Exploring the Possibility of Using Tropical Cyclone Numbers to Project Taiwan Summer Precipitation Patterns Mong-Ming Lu and Ru-Jun May Research and Development Center Central Weather Bureau
Jan 09, 2016
Exploring the Possibility of Using Tropical Cyclone Numbers
to Project Taiwan Summer Precipitation Patterns
Mong-Ming Lu and Ru-Jun May
Research and Development Center
Central Weather Bureau
Motivation
Taiwan summer rainfall prediction beyond one month has no skill.
Tropical cyclone numbers show promising predictability.
Can we use tropical cyclone numbers to project Taiwan rainfall patterns?
Available Information
NW Pacific tropical cyclone activity FORCASTS
• The IRI Typhoon Activity Forecast
(http://iri.columbia.edu/forecast/typhoon/)
• Korean Meteorological Agency’s Forecast
• Johnny Chan’s Forecast
(http://aposf02.cityu.edu.hk)
• Prof. Joe Kwon’s Forecast
NW Pacific tropical cyclone activity MONITORING
* Total TC number in January-May
Analysis Procedure Predictors
TC numbers in January - May
January - December
July - October
June – August
Regional Difference
Group the stations into 5 groups –
North (7 stations), Central (4), South (3), East (6) and All (20)
Contingency Table Method• Categorize Data
TC numbers (More, Normal, Less)
as close to tercile as possible,
the members in “Normal” must be more
than in “More” and “Less”
Precipitation (More, Normal, Less) - Tercile
• Significance Test - Mason and Goddard (2001)
Identify which months of precipitation with statistically significant signals at 85%, 90% levels of confidence
Limitations:
* Significance criterion is sensitive to the category member
numbers.
* Satisfactory test is difficult to do when stations are too few.
Identification Procedure and Criteria:
• Data length: 1959-2001
• The contingency tables are made for each station.
• Signal Identification :
The 20 stations are grouped into 5 groups. The signal magnitude is represented by the percentage number of stations showing the signal of satisfying the significance criterion of any category. The uncertainty is represented by the percentage number of stations showing no signal. If the signal magnitude is less than the uncertainty, we decide that this group has no signal. The relative signal magnitude is defined as the difference between the signal magnitude and uncertainty. Significant signal is identified only when the relative signal magnitude is larger than 33%.
**The signal in south Taiwan should be treated with extreme caution because there are only 3 stations in that group. The signal can be identified if one statio
n satisfies the signal magnitude and two stations are uncertain.
http://e-service.cwb.gov.tw/docs/V3.0/knowledge/typhone/ch2/005.htm
Typhoon Invading Paths (1960-1999)
DATA
• Tropical Cyclone -
JTWC best track data final v., 1959-2002
• Precipitation -
CWB 20 stations
RESULTSPredictor: Annual Total Number of TCs
More Normal LessAnnual TC Number Above Normal Annual TC Number Normal Annual TC Number Below NormalW: Wet, D: Dry, N: Normal, W: Wet, D: Dry, N: Normal, W: Wet, D: Dry, N: Normal,no *: 85% , with *: 90% significance level no *: 85% , with *: 90% significance level no *: 85% , with *: 90% significance level
N. TWNC. TWNS. TWNE. TWNAll TWN N. TWNC. TWNS. TWNE. TWNAll TWN N. TWNC. TWNS. TWNE. TWNAll TWNJun D D D D Jun N JunJul N* N* Jul W* JulAug W Aug AugSep Sep SepOct Oct D* Oct W W W
JJ JJ W* JJ D*
JA N* JA JA D
AS AS AS D
SO SO SO W W W W
JJA JJA W* JJA D
JAS JAS N JAS D* D
ASO ASO ASO
JFMAM TC Number Above Normal JFMAM TC Number Normal JFMAM TC Number Below NormalW: Wet, D: Dry, N: Normal, W: Wet, D: Dry, N: Normal, W: Wet, D: Dry, N: Normal, no *: 85% , with *: 90% significance level no *: 85% , with *: 90% significance level no *: 85% , with *: 90% significance level
N. TWNC. TWNS. TWNE. TWNAll TWN N. TWNC. TWNS. TWNE. TWNAll TWN N. TWNC. TWNS. TWNE. TWNAll TWNJun D* D* D* Jun W* Jun W N*
Jul N* Jul W* Jul W
Aug Aug W* W* Aug N*
