Analyses & Numerical Prediction

Operations Bulletin No. 48
Operational Implementation of TLAPS_PT375
22 September 1999

Introduction
Overall Scheme
Prediction Model
Operational Configuration
Model Performance
Product Availability

• Web Pages
• Difacs
• McIDAS
• Real-Time Database
• Archive

Future Developments
References

INTRODUCTION

An upgrade to the tropical region Limited Area Prediction System,  TLAPS, developed by the Regional Meteorology Group in BMRC, led by Dr. Kamal Puri and (during 1999) Dr. John McBride, has been introduced into operations at the NMOC Melbourne. TLAPS_PT375, an extended tropical version of LAPS_PT375 with tropical specific components, replaced the current version of TLAPS, which has been running operationally since June 1997.

Operational features of TLAPS_PT375 include an increase in resolution to 0.3750 in the horizontal and 29 levels in the vertical, a new  land surface scheme, improved resolution of the GMS cloud top temperature and bogus moisture profiling, and incorporates daily soil moisture analysis. The TLAPS_PT375 domain extends over the full Darwin RSMC Region with forecasts out to 48 hours being produced.

As with all new versions of the LAPS model, TLAPS_PT375 is Year 2000 compliant..
 

OVERALL SCHEME

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 in the GASP forecasts being removed and replaced with an idealised 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 to input into the analysis and prediction parts of the system.

Manual input is included by the insertion of tropical cyclone bogus observations supplied by the Darwin Regional Specialised Meteorological Centre (RSMC) when appropriate.
 

DATA ANALYSIS

The analysis is performed on sigma levels and operates on a latitude-longitude grid. The analysis method used is Multi-Variant Statistical Interpolation (MVSI), which has the feature of being able to make simultaneous use of geopotential and wind observations in three dimensions. This is the same analysis scheme as used in the global system, GASP. 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 geostrophic consistency over latitudes beyond 15^o from the equator. The moisture analysis is 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, remotely sensed GTS SATEM and GMS 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, and (c) synthetic GMS moisture data are used in the analysis. A new attractive feature of TLAPS_PT375, from a NMOC system perspective, is the streamlining of the observational data extraction procedure from the real time data base.
 

TROPICAL CYCLONE BOGUS

Tropical cyclone bogus data, prepared by Darwin RSMC, when tropical cyclones are in the analysis area, are used by the analysis. This data is used in the bogus program which (i) locates any circulations in the initialising GASP forecasts near the observed TC's, (ii) locally removes these by careful filtering, (iii) inserts an axisymmetric vortex at the observed locations, and (iv) builds large scale wind field asymmetries ("beta"-gyres) consistent with the past 12 hour motion of the storm. The modified 6 and 12 hour GASP forecasts, with the implanted TC vortex, is then used as the first guess for the objective analysis.
 

CLOUD TOP TEMPERATURE

Cloud top temperatures are extracted from hourly GMS satellite imagery averaged over 0.5 degree boxes over the region 80^oE to 180^o, 50^oS to 48^oN. This data is used in the initialization of synthetic moisture profiles in the dynamic nudging period of the model constraining the model integration close to the observed cloud and rainfall.
 

SOIL-MOISTURE ADJUSTMENT

A soil moisture and temperature adjustment is necessary for the use of the ECMWF land surface scheme in the model prediction component.
 

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 model prediction component.
 

INITIALISATION AND PREDICTION MODEL

The initialisation (controlling the generation of spurious gravity waves), 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 are a feature of the system. Detailed physical parameterisations, basically in line with those in GASP, 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. A new ECMWF land surface scheme has been introduced into TLAPS_PT375 which provides a detailed vegetation and soil-type specification and improved soil moisture initialisation. As mentioned above, the horizontal grid and vertical level structure of the forecast component is identical with that of the analysis component.

The actual prediction model commences 12 hours prior to the model run time from GASP analysis supplemented with bogus moisture data from GMS imagery. The model is then uses dynamic nudging as it steps towards target analyses at -6 hours and 0 hours. While moving towards the target analyses the rotational wind components are 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

The GASP forecast, from T-12 hours out to T+48 hours, are used to define the necessary lateral boundary conditions for TLAPS_PT375. 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 1.5⁰ latitude-longitude post-processed files from GASP (at present the T239/29L version).
 

OPERATIONAL CONFIGURATION

PERFORMANCE

Subjective

Random selection of TLAPS_PT375 charts through August and early September 1999, indicted an improved prediction ability of the new model over the current TLAPS model. This was seen in (i) better forecasting of wind strength at 10m, instead of the current over-forecasting by TLAPS, and (ii) better retention of upper troughs in the forecast period, where the current model tends to weaken then too quickly.

Objective
S1 skill scores, RMS errors, biases and anomaly correlations (AC) were calculated for mean sea level pressure (MSLP) and geopotential height (700 and 200 hPa), with bias and RMS calculated for upper level winds (700 and 200 hPa) for TLAPS_PT375 and TLAPS, over the period 20 August to 12 September 1999. Each system was verified against its own analysis. Results in table 2a, show that, for the whole period, there were good improvements in TLAPS_PT375 at MSLP. The upper levels (tables 2b-e)  show small improvement, at 200hPa level, with values at 700 hPa being marginally worse than the current system. This slight decline is partly due to the model resolution having decreased below 50km, where it is now producing more meso-scale structure and intensity in the low-level analyses.

