Analyses & Numerical Prediction

Analysis and Prediction Operations Bulletin No. 46
The Operational Implementation of LAPS_PT375 in NMOC
23 July 1999

Introduction

A new upgraded Australian region Limited Area Prediction System, called LAPS_PT375, and developed by the Regional Meteorology Group in BMRC, led by Dr. Kamal Puri and (during 1999) Dr. John McBride, is to be introduced into operations at the NMOC Melbourne. This new system will replace the current LAPS, which has been running in operations since July 1996.

LAPS_PT375 is basically a 4-dimensional intermittent data assimilation scheme consisting of an analysis-initialisation-forecast cycle formulated using the finite difference approach on a latitude/longitude grid and on sigma levels for both the analysis and forecast components. Features of LAPS_PT375 include an increase in resolution (to 0.375° in the horizontal and 29 levels in the vertical) and improvements to the analysis using archived data from LAPS. A new land surface scheme has also been introduced into the forecast component of the system.

Both synoptic and objective assessments have been carried out. The synoptic assessment has highlighted both positive and negative features of the new system, from a meteorological perspective. The S₁ skill scores indicate significant improvements in the upper levels and a slight improvement for mean sea level pressure.

An additional feature of LAPS_PT375 is its Year 2000 compliance.

OVERALL SCHEME

Figure 1 shows a simplified schematic representation of LAPS_PT375. Basically, the system can be considered as 3 cycles with an analysis and forecast component comprising the core. In the current configuration, observational data are inserted every 6 hours (LAPS_PT375 time). After each data insertion, an analysis is performed followed by a mass flux adjustment, a soil-moisture adjustment and then an initialisation and prediction. The prediction model is integrated forward to either the next analysis time or the end of a longer forecast. In LAPS_PT375, a cold start is performed at the beginning of cycle 1. In effect, this means that the GASP analysis becomes the first guess for the LAPS_PT375 analysis in the first cycle. After this, the first guess fields for the LAPS_PT375 analysis in the second and third cycle are provided by 6 hour forecasts from the prediction model in LAPS_PT375 itself. GASP also provides the boundary conditions for LAPS_PT375. The Pre-processor collates and converts GASP data on pressure levels to sigma levels for input into the analysis and prediction parts of the system.

Manual bogus observations, for mean sea level pressure and tropical cyclones, are incorporated in the new system.

DATA ANALYSIS

The analysis is performed on sigma levels and operates on a latitude/longitude grid (the same as that used by the forecast component of the system). The analysis method used is MVSI, which has the feature of being able to make simultaneous use of geopotential and wind observations in 3 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. (Surface pressure data are transformed to increments of geopotential before use.) Mass and wind increments are adjusted for geostrophic consistency. 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 "superobbing" - the combinations of closely spaced observations. Improvements to the analysis component of LAPS_PT375, based on archives from LAPS, include improved definition of structure functions and error characteristics.

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 (see Figure 2). 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. In addition, as mentioned above, the bogus mean sea level pressure data generated from the manual analyses (prepared in the NMOC) and tropical cyclone bogus data (prepared by Darwin RFC, when appropriate) are used by the analysis (see Figure 3). A new attractive feature of LAPS_PT375, from a NMOC system perspective, is the streamlining of the observational data extraction procedure from the real time data base.

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.

SOIL-MOISTURE ADJUSTMENT

A soil moisture and temperature adjustment is necessary for the use of the ECMWF land surface scheme in the following 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 LAPS_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.

BOUNDARY CONDITIONS

The GASP forecast, out to 60 hours, are used to define the necessary lateral boundary conditions for LAPS_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 1.5° latitude/longitude post processed files from GASP (at present the T239/29L version).

FIGURE 2. Typical spatial distribution of different types of observational data used in LAPS_PT375.

FIGURE 2 (continued).

FIGURE 3. An example showing Australian region PAOBS, entered by NMOC, and a NH cyclone, entered by Darwin RFC, for use in LAPS_PT375.

OPERATIONAL CONFIGURATION

• Domain: A modified Australian region: 17.125°N-65.0°S, 65.0°E-184.625°E (see Figure 4)
• Horizontal resolution (analysis and prognosis): 0.375° (220x320 latitude/longitude grid)

Figure 5 shows the grid points over Australia.

