# Presentation: 781165

## THE NESDIS TROPICAL CYCLONE FORMATION PROBABILITY PRODUCT:PAST PERFORMANCE AND FUTURE PLANSAndrea B. Schumacher, CIRAMark DeMaria, NESDIS/StARJohn A. Knaff, NESDIS/StARDaniel Brown, NOAA/NWS TPC

- 61st Interdepartmental Hurricane Conference, 3.6.08

## Motivation

- Need for objective, real-time TC formation guidance
- NHC needs objective guidance to help make Tropical Cyclone formation forecasts
- Numerical models provide objective guidance
- As resolution improves, forecast models gets better at handling TC formation
- However, still bias towards over-prediction (Beven 1999)

## Approach

- Compute 24-hour probability of TC formation over all 5o x 5o lat/lon grid boxes in domain by analyzing environmental & convective conditions

## Approach

- Compute 24-hour probability of TC formation over all 5o x 5o lat/lon grid boxes in domain by analyzing environmental & convective conditions

*“**A Needle in a Haystack”*
- Ratio of TC formation to non-formation points ~ 1:2000
- Maximum climatological formation probability ~1.8% (E. Pacific)

## Datasets

- NHC/DOD Best Tracks 1949-2005
- Atlantic, E. Pacific, Central Pacific & W. Pacific
- Subtropical/Extratropical cases excluded
- Unnamed depressions included since 1989
- NCEP Global Model Analyses 1995-2005
- Reanalysis 1995-1999 (2.5o grid)
- Operational Analyses 2000-2005 (1.0o grid)
- Geostationary Satellite Water Vapor Imagery (16 km Re-mapped Mercator projection)
- GOES-E    1995-2005
- GOES-W   1998-2005
- GMS-5 / GOES-9 / MTSAT-1R   2000-2005

**Notes:**
NCEP interp to 2.0 deg grid (compromise)

## Parameters

| ABBREV | Parameter Description | Source |
| --- | --- | --- |
| LAT | Latitude (°) | ATCF |
| PLAND | % land coverage | Land covrge |
| DSTRM | Distance to nearest TC (km) | ATCF |
| CSST | Climatological sea surface temp (°C) | Levitus |
| VSHEAR | 850-200 hPa vertical shear (kt) | NCEP GFS |
| CIRC | 850 hPa circulation (kt) | NCEP GFS |
| THDEV | Vertical instability (°C) | NCEP GFS |
| HDIV | 850 hPa horizontal divergence (m/s) | NCEP GFS |
| PCCOLD | % coverage by pixels colder than 40 C | Sat WV |
| BTWARM | Avg. cloud-cleared brightness temp (°C) | Sat WV |
| CPROB | Climatological formation probability | Best Track |

## Product Algorithm

- _Screening Step_:
- Remove all points where TC formation is highly unlikely
- < 5% TCG points removed
- 75-80% non-TCG points removed

**Notes:**
Purpose:  Remove all points from dataset for which TC formation is highly unlikely
Approximately 5% of TC formation cases removed, while 75-80% of non-formation cases removed
Ratio of TCG to NTCG cases ~ 1 to 300-600

## Product Algorithm

- _Linear Discriminant Analysis Step_:
- Uses developmental dataset parameter values
- Solves for coefficients a0,..,an so that the distance between parameter means for each group (TCG and non-TCG) are maximized in standard deviation units
- For each independent data point with parameter values x1,...,xn, the LDF determines group membership, where
- LDF = a0 + a1x1 + ... + anxn
- Deterministic (yes/no)
- *Perrone & Lowe (1986), Hennon & Hobgood (2003), Knaff et al. (2008)

**Notes:**
Purpose:  Remove all points from dataset for which TC formation is highly unlikely
Approximately 5% of TC formation cases removed, while 75-80% of non-formation cases removed
Ratio of TCG to NTCG cases ~ 1 to 300-600

## Product Algorithm

- _Probability Determination Step_:
- Use dependent dataset to develop a relationship between LDF and TC formation probability
- Coefficients and LDF/probability relationship defines the TCFP product

