Longwave Cloud Forcing Due Longwave Cloud Forcing Due to Tropical Cyclone Presence: to Tropical Cyclone Presence:

August 1995August 1995

Cristy KingCristy King

What I want to knowWhat I want to know

If tropical cyclone (tropical If tropical cyclone (tropical depression, tropical storm, and depression, tropical storm, and hurricane) cloud cover affects the hurricane) cloud cover affects the outgoing longwave radiationoutgoing longwave radiation

DataData

NOAA Interpolated Outgoing NOAA Interpolated Outgoing Longwave Radiation at TOA from the Longwave Radiation at TOA from the NOAA14 satelliteNOAA14 satellite

Spatial Coverage: 2.5 X 2.5 degree Spatial Coverage: 2.5 X 2.5 degree gridgrid

Data ContinuedData Continued

Spatial:Spatial: Tropical Atlantic Tropical Atlantic

75W to 20W75W to 20W 10N to 30N10N to 30N

Temporal:Temporal: Daily OLRDaily OLR August 22, 1995 to August 31,1995August 22, 1995 to August 31,1995

4 cyclones present 4 cyclones present

Storm TracksStorm Tracks

8 Humberto

9 Iris

11 Karen

12 Luis

August 27,1995August 27,1995

Anomalous OLRAnomalous OLR

ThresholdThreshold

200 210 220 230 240 250 260 270 280 2900

10

20

30

40

50

60

70

80

90

100

Mean OLR Frequency

OLR W/m^2

Fre

qu

ency

(#)

Hartmann et al. Hartmann et al. 2001 used a 2001 used a threshold of 30 threshold of 30 W/m^2W/m^2

Test statistic Test statistic revealed at 95% revealed at 95% confidence level confidence level a range of a range of -28 < mean < -28 < mean < +28+28

Threshold=30 Threshold=30 W/m^2W/m^2 96.4% of longterm mean OLR lie between 220 – 280

with a mean value of 260 W/M^2

In any direction about the

core of the cyclone, TOA

OLR increases

By offsetting the above

OLR values by 30 (the

threshold) we obtain the

LWCRF due to the

presence of the cyclone

TOA OLR with radius about Humberto

-160

-140

-120

-100

-80

-60

-40

-20

0

-4 -2 0 2 4

Radius 1 rad=242km

OL

R W

/M^2

) N-S

E-W

NW-SE

SW-NE

LWCRF due to Humberto

-140

-120

-100

-80

-60

-40

-20

0

-4 -2 0 2 4

Radius 1 rad=242km

LW

CR

F (

W/m

^2)

N-S

E-W

NW-SE

SW-NE

Distribution of OLR about the Distribution of OLR about the corecore

-59.73-59.73 -49.27-49.27

-47.58-47.58 -61.915-61.915 -55.0933-55.0933 -57.46-57.46 -50.72-50.72

-61.65-61.65 -61.16-61.16 -70.955-70.955 -60.41-60.41 -61.37-61.37 -54.86-54.86

-47.42-47.42 -66.87-66.87 -120.78-120.78 -102.25-102.25 -58.0625-58.0625 -42.67-42.67

-47.295-47.295 -76.3683-76.3683 -149.39-149.39 -104.748-104.748 -39.355-39.355

-45.88-45.88 --74.0274.02 -114.14-114.14 -98.1-98.1 -34.195-34.195

-52.01-52.01 -70.61-70.61 -85.51-85.51 -60.024-60.024 -30.88-30.88

-38.26-38.26 -66.43-66.43 -42.415-42.415

-38.83-38.83

Of total LWCRF:Of total LWCRF: 37% comes from within a radius of 1 37% comes from within a radius of 1

grid box (242km)grid box (242km) 65% comes from within a radius of 2 65% comes from within a radius of 2

grid boxes(484km)grid boxes(484km) 82% comes from within a radius of 3 82% comes from within a radius of 3

grid boxes(726km)grid boxes(726km)

Correlation Correlation

TOA OLR w/ Storm Category = -.15TOA OLR w/ Storm Category = -.15 TD=1 TD/TS= 1.5 TS=2 TS/H=2.5 TD=1 TD/TS= 1.5 TS=2 TS/H=2.5

H=3 H=3 TOA OLR w/ SST = .23TOA OLR w/ SST = .23

This is expected because if This is expected because if correlation w/ SST was strong, we correlation w/ SST was strong, we would conclude that there is no net would conclude that there is no net LWCRF induced by the cycloneLWCRF induced by the cyclone

Conclusions/CommentsConclusions/Comments

Tropical cyclone cloud cover does significantly Tropical cyclone cloud cover does significantly affect the outgoing longwave radiationaffect the outgoing longwave radiation Similar results were found for all 4 stormsSimilar results were found for all 4 storms

SWCRF may balance the LWCRF leading to a SWCRF may balance the LWCRF leading to a NET CRF close to 0 but SW values are needed NET CRF close to 0 but SW values are needed on a daily timescale as opposed to monthlyon a daily timescale as opposed to monthly Contours of monthly OLR washed out much of the Contours of monthly OLR washed out much of the

signalsignal