• Vertical resolution (analysis and prognosis): No. of sigma levels=29

Levels=0.9988, 0.9974, 0.9943, 0.9875, 0.9750, 0.9625, 0.9500, 0.9250, 0.9000, 0.8750, 0.8500, 0.8000, 0.7500, 0.7000, 0.6000, 0.5000, 0.4500, 0.4000, 0.3500, 0.3000, 0.2750, 0.2500, 0.2250, 0.2000, 0.1750, 0.1500, 0.1000, 0.0700, 0.0500

Figure 6 shows the vertical level distribution of levels and Table 1 gives the approximate heights of each of the levels.

• Topography: as shown in Figure 4 also (derived from a 0.1° resolution data set)
• Data insertion frequency: every 6 hours
• Data cut off: approximately 2 hours at 00 and 12UTC, 5.5 hours at 06 and 18UTC
• Manual intervention: MSLP PAOBS, TC bogus
• Initialisation: digital filtering technique
• Timestep: 40 seconds

• Nesting: lateral boundary conditions derived form +6 to +60 hour GASP forecasts
• Output: 6 hourly analyses and forecasts out to 48 hours from 00 UTC and 12 UTC daily
• Climatology: albedo
• Soil Moisture Analysis: Daily 0.25°x 0.25°
• Sea Surface Temperature Analysis: Weekly 1° x 1° O.I (generated in NMOC Melbourne)



• NEC SX-4 supercomputer resources:

analysis (including mass-flux and soil-moisture adjustments):- elapsed time: 9 minutes (approx.), number of processors: 4, memory: 2.2GB

48 hour prognosis:- elapsed time: 18 minutes (approx.), number of processors: 16, memory: 1.9GB

• Cold start: off GASP
• NMOC products driven by LAPS_PT375:

Seastate model (Australian region WAM): driven by 10 m boundary layer winds derived from LAPS_PT375.

MOF

EER

• Backup: LAPS (0.75°/19L) will remain as a backup system initially.

If the expected GASP output is not available, LAPS_PT375 will use the previous GASP output - otherwise it will be warm run in a fixed boundary mode.

(xix) Feed-back: Comments on LAPS_PT375 can be emailed to: laps_feedback@bom.gov.au

TABLE 1. Approximate heights corresponding to sigma () levels in LAPS_PT375.

PERFORMANCE

Synoptic

Terry Skinner (MSDSS, NMOC) undertook a synoptic assessment of LAPS_PT375 for the period: May 1999 - July 1999, comparing mainly the mean sea level pressure analysis and prognosis products with the corresponding NMOC manual operational analysis charts. In addition, he compared the rainfall forecasts from LAPS_PT375 and LAPS, making use of the objective rainfall verification system developed (and currently run) by Beth Ebert of BMRC, whose assistance is very gratefully acknowledged. (The period considered for the rainfall verification was February 1999 - July 1999.) The following features emerged from the assessment:

Mean Sea Level Pressure Analyses:

• Unsmoothed full resolution displays of analyses occasionally noisy.

• Observations over Central Australia, Pilbara and Kimberley regions of WA on occasion are not paid, especially at 00 UTC, resulting sometimes in a positive pressure bias over elevated regions.

• Cut off low pressure systems generally not deep enough; MSLP pseudo observations have little effect.

Mean Sea Level Pressure Prognoses:

• Good phase predictions of troughs and ridges in westerlies.

• Cut off lows in the Bight and the Tasman Sea are well predicted but depth of cyclogenesis continues to be underestimated; pressure errors of such centres are generally between 5 and 10 hPa.

• Unsmoothed full resolution prognoses from +12 to +48 hours occasionally noisy.

• Positive pressure bias is again noticed over continental interior (Central and WA) which tends to increase with the forecast period.

Surface Temperature Forecasts:

Unsmoothed full resolution displays of 24-hour forecasts of screen temperature very noisy.

Rainfall Predictions:

• Good definition of east coast rainfall with onshore streams, particularly when associated with Tasman Sea cut off lows.

• Frontal rainfall over South West WA well defined.

• Better definition of heavier rainfall rates over Tasmanian west coast; however, LAPS_PT375 overall generally underestimates the rainfall area.

• In the last 24 hours of the 48-hour forecast, the rainfall area is under-predicted; there is a general negative bias in the first 24 hours which becomes more accentuated in the last 24 hours of the forecast.

• Occasional overprediction of coastal rainfall in Queensland tropics.

• A variety of objective skill scores (eg Critical Success Index or Threat Score, Hansen-Kuipers Score - see Ebert and McBride (1997)) were considered for the period and showed that LAPS_PT375 performed better with respect to maximum rainfall rates, although some objective tests showed that LAPS scored better than LAPS_PT375, as already indicated above. However, a feature of LAPS_PT375 is that it shows more detail in the rainfall.

