Assessment and inter-comparison of five GHRSST products in the coastal and

shelf sea around China

Jiping Xie1,3

, Jiang Zhu1, and Yan Li

2,3

1. Institute of Atmospheric Physics, CAS2. National Marine Environmental Forecasting Center, Beijing

3. Graduate School of the Chinese Academy of Sciences

Melbourne, Australia, May 2007

1. Background

2. Data Processing

3. Inter-comparison GHRSST products

4. Assessment by in situ observations

5. Conclusions

Outline

GRHSST L4 Products : MWOISST (RSS ), NGSST (UT, Japan), MGDSST (JMA), OSTIA (NCOF), FSTIA (FNMOC)

Annual SST:

(2005.10~2006.9)

OSTIAMGDSST

However, we notice there are some … …

Questions

I. How about the differences and errors of the five GHRSST products in the coastal and shelf sea around China?

II. How to improve the GHRSST products as a major input for data assimilation in this region?

III. Can we use the ensemble mean method to improve the presently GHRSST products?

116-135ºE, 20-42ºN

Global

Global

Global

116-166ºE, 13-63ºN

Global

Coverage

[synthesis]10 km, dailyEMSST

NOAA/AVHRR10 km, 6 hourlyFSTIA

ENVISAT/AATSR, NOAA/AVHRR, MSG/SEVIRI, AQUA/AMSR-E,

TRMM/TMI

5 km, dailyOSTIA

NOAA/AVHRR, AQUA/AMSR-E

NOAA/AVHRR, AQUA/MODIS, AQUA/AMSR-E

TRMM/TMI, AQUA/AMSR-E

Satellite/sensor

25 km, daily

2 km, daily

25 km, daily

Resolution

NGSST

MGDSST

MWOISST

116-135ºE, 20-42ºN

Global

Global

Global

116-166ºE, 13-63ºN

Global

Coverage

[synthesis]10 km, dailyEMSST

NOAA/AVHRR10 km, 6 hourlyFSTIA

ENVISAT/AATSR, NOAA/AVHRR, MSG/SEVIRI, AQUA/AMSR-E,

TRMM/TMI

5 km, dailyOSTIA

NOAA/AVHRR, AQUA/AMSR-E

NOAA/AVHRR, AQUA/MODIS, AQUA/AMSR-E

TRMM/TMI, AQUA/AMSR-E

Satellite/sensor

25 km, daily

2 km, daily

25 km, daily

Resolution

NGSST

MGDSST

MWOISST

Table 1. Specification of different GHRSST L4 products

Cover: 116-135ºE, 20~42ºN; period: 2005.10~2006.9

2. Data Processing GHRSST L4 Products

In Situ Observations

Drifting buoy and Ship reports are collected by Near-Goos (http://near-goos1.jodc.go.jp) from GTS.

DDGR: Jun 2006

I. Pre-process

1) Excluding the reports with invalid ship signal.

2) Excluding the redundant reports.

3) Position check by tracking consistency method as Kent and Challenor(2006).

Night SST : SHIP ~ 3167; BUOY ~ 7284

II. Quality Control1) Comparing the monthly SST for a ship or buoy with

climatology, if outside of 3 standard deviation, the observations in this month will be rejected.

2) Excluding any record outside of 4.5 standard deviation referring to monthly climatology from GDEM-V3.0.

3) Consistency checking by the EMSST linear regression with 99% confidence level.

SHIP : 2715 BUOY : 6835

6.2%14.3%

Depth (m) : • <40 • 40~80 • 80~200 • >200

Distributions of in situ daily SST observations through data quality control

Num:120198573

5944

Num:115426379

1795

3. Inter-comparison of GRHSST

Difference of annual SST referring to EMSST during the period of 2005.10~2006.9a.) MWOISST (b.) NGSST (c.) MGDSST (d.) OSTIA (e.) FSTIA

Spatial distribution of the annual difference spread relative to EMSST (left) and the averaged difference spread as a function of depths (right).

Note: if the sample is over 40 in a depth interval, a red circle can be shown.