Sep Sep W SepOct N* N* Oct Oct W
JJ D* JJ JJJA D* N* JA W* JA W*
AS D* N AS N* ASSO D* SO N* SOJJA D* D* JJA W* JJA W
JAS D* JAS W* JASASO D* D* D ASO N* ASO
Predictor: January-May Total Number of TCs
More Normal Less
Predictor: July-October Total Number of TCs
JASO TC Number Above Normal JASO TC Number Normal JASO TC Number Below NormalW: Wet, D: Dry, N: Normal, W: Wet, D: Dry, N: Normal, W: Wet, D: Dry, N: Normal,no *: 85% , with *: 90% significance level no *: 85% , with *: 90% significance level no *: 85% , with *: 90% significance level
N. TWNC. TWNS. TWNE. TWNAll TWN N. TWNC. TWNS. TWNE. TWNAll TWN N. TWNC. TWNS. TWNE. TWNAll TWNJun D* D* D* Jun W Jun W N*
Jul Jul W W W W Jul D* D* N D*
Aug Aug W Aug N* N*
Sep Sep Sep N
Oct N Oct Oct N
JJ JJ W* W W* W JJ N D*
JA N* JA W* JA D*
AS AS W AS N D*
SO N N SO SOJJA N D* D* D* JJA W W* W JJAJAS JAS W W JAS D*
ASO N ASO W W ASO
More Normal Less
RAINFALL PROJECTIONS
What did we know in June 2003 ? 1. Jan-May total number (6) - above normal. What can we project ? High possibility for North Taiwan to have a DRY summer (June-October).
2. Dr. Johnny Chan’s Forecast: Annual total TC number will be normal / slightly below normal (263). What can we project ? None.
3. IRI’s Forecast: July-October total (named) TC number will be normal / slightly above normal. (reference Normal: 17-20)
Dr. Joe-H. Kwon’s Forecast: JJA total number will be below normal. (6.3±1) KMA: JJA total number below normal (?) What can we project ? normal / slightly above normal ? (Normal Wet; Above Normal Dry)
below normal High possibility for East Taiwan to be DRY.
June 2003 IRI Typhoon Activity Forecast
There is a slightly enhanced probability (approximately 40%) that the number of named tropical cyclones in the western North Pacific during the 2003 peak season (July to October) will be in the normal category, and also a 40% that the number of cyclones will be in the above normal category. The normal category is defined as between 17 and 20 named tropical cyclones. These probabilities are slightly greater than the long-term average probability of 33%. The accumulated cyclone energy (ACE*) index during these months also has an enhanced (approximately 45%) probability of being in the above normal range. Furthermore, a slight shift in the average longitude (westward) and latitude (nortward) of tropical cyclone tracks is predicted. This forecast is consistent with the possible onset of La Niña sea surface temperature anomalies that could develop, with a region of above normal SST in the central and western North Pacific. Normally, La Niña conditions would result in below normal SSTs in both the eastern and central tropical Pacific. Our SST forecast, however limits the below normal SST to east of the date line.
TC NUMBER VERIFICATION
Jan-May total number (6) of the Western North Pacific TCs this year is above normal.
Annual total TC number will be normal / slightly below normal (263).
up-to-now 19 named
x IRI’s Forecast: July-October total TC number will be normal / slightly above normal.
actually 13 named Dr. Joe-H. Kwon’s Forecast: JJA total number will be below normal
(6.3±1).
actually 8 named
Taiwan Precipitation Percentile Rank
Jan-May TC
MORE
SLP Ano. Composite - June
2003 June
SLP Anomalies
JASO TC
LESS
SLP Ano. Composite –
July
2003 July
SLP Anomalies
SUMMARY• TC numbers may be useful for projecting Taiwan summer pr
ecipitation patterns.• January-May total number of TCs shows most consistent sig
nals in the precipitation of northern Taiwan. More Dry Normal Wet Less signals are weak• Peak season (July-October) total number of TCs also show c
onsistent signals in the precipitation of eastern Taiwan. More, Less Dry Normal Wet• No Taiwan precipitation signals can be extracted based on
the annual total number of TCs.
Questions
• Where does the predictability of TC numbers come from?
• How have the TC number predictions been used? (Who are your customers?)