# Presentation: 534420

## MODIS Level-3 Application: Cloud Susceptibility (Oreopoulos, Platnick)

- Uses operational **Collection 5 MODIS global daily Level-3 joint histograms of liquid water cloud optical thickness (**********c****) and effective radius (*****r*****e****)**.
- Individual* ***c, *r*e combinations from joint histograms and <*T*c>, *T*(*z*), *q*(*z*), *A*sfc,** input to broadband code for each grid.
- Difference between unperturbed and perturbed (due to *r*e changes)  albedo gives susceptibility.
- Daily susceptibilities aggregated to provide monthly means.
**Ancillary data sets same as used in MODIS cloud algorithm** (NCEP GDAS, surface spectral albedo maps from *Moody et al.*, 2005)
- Calculations assume no change in water amount with microphysical changes
- Does not address:
- Current day vs. pre-industrial changes (not a sensitivity but a  climate change question that includes feedbacks)
- Cloud amount and precipitation sensitivities (requires statistical and/or modeling studies to eliminate dynamic/thermodynamic sensitivities)

- MODIS Level-3 Application: Cloud Susceptibility (*Oreopoulos, Platnick*)

**Notes:**
On-going effort. Susceptibility has been used to explicitly refer to the microphysical sensitivity. There are of course many other sensitivities - sensitivity to cloud amount being obvious - and any number of dynamics and thermodynamic sensitivities. (Sensitivity used by Arking, BAMS, 1991). Some practical implications to how delta_re is specified.

RT using modified Ming-Dah Chou BB code.

Strategy:

MODIS Collection 5 Level-3 Terra (MOD08) and Aqua (MYD08) daily files
joint histograms of liquid water cloud optical thickness (tc) and effective radius (re)
mean grid cloud-top temperature (<Tc>) from joint liquid water tc-Tc histogram
Ancillary data sets
Atmospheric state (NCEP GDAS) and global broadband surface albedo maps (~1 km, Moody et al., 2005) used by the MODIS retrievals
Broadband shortwave radiative transfer code (modified from M.-D. Chou et al.)
Individual tc, re combinations from joint histograms and <Tc>, T(z), q(z), dual band Asfc input to broadband code for each grid
difference between unperturbed and perturbed (due to Dre changes)  albedo gives susceptibility
Daily global susceptibility (at time of overpass) aggregated to provide monthly means

## Susceptibility (S), N=1 cm-3, LWC=0.3 gm-3, MODIS Terra

- April 2005

- July 2005

- October 2005

- Susceptibility (*S*), *N*=1 cm-3, *LWC*=0.3 gm-3, MODIS Terra

**Notes:**
S ~ tau * dR/dtau * re^3
Smaller S over bright surfaces (BB solar albedo): TOA S is higher over ocean not just because the clouds are inherently more susceptible

## Relative Susceptibility (Srel), N/N=10%, MODIS Terra

- Relative Susceptibility (*S*rel), *N*/*N*=10%, MODIS Terra

- April 2005

- July 2005

- October 2005

- January 2005

**Notes:**
S_rel ~ tau * dR/dtau * deltaN/N, tau * dR/dtau has a “sweet spot” at a tau~10 or so

10 = .01 absolute change in Albedo
Cloud albedo (Acloud) = column albedo

2D joint tau-re histograms has bigger effect on S_rel than on S.

## Global Susceptibility Forcing Examples

**Notes:**
Cloud fraction effect: large S correlated with low cloud fraction over the ocean, for S_rel, sweet spot not correlated with low cloud fraction.
Though the relative relationships look very similar, note different scales. In these examples the relative susc forcing  is about 80% larger than for the absolute susceptibility.