Berry(1986)

BandR(Berry and Reinhardt

autoconversion 1974)

e.CCN spectra:Maritime, Continent1, and Continent2

experiment

Maritime(Nc=50cm-3)

Continent1(Nc=200cm-3)

Continent2(Nc=500cm-3)

f.Treatment of graupel: Graupel1

and Graupel2 experiment

Graupel1:original exponential distribution, No,g=4 106m-4, and Reisner rimed snow-to-graupel conversion

Graupel2:Riming growth of snow must be 3 times larger then depositional growth before creating graupel

g.Version2

• 把 Fletcher, Kessler5, Graupel1 一起使用

• 可以得到一個非常接近 Graupel1 的結果，所以在這三組實驗中以 Graupel1 比較敏感

h.Snow intercept parameter:SON and SONV experiment

SON(No,s=2 107m-4)

SONV(No,s=f(T))

i.Rain intercept parameter:RON

and RONV

RON(No,r=11010m-4)

RONV(No,r=f(qr))

Note : combine SNOV

j.Final experiment

BandR, Graupel2, SONV, RONV 一起使用

k.Deeper/colder cloud system

• CCT=-25℃• CCT=-60℃• Seeder and feeder system

CCT=-25℃

CCT=-60℃

Seeder and feeder system

上層都是由冰晶跟雪組成的溫度較低的雲

下層是混和雲

在經過兩小時之後上層的雲開始影響下層

在下層會有一些過冷毛雨被冰覆蓋落在地面

Conclusion • Primary ice nucleation using Copper(1986), replaces the

Fletcher(1962) curve• Autoconversion using Walko et al.(1995), replaces the

Kessler scheme• A generalized gamma distribution for graupel (eq. 8)

replaces the exponential distribution; the intercept parameter depends on mixing ratio (eq. 10) instead of constant; and riming growth of snow must exceed depositional growth of snow by a factor 3 before rimed snow transfers graupel

• The intercept parameter of snow size distribution depends on temperature replacing the mixing ratio dependency

• The intercept parameter for rain size distribution depend on mixing ratio, thereby simulating the fall velocity of drizzle drops as well as raindrops