# Presentation: 216400

## Suppression of Deep Convection by the Saharan Air Layer (SAL) over the Tropical North Atlantic

- Sun Wong1, Andrew E. Dessler1, Peter Colarco2
- 1ESSIC, University of Maryland, College Park
- 2Goddard Space Flight Center, Maryland, Greenbelt
- March 24, 2005
- (MODIS Meeting)

## Africa

- Saharan Air Layer
- Warm
- Dry
- Dusty

- Atlantic

- 5 km

- 0.9 km

**Cool, moist**

- Easterlies

## Why does the SAL concern us?

- Evidence has shown linkage between the SAL and the tropical cyclone (TC) activity
- The SAL is associated with the transport and distribution of dust over Atlantic.  Dust can cool the surface and warm the atmosphere (Global mean forcing at surface from dust: –0.96 Wm-2)

## Purposes

- Using NASA’s satellite data (including MODIS) to study the thermodynamic structure of the SAL
- Evaluate simulation of the SAL by NASA’s model (fvGCM) using the satellite data
- Understanding the detailed physics using the model

## Data Resources

- MODerate resolution Imaging Spectroradiometer (MODIS) aerosol optical thickness (AOT,0.55 μm, MOD04 level-2 product from Terra)
- Brightness T (11 μm, MOD06 level-2 product)
- NCEP/NCAR Reanalysis T, RH (2.5°×2.5°)
- Time: Daily instantaneous samples at around noon for Aug-Sep 2002
- Region: 40°-20°W, 10°-20°N

## Monthly Mean AOT

**Monthly Mean AOT**

## For each grid and each day in the region

- 1°

- 1°

- AOT

- 10 km

- BT

- 1 km

**For each grid and each day in the region**

- Range: 190-300 K
- Bin width: 10 K

-  Grid-averaged AOT

-  BT histogram
-   Total BT pixels no. > 100

## Regional Statistics for Aug-Sep 2002

- 0-0.3

- 0.9-3

## Composites of T Anomaly Profiles 

- 0.9-3

- 0.3-0.5

- 0.5-0.7

- 0.7-0.9

- 0.3-0.5

- 0.5-0.7

- 0.7-0.9

## A

- Virtual Temperature

- Altitude

**CAPE and Convection Barrier**

- LCL

- LFC

- LNB

- Air parcel T

- Non-SAL T

- B

- A: Energy released in convection
- B: Convection barrier
- CAPE = A – B

**Lifted LFC**

- SAL T

## Summary for East Atlantic

| AOT Range | Ave. AOT | Conv. Barrier
(m2/s2) | P(BT<230 K)
% | P(BT<250 K)
% |
| --- | --- | --- | --- | --- |
| 0-0.3
0.3-0.5
0.5-0.7
0.7-0.9
0.9-3 | 0.21
0.41
0.60
0.79
1.21 | 6.65
16.10
27.84
37.63
54.27 | 1.05
0.19
0.20
0.00
0.00 | 2.70
1.10
1.04
0.15
0.05 |

**Summary for East Atlantic**

## AOT EOF-1

## T at 850 hPa EOF-1

## Conclusion

- The SAL is associated with a suppression of deep convection.  Frequency of clouds with BT less than 250 K is largely reduced
- The SAL suppresses deep convection by lifting the LCL (less moisture), LFC (warmer temperature), and strengthening the convection barrier

## FVGCM + NCEP/NCAR + Peter Colarco

**FVGCM + NCEP/NCAR + Peter Colarco**

## Monthly Mean AOT and Air-Mass Convergence

## GOES SAL Tracking Imagery, BT11-BT12 (Dunion and Velden, 2004)

**GOES SAL Tracking Imagery, BT11-BT12 (Dunion and Velden, 2004)**

## GPS Moisture Sounding Composites (Dunion and Velden, 2004)

**GPS Moisture Sounding Composites (Dunion and Velden, 2004)**

## MODIS Satellite True Color Image (August 11, 2002)

**MODIS Satellite True Color Image (August 11, 2002)**

## For each grid and each day in the region

- 1°

- 1°

- AOT

- 10 km

**For each grid and each day in the region**

-  Grid-averaged AOT

## Regional Statistics for Aug-Sep 2002

| AOT | P(BT<230 K) in % |
| --- | --- |
| 0-0.3
0.3-0.5
0.5-0.7
0.7-0.9
0.9-3.0 | 1.05
0.19
0.20
0.00
0.00 |

- East Atlantic (Region 1):

## Summary for Central Atlantic

| AOT Range | Ave. AOT | Conv. Barrier
(m2/s2) | P(BT<230 K)
% | P(BT<250 K)
% |
| --- | --- | --- | --- | --- |
| 0-0.3
0.3-0.5
0.5-0.7
0.7-0.9
– | 0.21
0.40
0.59
0.78
– | 6.04
11.48
15.42
12.34
– | 0.81
0.51
0.13
0.18
– | 2.37
2.27
0.90
1.42
– |

**Summary for Central Atlantic**

## Composites of q Anomaly Profiles

## Composites of T Anomaly Profiles 

- 0.9-3

- 0.3-0.5

- 0.5-0.7

- 0.7-0.9

## A

- Virtual Temperature

- Altitude

**CAPE and Convection Barrier**

- LCL

- LFC

- LNB

- Air parcel T

- Environmental T

- B

- A: Energy released in convection
- B: Convection barrier
- CAPE = A – B

## Discussion and Future Direction

- How about the wind shear at the southern border of the SAL?
- Are dust and warm anomalies really advected coherently?
- The quality of NCEP/NCAR reanalysis? (Especially moisture!!!)