5

1

25

1

)(1

i

M

jj

ij

ii

spread

i

EMSSTSSTM

DEvaluate the difference about the five products:

MWOISST NGSST MGDSST OSTIA FSTIA EMSST

Bias 0.23 0.35 0.12 -0.04 0.06 0.15

RMSE 0.80 0.94 0.88 0.31 0.26 0.52

Table 2. Comparison of Bias and RMSE of the different SST products referring to buoy observations (ºC)

4. Assessment by in situ observations

Comparison of bias and RMSE of SST products relative to buoy and ship observations in different depth interval. The histogram denotes the bias, and the error bar denotes the corresponding RMSE.

)exp( 000)( dCBAT d Correct Increment:

Depth < 40 m

Comparison of Bias and RMSE through the Correction

Comparison of bias and RMSE of SST products through correction according to the depth scale. The histogram denotes the bias, and the error bar denotes the corresponding RMSE.

Bias (ºC)OSTIA/New

0.2/-0.01

-0.03/-0.04-0.02/0.02-0.05/0.0

Bias (ºC)FSTIA/New

0.4/0.070.07/-0.040.08/0.020.06/0.0

Bias and RMSE

OSTIA FSTIA New_OSTIA New_FSTIA

Te

mp

era

ture

(o

C)

-.2

-.1

0.0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1.0

1.1

1.2

1.3

1.4

Buoy Ship Bias RMSE

OSTIA -.039 0.311

FSTIA .064 0.256

New_OSTIA -.005 0.308

New_FSTIA -.003 0.247

Table 3. Changes of Bias and RMSE through correction to Buoy SST (ºC)

Bias RMSE

OSTIA -.040 1.212

FSTIA -.014 1.289

New_OSTIA -.019 1.212

New_FSTIA -.093 1.294

Table 4. Changes of Bias and RMSE through correction to Ship SST (ºC)

Referring to buoy SST, the bias can be decreased an order, but the RMSE can be decreased by 1% (OSTIA) and 4% (FSTIA) of RMSE.

Comparison of Bias and RMSE through the Correction

Coatal Station ID

11201146

21234130

41344144

41526452

74157416

74249710

RM

SE

(o

C)

1.01.52.02.53.03.54.04.55.05.56.0

OSTIANew_OSTIAFSTIANew_FSTIA

BIA

S (

oC

)

-1.8

-1.2

-.6

0.0

.6

1.2

1.8

2.4

3.0

OSTIANew_OSTIAFSTIANew_FSTIA

Validation of the corrected SSTs by night SST Observations of 12 Coastal Stations

Comparison of bias and RMSE of OSTIA and FSTIA by exponent correction relative to costal station observations. On average, the RMSE can be decreased about 5% and 3%, respectively to OSTIA and FSTIA.

Mean: (ºC)

1.61.00.8-0.2

Mean: (ºC)

3.53.32.32.2

74157416

7424

5. Conclusions

Looking for a major data source to feed our data assimilation system for preoperational ocean forecast, we investigate the five GHRSST Level 4 products in the coastal and shelf sea around China. The five GHRSST products are compared to each other and to the in situ SST observations in the period of 2005.10~2006.9. Some efforts to correct the bias near coastal regions are made. The main results are:

1) These products are agreed with very well when water depth is larger than 80m. However, near the coast around China, large differences exist. For example, as the depth is less than 40m, the annual mean spread of them is nearly to 0.9ºC.

2) Comparing to in situ observations, most of them are excellent for reasonable bias and RMSE. Especially the RMSE of OSTIA and FSTIA (reference to buoy SST observations) are about 0.3ºC. However near the coast, i.e, the depth is less than 40m, the warm bias (>0.2ºC) and the considerable RMSE (>0.6ºC) are prevalent in the five products.

3) A bias correction scheme that uses an exponent function of depth is applied to two products (OSTIA and FSTIA). The correction can reduce bias effectively, however, the RMSE doesn’t reduced so effectively.

4) The current GHRSST L4 products, especially some of them, can be safely used as a major input of data assimilation system in the shelf sea around China. However, near the coastline where depth is less than 40m, they are not ready to be assimilated. It is necessary to make further correction or improvement.

Thank you !