**Notes:**
Purpose:  Remove all points from dataset for which TC formation is highly unlikely
Approximately 5% of TC formation cases removed, while 75-80% of non-formation cases removed
Ratio of TCG to NTCG cases ~ 1 to 300-600

## Recent Product Updates

- Added 2004 & 2005 to dependent dataset
- Expanded to cover Central and Western N. Pacific basins
- Uses GOES-E, GOES-W and MTSAT-1R water vapor imagery, divides domain into 3 basins (below)
- Added HDIV (850 hPa horiz. divergence) parameter
- Scheduled to replace operational product by Summer 2008

## Parameter Contributions By Basin

| ABBREV | ATLC | EPAC | WPAC |
| --- | --- | --- | --- |
| LAT |  |  |  |
| PLAND | -0.21 | -0.27 | -0.20 |
| DSTRM | 0.80 | 0.68 | 1.15 |
| CSST |  |  |  |
| VSHEAR | -0.37 | -0.22 | -0.17 |
| CIRC | 1.33 | 1.41 | 1.59 |
| THDEV |  |  |  |
| HDIV | -0.22 | -0.39 | -0.33 |
| PCCOLD | 0.63 | 0.29 | 0.56 |
| BTWARM |  |  |  |
| CPROB | 1.47 | 1.67 | 0.88 |

**Notes:**
ATLC & EPAC:  cpob....circ...dstrm (climatology...LL disturbance...proximity to locally inhibitive conditions
WPAC:  circ...dstrm...cprob, different bc dependence of timing/location of monsoon trough environment and hence disturbances ??

## Product Example:  WP11 2006

**Notes:**
WP 11 (TS Wukong)

## Reliability Diagrams

- Both dependent years (orange) and independent years (green) show good agreement between predicted and observed TC formation probabilities
- E. Pacific basin diagram suggests the actual TC formation probability may be greater than what the TCFP product predicts at higher probabilities – may need correction

**Notes:**
Plotted using bins of equal population

## Verification – Relative Operating Characteristic and Brier Scores

- Typical metrics are not valid for such rare events.  However, we can use metrics that compare TCFP product to climatology
- Brier Skill Score:  BSS = 1 - BS/BSref
- ROC Skill Score:  RSS = ROC/ROCref - 1

- Brier Score, BS = 1/N Σ* i=1,n (p**i**-o**i**)**2*, where *p**i* and *o**i* are the predicted and observed probability for case *i*, respectively
- ROC is the area under the plot of hit rate vs. false alarm rate for various probability thesholds.

## Skill Scores & Interpretation

- BSS positive, but small
- RSS for E. Pacific much smaller than others  hard to beat climatology in that basin

|  | Dependent Years  |  |  | Independent Years |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Basin: | ATLC | EPAC | WPAC | ATLC | EPAC | WPAC |  |
| BSS | 0.012 | 0.024 | 0.025 | 0.010 | 0.032 | 0.021 | All BSS > 0  skill over climatology |
| RSS | 0.21 | 0.01 | 0.22 | .11 | .01 | .17 | All RSS > 0  skill over climatology |

**Notes:**
Climatology hard to beat, reinforces cprob as significant predictor

## Summary

- The current TCFP has run operationally since 2004 and has shown skill with respect to climatology (not shown)
- The TCFP has been successfully updated and extended to cover the Central and Western N. Pacific
- 2007 verification of the updated TCFP demonstrates it has skill above climatology alone in all 3 basins
- Extended TCFP product to be implemented operationally by summer 2008

## Future Plans & Improvements

- Expand domain to include S. Hemisphere & Indian Ocean
- Extend forecast period from 24 h to 48 h +
- Use GFS forecast fields
- Analyze global water vapor strip to identify upstream predictors, particularly convective signatures associated with tropical waves (Frank and Roundy 2006)
- Identify new parameters to add to algorithm
- TAFB invest locations and intensities (D. Brown)