A Cloud Resolving Modeling Study of Tropical Convective and Stratiform Clouds

C.-H. Sui and Xiaofan Li

Introduction

• Partition of clouds and precipitation into convective and stratiform types is an important part of the study towards understanding of clouds and associated microphysics and thermodynamics and their impacts on tropical hydrological and energy cycles.

• Churchill and Houze (1984) used the radar observational data to study convective-stratiform rain participation.

• Alder and Negri (1988) developed a convective-stratiform technique for analysis of satellite infrared data locates all local minima in the brightness temperature field.

• A version of the separation method used in the Goddard Cumulus Ensemble (GCE) model is adopted in Sui et al. (1994).

Model and experiment

• The 2-D version of the model used by Sui et al. (1994, 1998) and further modified by Li et al. (1999) is used in this study.

• The cloud microphysics parameterization schemes are referred to Li et al. (1999, 2002b) and Sui and Li (2002).

• Based on the 6-hourly TOGA COARE observations within the Intensive Flux Array (IFA) region.

• The model is integrated from 1992/12/19/0400 LST to 1993/01/09/0400 LST (21 days total).

• The horizontal domain is 768 km, the grid mesh of 1.5 km and time step of 12 seconds.

xxxTxxx DqSqww

zx

uq

t

q

)(1)(

][][][

qxsxx SPCONVqt

q

Mean cloud budgets in stratiform and convective regions based on an existing separation method

The cloud budget equation:

The vertically integrated cloud budget equation:

qx = (qc, qr, qi, qs, qg),

is a mean density, which is function of height;wTx = (wTr, wTs, wTg) are terminal velocity for qr, qs, qg;Sqx are the source and sink of various hydrometeor species;D’s are turbulent dissipation terms;[CONVqx] is hydrometeor convergence; is surface rain rate.

sTVs CwP )(

Cloud Ratio: CR = ( [qi] + [qs] + [qg] ) / ( [qc] + [qr] ) Ice Water Phase: IWP = [qi] + [qs] + [qg] Liquid Water Phase: LWP = [qc] + [qr]

Rate Ratio: RR = ( [PDEP] + [PSDEP] + [PGDEP] ) / [PCND]

Cloud Microphysics Precipitation Efficiency: CMPE = Ps / ( [PDEP] + [PSDEP] + [PGDEP] + [PCND] )

A new separation method for stratiform and convective regions and the corresponding mean cloud budgets

Rate Ratio (RR) Cloud Ratio (CR)

CR > 1 CR < 1

IWP > LWP stratiform clouds developed

IWP < LWP convective clouds developed

Sui, 1994 (fcrsc1, fcrcc1)RR (fcrsc2,fcrcc2, 0.1, +)CR (fcrsc3,fcrcc3, 0.4, o)

Stratiform Convective

Stratiform

CR = 0.83RR = 0.44CMPE = 0.87

Convective

CR = 0.26RR = 0.07CMPE = 0.62

Sui, 1994

The LWP and associated microphysical conversion rates are significantly larger in the convective regions than in the stratiform regions,

whereas the IWP and associated microphysical conversion rates do not change much in the two regions.

(a) stratiform, CR = 2.1, RR = 2.1, CMPE = 0.48(b) mixture conditions, CR = 0.6, RR = 0.3, CMPE = 1.01(c) convective, CR = 0.13, RR = 0.03, CMPE = 0.72

Cloud Ratio:

1. It is specifically linked to microphysical processes.

2. It does not need cloud information from surrounding areas for determining rain types.

3. IWP and LWP are now available routinely from satellite measure- ments by sensors like TRMM.

Budget analysis for dominant processes changing the cloud ratio

))(,(1

IWPLWP

IWPLWPIWPLWPC

LWP

CONV

IWP

CONV

t

CR

CRLWPIWP

LWP

P

LWP

P

LWP

P

IWP

PREVPsfcCNDDEP ][][][

Process 1 Process 2

Process 3 Process 4

Process 1 denoting the contribution from the convergence of cloud hydrometeors.

Process 2 is determined by the conversion between LWP and IWP.

Process 3 are the condensation and deposition time scales. Ex: PCND occurs slower(faster) than the PDEP, CR increases (decreases), and stratiform clouds develop faster (slower) than the convective clouds.

Process 4 always enhance CR since rainfall and the PREVP consume raindrop.

LWP

CONV

IWP

CONV LWPIWP Process 1

Process 1 has the same order of magnitude as the sum of the other processes.

CC= 0.76

CC= 0.08

CC= 0.25

The Process 3 and Process 4 are insensitivity to CR , they have no significant impacts on the variations of clouds.

Process 2A causes the decrease of CR and the development of convective clouds, Process 2C cause the increase of CR and the development of stratiform clouds.

2A : PGMLT (+)2B : PSACW (o) 2C : PGACW (Δ)

IWP 　　 LWP

disspating stage with the development of anvil clouds

The relative amounts of IWP and LWP may depend on the vertical profiles of upward motion.

0800 LST 20 (CR=0.2)

2200 LST 20 (CR=0.8)

0400 LST 23 (CR=2.1, anvil cloud)

CRC (+), CRM (o), CRS (Δ)

Summary

• The conversion between LWP and IWP determines the variation of cloud ratio through the melting of graupels and the accretion of cloud water by graupels.

Convective Mixed Stratiform

CR < 0.4 0.4 ~ 1.0 > 1.0

RR < 0.1 0.1 ~ 1.0 > 1.0