Studying the Impact of Saharan Dust on Tropical Cyclone Evolution

using WRF/Chem and EnKF

Jianyu Liang (York U.)Yongsheng Chen (York U.)

Zhiquan Liu (NCAR)

Acknowledge: Avelino Arellano, Ziqiang Jiang, Yongxin Zhang

Image: NASA

Definition: elevated layer of Saharan air and mineral dust, warm, dry, and enhanced easterly jet to the south

Origin: begin from near the costal of Africa. Under the influence of African easterly waves, the air mass often moved towards west from the North African coast ( Burpee 1972)

Duration : The SAL usually originate form late spring and remain exist to early fall.

Coverage : It cover a very large region in the North Atlantic Ocean

Vertical extend : During the summer, the dry ,well mixed SAL can reach around 500 hPa height (Calson and Prospero, 1972).

Saharan Air Layer (SAL)

Positive impact:Enhance easterly waves growth and potentially cyclongenesis(eg., Karyampudi and Carison, 1988)

Negative impact:1)Bring dry and warm air into tropical storms, thus increase stability2)Enhance the vertical wind shear to suppress the developments of tropical storms(eg., Dunionand Velden2004; Sun et al. 2009)

Objectives:Use WRF-CHEM and DART to quantify the impact of SAL on TCs.Hurricane Earl (2010) is chosen to be the first case.

Impact of SAL on Tropical Cyclones

Methodology

1)WRF-CHEM model• The chemistry component including dust variables in addition

to the meteorological component;• both components use the sametime steps, grid , transport

schemes, and the same physics schemes for subgrid-scale transport (Grell, etc. 2005).

• GOCART dust

2) DART• Assimilate MODIS aerosol optical depth (AOD) at 550 nm in

addition to conventional observations• Localization in variables and space• Fixed prior covariance inflation

Hurricane Earl case

Figure 1 Hurricane Earl best track from 25th , August to 4th September, 2010. ( FromCangialosi 2011)

Figure 2. Forecast from the model from 0000 UTC 26th , August to 0000 UTC 30, August. ( From Cangialosi 2011)

Figure 3. +METEOSAT-7/GOES-11 combined Dry Air/SAL Product (source: University of Wisconsin-CIMSS) ,red A indicate the position of hurricane Earl .

(b) 26th, August.

(a) 25th, August.

Temperature (oC) from AIRS. at 1000hPa

Temperature (oC) from AIRS. at 850hPa

Relative humidity from AIRS. at 1000hPa

Relative humidity from AIRS. at 850hPa

Optical_Depth_Land_And_Ocean_Mean(0~1) from MODIS L3 product . a) 23, August . b) 24th, August

Resolution: 36 km West-east: 310; North-South: 163; Vertical: 57GOCART simple aerosol scheme , RRTMG radiation scheme, Mellor-Yamada Nakanishi and Niino Level 2.5 PBL, Grell 3D cumulus, Lin microphysics schemeEnsemble: 20 members

Experiment Design

1) Generating ensemble perturbations in chemistrya. spin up for 20 days starting from 00UTC, 01 August 2010b. updating meteorological fields by FNL every 6 hoursc. spin-up cycle stops at 20,August , 2012

2) Generating ensemble perturbation in meteorological fieldsRandomly draw from 3DVAR error covariance

3) Data assimilation cycles and forecastFirst, assimilate conventional observations 6-hourly for 1 dayThen, cycle 6-hourly for 4 days

a) Assimilate conventional observations onlyb) Assimilate conventional and MODIS AOD observations

Finally, forecast with/without chemistry using WRF-CHEM

Standard deviation of Modis AOD from model at 00UTC, 21 August 2012.

average observaton error~ 0.2

12UTC, 24,August, 2010

Dust size bin 1

with modis - without modis

Dust size bin 1 (assimilate modis, level 11)

Relative humidity (assimilate modis , level 11)

12UTC, 24,August, 2010

Relative humidity difference

with modis - without modis

12UTC, 24,August, 2010

Temperature difference

with modis - without modis

Temperature (assimilate modis , level 11)

Compare hurricane evolution in different experiments

MU

Assimilate MODIS, with chemistry

Assimilate MODIS, no chemistry

No MODIS assimilation , chemistry

00UTC 27, August ,2010 Surface dry pressure perturbation

00UTC 28 Surface wind speed

Assimilate MODIS, with chemistry

Assimilate MODIS, no chemistry

No MODIS assimilation , chemistry

Summary 1)Simple GOCART scheme in WRF/CHEM can represent the SAL to some extend.2)MODIS AOD product can be assimilated into the model. It can change the chemistry field and impact on the meteorological field through the chemistry interaction with meteorological field 3)Dust can influence the hurricane intensity significantly in this case

Future work1)Use different chemistry schemes such as MOSAIC , which includes interaction between the aerosols and the microphysics processes2)Conduct more case study and understand the physical mechanism of dust impact on the tropical cyclone formation and evolution .