Darwin Regional Specialised Meteorological Centre
(Darwin RSMC)

Analysis and Prediction

Operations Bulletin No. 41

The Tropical Limited Area Prediction System (TLAPS)

INTRODUCTION

A new upgraded tropical analysis and prognosis system, developed by the Regional Meteorology Group, led by Dr. Kamel Puri, in BMRC, has been introduced into operations at the NMC Melbourne. The new Tropical Limited Area Prediction System (TLAPS) has replaced the TAPS system, which has been operational since November 1992. TLAPS is an extended tropical version of LAPS with the tropical specific components, such as diabatic nudging, GMS moisture data and tropical cyclone bogus data, all of which were developed under TAPS. Major improvements over TAPS are: higher horizontal resolution, improved numerics and the use of mass flux convection scheme. Additionally it incorporates daily soil moisture analysis, developed by Neil Prescod.

Operational features include a horizontal resolution of 0.75o and a vertical resolution of 19 levels for the analysis and the forecast. The TLAPS domain extends over the full Darwin RSMC Region with forecasts out to 48 hours being produced.

OVERALL SCHEME

Figure 1 shows a simplified schematic representation of TLAPS.

Analyses are performed at time T-6 and T0 hours. First guess for these analyses are GASP +6 and +12 hour forecasts, with any pre-existing tropical cyclone circulation removed and replaced with am odealised vortex at the observed location. GASP also provides the boundary conditions for the model forecasts. The pre-processor collates and converts GASP data from pressure levels onto sigma levels prior tp input into the analysis and prediction parts of the system.

Manual input is included by inserting tropical cyclone 'PAOBS' from Darwin RSMC when appropriate.

DATA ANALYSIS

The analysis is performed on sigma levels and operates on a latitude/longitude grid. The analysis method used is MVSI, which has the feature of being able to make simultaneous use of geopotential and wind observations in three dimensions. The MVSI scheme interpolates the observed increments (ie the deviations from the first guess field) of geopotential heights, thicknesses and winds to produce increments of geopotentials and winds at the grid points. Mass and wind increments are adjusted for geopotrophic consistency over latitudes beyond 15o from the

equator. The moisture analysis is now carried out using univariate statistical interpolation. Gross error checking and a comprehensive "cross-validation" is carried out in the analysis. Use is also made of "super-observations" - the combination of closely spaced observations.

OBSERVATIONAL DATA USED

The analysis uses a variety of observational data which includes: surface SYNOPs, ship and drifting buoy reports, radiosonde and rawinsonde observations, TOVS soundings, GMS cloud drift winds, and winds from aircraft. It is noted that (a) significant and mandatory level wind and moisture data, (b) locally processed satellite sounding data and locally derived cloud drift winds, (c) synthetic GMS moisture, and (d) real-time soil moisture data are used in the analysis. In addition, as mentioned above, tropical cyclone bogus data (prepared by Darwin RSMC) are used by the analyses.

MASS FLUX ADJUSTMENT

An adjustment to winds, at the boundaries, is made to balance the mass flows after completion of the pre-processing and analysis stages, prior to input into the prediction model component.

INITIALISATION AND PREDICTION MODEL

The initialisation based on a digital filtering technique, is incorporated in the prediction model component. The forecast component is basically a hydrostatic primitive equation model formulated on sigma levels for a non-staggered ("Arakawa A") latitude/longitude grid. Higher order numerics (than those used in TAPS) are a feature of the system. Detailed physical parameterisations are now more in line with those in GASP and include: a mass-flux convective scheme (for deep, mid-level and shallow convection), large-scale rain, radiative transfer with a diurnal cycle, diagnostic clouds, stability dependent surface fluxes, and interactive soil moisture.

The actual prediction model commences 12 hours prior to the model run time from GASP analysi supplemented with moisture data from GMS imagery. The model is then nudged towards target analyses at -6 hours and 0 hours with the rotational wind component preserved. During 'nudging' the convective heating is defined by the cloud top temperatures from GMS imagery. The resulting 'nudged' analysis is not identical to the target analyses but will have generated vertical motion fields that are consistent with the GMS observed tropical convection. The prediction model then integrates out to 48 hours.

BOUNDARY CONDITIONS

GASP forecasts (from the T-12 hours base time) are used to define the necessary lateral boundary conditions for TLAPS. Absolute values of the mean sea level pressure, wind components, temperatures and mixing ratios are used at 6-hourly intervals throughout the nesting procedure. The nesting files are currently derived from the 2.5o latitude/.longitude post-processed files from

GASP (at present the T79/19L version).

OPERATIONAL CONFIGURATION

1. Domain: Australian Tropical Region: 44.25oN-45.0oS,70.0oE-189.25oE
2. Horizontal resolution (analysis and prognosis): 0.75o (120x160 latitude/longitude grid)
3. Vertical resolution, number of sigma levels: 19

Levels= 0.991, 0.975, 0.950, 0.900, 0.850, 0.800, 0.750, 0.700, 0.600, 0.500, 0.400, 0.350, 0.300, 0.250, 0.200, 0.150, 0.100, 0.070, 0.050

1. Topography: as shown in Figure 2 (derived from a 0.1o resolution data set)
2. Data insertion: at T-6 and T0
3. Manual intervention: TC bogus
4. Initialisation: digital-filtering technique
5. Diabatic Nudging: 12 hours
6. Timestep: 90 seconds
7. Nesting: lateral boundary conditions derived from +0 to +60 hour GASP forecast
8. Output: 6 hourly analyses and forecasts out to 48 hours from 00 UTC and 12 UTC daily
9. Climatologies: albedo and ground wetness
10. Sea Surface Temperatures: weekly 1ox1o O.I. (NMC Melbourne)
11. Cray YMP resources:

Analysis: cpu time (1 processor): 350 seconds (approx.) per analysis

memory: 26 Mw

60^th Prognosis: cpu time (max. 4 processors): 4500 seconds (approx.)

memory: 26 Mw

1. Backup: Due to time constraints on the cray YMP, no backup is available.

OBJECTIVE PERFORMANCE

Objective verification was carried out between TAPS and TLAPS from late May until June 1997 for their respective 24 and 48 hour forecasts against their own analyses. S1 skill scores, RMS and bias were calculated for mean sea level pressure (MSLP) (table 1) and the 500 hPa geopotential (table2). With the RMS and bias calculated for the 850 hPa (table 3) and 200 hPa (table 4) winds.

The tables show that there is a marked improvement of TLAPS over TAPS at MSLP, with improvement at 48 hours for the 500 hPa geopotential. RMS errors for the wind components at 850 hPa are reduced in TLAPS. However, for the 200 hPa level the results were much the same, although slightly in favour of TAPS at 24 hours.

Table 1 - MSLP (hPa)

	S1 +24h	S1+48h	RMS +24H	RMS +48H	Bias+24H	Bias+48H
TAPS	46.8	53.6	1.8	2.2	-1.5	-1.7
TLAPS	41.4	49.7	1.5	1.6	-1.3	-1.0

	S1 +24h	S1+48h	RMS +24H	RMS +48H	Bias+24H	Bias+48H
TAPS	36.9	45.1	17.6	20.0	-15.6	-15.8
TLAPS	37.2	42.2	16.3	17.7	-14.9	-14.4

Table 3 - 850 hPa Wind (ms-1)

	S1 +24h	S1+48h	Bias +24H	Bias +48H
TAPS U	2.4	3.3	1.2	0.5
TAPS V	2.5	3.2	0.1	-0.3
TLAPS U	2.2	2.8	-0.3	-0.3
TLAPS V	2.1	2.7	-0.1	-0.4

Table 4 - 200 hPa Wind (ms-1)

	S1 +24h	S1+48h	Bias +24H	Bias +48H
TAPS U	4.2	5.8	2.6	0.0
TAPS V	4.6	6.3	-1.7	-0.9
TLAPS U	4.6	5.7	-0.1	-0.6
TLAPS V	4.9	6.3	-0.2	0.8

PRODUCT AVAILABILITY

Difacs:

TLPS charts have replaced all TAPS charts on the Difacs system.

Australian Region:

MSLP/Thickness 275-277 Wind/Height 324-318

850 hPa Vorticity 250-251 Upmotion 257-259

Relative Humidity 260-262, 326 Total-Totals Index 263

Cyclone Tracks 264 Isallobars 265

Precipitation 256 850 hPa Change 323

850-200 hPa Shear 324,325

Full Domain

MSLP 240, 241, 330 Deep Layer Means 267-270

Upper Winds 242-253, 331-335 Precipitation 254

Only the latest TLAPS charts will be displayed on Difacs.

Unit Conventions on Difacs:

Geopotential Heights and thicknesses will be displayed in geopotential metres.

Isotachs and wind barbs are displayed in knots.

Positive relative vorticity refers to cyclonic motions.

McIDAS:

TLAPS fields are available through MCIDAS at GRID 1061-1069, for pressure level data. GRID1061 contains the 11UTC or 23UTC analysis data, with GRID1062-1069 containing the forecast fields out to +48 hours, in 6 hourly increments. The addition of sigma level GRIDs is expected in the near future.

Currently, the following TLAPS fields are available on MCIDAS (out to 179oE):

Pressure Level data: Z, U, V, T, MIX, OMGP TD, RH;

Single level data; MSLP, PSUR, TSUR, TOTG,THIK, PPTN.

Other fields are derivable through use of MCIDAS macros.

Real Time Database (rtdb):

Complete sigma level output from TLAPS is written to the real time database (rtdb). Analysis and forecast fields (out to +48 hours, in 6 hourly intervals) are available for the full domain (ie 120x160 grid). Pressure level data is also available. The data is also available. The data base currently holds TLAPS fields for the last 5 days.

ARCHIVES:

Data from TLAPS is archived on epoch in the directory:

/epmodel/Yyyyy/Mmm/tlaps

and into the new SAM archive system in the directory:

/samnmc/trop

In this archive, fields are stored in NetCDF form and observational data in "box form".

FUTURE DEVELOPMENTS

Like most NMC systems, TLAPS will continually evolve and be upgraded. Increased horizontal and vertical resolution, and additional use of satellite data (IR imagery and cloud drift winds) may be expected in the near future. Longer term developments are likely to be improved representations of surface processes and large scale rain.

REFERENCES

Diabatic Nudging:

Davidson, N.E. and Puri, K. 1992: "Tropical prediction using dynamical nudging, satellite-defined convective heat sources and cyclone bogus.", Mon.Wea.Rev,. 120.

Analysis:

Seaman, R., Bourke, W., Stenile P., Hart T., Embery G., Naughton M. and Rikus L. 1995: "Evolution of the Bureau of Meteorology's Global Assimilation and Prediction system. Part 1: analysis and initialisation." Aust.Met.Mag. 44, 1-18.

Forecast model:

Puri K., Dietachmayer G., Mills G., Davidson N., Bowen R., Logan L., and Leslie L., 1996: "The new BMRC Limited Area Prediction System: A discussion of the numerics." BMRC Research Report No. 54, 41.