Warm rain variability and its association with cloud mesoscale structure and

cloudiness transitions

Robert Wood, University of Washingtonwith help and data from Louise Leahy (UW), Matt Lebsock (JPL),

Irina Sandu (ECMWF)

Photo: Mingxi Zhang

A paradigmof strongly

precipitating open cells and

weakly precipitating closed cells?

Observations from VOCALS, 2008

Canonical modes of mesoscale

variabilityin subtropical and tropical marine low

clouds

No mesoscale cellular convection

Closed mesoscale cellular convection

Open mesoscale cellular convection

Cellular but disorganized

Wood and Hartmann (2006), J. Climate

SE PacificNE Pacific

Frequency of occurrence of

different mesoscale

modes

Stratocumulus to trade cumulus transition

Decoupling and Sc to Cu transition:

“Deepening-warming mechanism”

• Well-mixed MBL deepens by entraining FT air which is positively buoyant.

• Higher SST increases LHF stronger buoyancy flux more TKE more entrainment

• Insufficient LW cooling to allow entrained air to mix down, so pools near MBL top

• Surface moisture source cut off cloud breakup

qt [g kg-1]

qL [K]

Wyant et al. (1997, JAS)

Data• CloudSat: new warm rain retrievals from Matt

Lebsock (JPL)– column maximum precipitation rate for clouds with tops <

3 km derived from blend of Z-R and attenuation-based (2C-Precip) retrievals

• CALIPSO: low cloud fractional coverage (high cloud cleared) and cloud top height

• MODIS: Vis/NIR liquid water path retrievals to determine cloud mesoscale morphology for 256x256 km daytime subscenes using trained neural network (Wood and Hartmann 2006). – Where CloudSat swath passes through the MODIS scene

we store precipitation statistics

CloudSat column maximum precipitation rate from low clouds (ztop<3 km)

Precipitation rates maximize in the Sc to Cu transition regions

Low cloud fraction (CALIPSO)

Does drizzle formation play a role in Sc to Cu transiton?

Regions of max drizzle

Trajectories from Sandu et al. (2010, ACP)

Mean cloud top height for low clouds (CALIPSO, ztop < 3 km)

Regions of max drizzle

Precipitation maximizes in regions of maximum low cloud top height

COSMIC GPS-RO boundary layer depth

• Using refractivity gradient. Primarily detects hydrolapse, but also thermal inversion at higher latitudes

• Frequency of occurrence of closed and open cells (MODIS, Annual 2008)

• Open cell frequency is well correlated with precipitation rate from low clouds

Precipitation rate (ztop<3 km)

Precipitation and mesoscale cellularity

• Frequency of occurrence of closed and open cells (MODIS, Annual 2008)

• Open cell frequency is well correlated with precipitation rate from low clouds

Precipitation and mesoscale cellularity

Cell type frequency and CloudSat column max precipitation rate along Sc-Cu transition trajectories

Closed cells

Open cells

• Rising precipitation rate coincides with decrease in closed cells and increase in open cells

• Open cells appear to lag precipitation

Trajectories courtesy of Irina Sandu (MPI, now ECMWF)

NE Pacific

Cell type frequency and CloudSat column max precipitation rate along Sc-Cu transition trajectories

Closed cells

Open cells

• Rising precipitation rate coincides with decrease in closed cells and increase in open cells

• Open cells appear to lag precipitation

• SE Pacific shows double max in precipitation and open cell frequency

Open cells

Closed cellsSE Pacific

Trajectories courtesy of Irina Sandu (MPI, now ECMWF)

Distribution of mean precipitation rate for open and closed cells

• Differences in distributions of mean precipitation rates in open and closed cells

• Open cells appear to have a narrower distribution of precipitation rates, consistent with robust self-organized system discussed by Feingold

• Heaviest precipitation rates appear to occur in closed cells

Closed cells Open cells

Precipitation and stratocumulus to trade cumulus transition

Precipitation maxima Mean cloud top height

Height of thickest clouds

<1 km

3 km

Nature of precipitation along Sc-Cu trajectory

• Initial increase in precipitation explained by increase in precipitation frequency

• Precipitation rate continues to rise after frequency peaks and begins to fall

• Heavier but less frequency precipitation in trade Cu regions

dBZ histogram

How can drizzle cause decoupling?

Results from EPIC/VOCALS cruises (Bretherton et al. 2004)

Entrainment warming

Does drizzle help decouple the MBL?

• Strongly drizzling cases tend to be decoupled

• However, decoupling primarily related to layer thickness between inversion and LCL

• Difficult to separate deepening-warming impacts on decoupling from effects due to drizzle Jones et al. (2011, ACPD)

z i – z

LCL [m

]

Bretherton and Wyant minimal model

• Mixed layer model: dry and moist static energy constant with height

• Longwave cooling drives mixing and TKE production in the Sc-topped boundary layer (STBL). Assume surface fluxes do not impact TKE

• Entrainment depends upon buoyancy flux, which is strongly related to surface LHF, so warmer SST more entrainment

• Entrainment brings in buoyant air from the FT making it difficult to mix to surface leading to decoupling

• Can express decoupling criterion using ratio of LHF to radiative flux divergence F across the MBL:– LHF/F > [A h/zi]-1 for decoupling, where h is cloud thickness (zi – LCL), and

zi is the MBL depth, and A is the entrainment efficiency (1)

• Drizzle not accounted for

Modified Bretherton and Wyant minimal model

• Incorporate drizzle into mixed layer model• Decoupling criterion is 3-way balance between F, LHF, and precipitation

rate at cloud base Rcb and fraction fevap evaporating . For decoupling:

A [h/zi] LHF + LvRcb(1+1.4fevap) > F

fevap 0.75 is a reasonable parameterization of drizzle evaporation • Given that LCL is typically 600 m over ocean (Betts argument),

h zi 600, we can estimate:

zi =1.0 km: 0.4 LHF + 2.0 LvRcb > F

zi =1.5 km: 0.6 LHF + 2.0 LvRcb > F

zi =2.0 km: 0.7 LHF + 2.0 LvRcb > F

This means that 1 mm day-1 of cloudbase precipitation flux exerts as much “decoupling potential” as 80-140 W m-2 of LHF

Summary

• Precipitation rates from subtropical/tropical low clouds maximize in the transition regions between Sc and trade Cu

• Maximal rates correspond with transition from closed to open mesoscale cellular convection

• Frequency of occurrence of open cells occurs slightly downstream of maxima in precipitation rate. Heaviest precipitation rates tend to be found in deep closed cell cases prior to breakup

• Rates in these regions are 1 mm day-1, sufficient to compete with surface latent heat fluxes as a driver of decoupling and Sc-Cu transition

Precipitation and cloud optical depth

SE Pacific70-110oW, 40oS-0oS

dBZ Precip L.Heat [mm day-1] [W m-2] -15 0.15 5-7.5 0.7 20 0 2.0 60

Diurnal cycle of precipitation from

low clouds (ztop<3km)

NIGHT [1:30am]

DAY [1:30pm]

NIGHT - DAY

EPIC Cruise, SE Pacific

“Background” cloud droplet concentration critical for determining aerosol indirect effects

Quaas et al., AEROCOM indirect effects intercomparison, Atmos. Chem. Phys., 2009

Low Nd background strong Twomey effectHigh Nd background weaker Twomey effect

A ln(Nperturbed/Nunpertubed)

LANDOCEAN

C-130 flight path (grey)Cloud base (lidar-derived)LCL (“well-mixed cloud base”)

Radar reflectivity(drizzle proxy)

We use vertical profiles and subcloud level legs