Australian Region verification statistics (Operations Bulletin No. 48 supplement Operational Implementation of TLAPS_PT375) indicate improvements for MSLP and all levels of geopotential heights (700 and 200 hPa). Wind verification statistics remained mostly the same over the verification period, varying in the range -0.1 to +0.2 m/s for the bias, and -0.2 to +0.3 m/s for the RMS error.

When verification against observations is performed over the tropics, TLAPS_PT375 shows significant improvements in the RMS vector errors over TLAPS at low and high levels (tables 3a,b,d, fig. 3a,b,d), with values in the middle levels (500hPa, table/fig. 3c) remaining similar.

Note TLAPS375 values in green indicate a value that is greater or equal to TLAPS, while red indicte a lesser value
 

PRODUCT AVAILABILITY

Web Page

The main TLAPS web page on the internal web have been updated.

Usually only the latest run products are available, however a selection of full domain and Australian Region charts will be displayed for the past week.

Table 3a: Asian Tropics: 40N-40S, 70E-180

(i) RSMC Analyses

(ii) RSMC Forecast Output

(iii) BMRC Forecast Charts (Full Domain)

Australian Tropics
The standard display for the Australian Region is 6 hourly gif images, an animated gif and a Java loop. These are available for the fields shown in table 3b, below.
 

Other fields including MSLP/Thickness, 850 hPa wind surge, 24h precipitation and vortex tracking are also available for the Australian Region.

Regional Charts
The charts listed in table 4c are available for the following regions: Northern Territory, Queensland, Western Australia, South East Asia, Philippines, Western Indonesia, Eastern Indonesia, Papua New Guinea & Solomon Islands, and South West Pacific. These domains with topography are shown in figures below. Each region covers an area 35 degrees west-east and 17 degrees north-south.
 

External Web Page

A large selection of the images from the TLAPS Web Page is also available externally to the Bureau to registered users through the Darwin RSMC web page at:http://www.bom.gov.au/weather/nt/rsmc/

Difacs

When TLAPS_PT375 becomes operational, all DIFACS slots currently being filled by TLAPS will be replaced by TLAPS_PT375 charts, these will continue to have the heading "TLAPS". Only charts from the latest run of TLAPS_PT375 will be displayed on DIFACS.

The relevant DIFACS slots are as follows:
Full Domain

Australian Region:

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 in the Southern Hemisphere and anti-cyclonic motion in the Northern Hemisphere.

Tropical DIFACS charts are also available through the web. Any requests concerning DIFACS should be sent, by email to: Difacs@bom.gov.au

McIDAS:

For the present time, TLAPS_PT375 fields will continue to be made available internally through MCIDAS at GRID1060-1069, for pressure level data, and at GRID1050-1059, for sigma level data. These grids contain forecast fields out to 48 hours, in 6 hourly increments. Although derived from TLAPS_PT375 output, the grids will continue to have the same resolution as for the current TLAPS, ie 0.75 degrees.

The following TLAPS_PT375 fields will be available through these MCIDAS GRIDs:

 Other fields are derivable through use of MCIDAS macros.

It is envisaged that in the near future, users will move to the MCIDAS ADDE method of accessing TLAPS_PT375 fields using the real time data base (rtdb) directly. Additional single level fields (as shown in table 3b) , will be available through this method at full resolution.

Real Time Data Base (rtdb):
Sigma level output from TLAPS_PT375 is written to the real time data base (rtdb). Analysis and forecast fields (out to +48 hours, in 6 hourly intervals) are available for the full domain. Pressure level data is also available. The data base currently holds TLAPS_PT375 fields for the last 10 days.

At the present time, TLAPS_PT375 runs on a 320x240 horizontal grid and on 29 sigma levels in the vertical. However, in view of resource limitations (both in terms of CPU power and disc storage), the multi-level fields are put into NMOC's real-time data base (rtdb) at a coarse horizontal resolution of 160x120 and only a small number of single-level fields are put into rtdb at the full horizontal resolution of 320x240. Tables 4a and 4b show the various TLAPS_PT375 fields available through rtdb.

(i) Details of TLAPS_PT375 NMOC's real-time data base (rtdb) - coarse resolution representation.

(ii) Details of TLAPS_PT375 NMOC's real-time data base (rtdb) - high (full) resolution representation.

FUTURE DEVELOPMENTS

Immediate plans include the operational implementation of the tropical cyclone centric, TC_LAPS_PT125, which is based on LAPS_PT375 with dynamical nudging as in TLAPS_PT375 and horizontal resolution of 0.125o. After this, there are plans to incorporate new types of observational data including METARs and locally derived hourly water vapour and high resolution visible winds. Upcoming changes to the model will include the Semi-Lagrangian Semi-Implicit (SLSI) and non-hydrostatic formulations and improvements in the parameterisation of moist processes. There will also be some minor changes to the model data storage and manipulation with the upgrade of the NEC SX-4 to the SX-5.
 

REFERENCES

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

Ebert, E.E. and McBride, J.L.: "Methods for Verifying Quantitative Precipitation Forecasts: Application to the BMRC LAPS Model 24-hour Precipitation Forecasts." BMRC Techniques Development Report, No 2, May 1997.

Puri, K., Dietachmayer, G., Mills, G.A., Davidson, N.E., Bowen, R.A., and Logan, L.W.: "The new BMRC Limited Area Prediction System, LAPS." Australian Meteorological Magazine Vol 47, No 3, 203-223, 1998.