TABLE 2. S₁ skill scores comparing LAPS_PT375 with LAPS.

(B) 24 hour prognoses:

(C) 36 hour prognoses:

(D) 48 hour prognoses:

Objective

S₁ skill scores were calculated for mean sea level pressure and geopotential height (at 850, 500 and 300 hpa) for LAPS_PT375 and LAPS, for the period 1 April to 18 July 1999. Each system was verified against its own analysis. The results, in Table 2, show that, for the whole period, there were good improvements at upper levels and only a slight improvement for mean sea level pressure. The improvements in the uppers were most marked for the first 24 hours.

PRODUCT AVAILABILITY

DIFACS

When LAPS_PT375 becomes operational, all Difacs slots currently being filled by LAPS (ie with headings "LAPS") will be filled by LAPS_PT375 (with headings "LAPS_PT375"). Only charts from the latest run of LAPS_PT375 will be displayed on Difacs. The relevant Difacs slots are as follows:

Analysis charts: 78, 79, 82, 83, 99, 100, 101, 605 and 610

12 hour Prognosis slots: 131, 132, 133, 134, 135, 136, 137, 138, 171 and 611

24 hour Prognosis slots: 86, 88, 89, 90, 92, 93, 94, 95, 143, 144, 145, 172, 606 and 612.

36 hour Prognosis slots: 87, 151, 152, 153, 154, 155, 156, 157 and 173.

48 hour Prognosis slots: 607, 608 and 609.

Unit Conventions on Difacs:

Geopotential heights and thicknesses will be displayed in geopotential metres.

Isotachs will be displayed in knots.

Positive vertical motions will refer to downward motion.

Positive relative vorticity refers to cyclonic motions.

Figures 7 and 8 show typical examples of basic charts from LAPS_PT375, as they appear on Difacs.

MCIDAS:

For the present time, LAPS_PT375 fields will continue to be made available internally through MCIDAS at GRID1300-1310, for pressure level data, and at GRID1320-1330, for sigma level data. (Some single level data is found in each group of GRIDs). GRID1300 and GRID1320 contain either the 05UTC or 17UTC analyses. GRID1302 and GRID1322 contain the 11UTC or 23UTC analysis data. GRID1303-1310 and GRID1323-1330 contain the forecast fields out to 48 hours, in 6 hourly increments. These GRIDs, derived from LAPS_PT375 output, will continue to have the same resolution as for LAPS.

However, it is envisaged that users will move to the MCIDAS ADDE method of accessing LAPS_PT375 fields using the real time data base (rtdb) directly. Some additional single level fields, at full resolution, will be available through this method. Figure 9 shows an example of surface winds (coming from the boundary layer formulation) which are available through ADDE accessing the data base.

TABLE 3A. LAPS_PT375 fields in NMOC's real-time data base (rtdb) - coarse resolution representation.

Horizontal grid: 160x110

29 Vertical sigma levels (ie sgma_lvl): 0.9988, 0.9974, 0.9943, 0.9875, 0.9750, 0.9625, 0.9500, 0.9250, 0.9000, 0.8750, 0.8500, 0.8000, 0.7500, 0.7000, 0.6000, 0.5000, 0.4500, 0.4000, 0.3500, 0.3000, 0.2750, 0.2500, 0.2250, 0.2000, 0.1750, 0.1500, 0.1000, 0.0700, 0.0500

13 Vertical pressure levels (ie isbr_lvl): 1000, 950, 900, 850, 700, 500, 400, 300, 250, 200, 150, 100, 50 hpa (Note: Dew point temperatures are only ingested to 300hpa.)

Time levels: 3-hourly from 00 to +48 (at 11 and 23UTC)

TABLE 3B. LAPS_PT375 fields in rtdb - high (ie full) resolution representation.

Horizontal grid: 320x220

At present, only single level fields are ingested at high resolution.

Time levels: 1-hourly from 00 to +48 (at 11 and 23UTC)

REAL TIME DATA BASE

Sigma level output from LAPS_PT375 is written to the real time data base (rtdb). Analysis and forecast fields (out to +48 hours, in 3 hourly intervals) are available for the full domain. Pressure level data is also available. The data base currently holds LAPS_PT375 fields for the last 10 days.

At the present time, LAPS_PT375 runs on a 320x220 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 (and some single-level) fields are put into NMOC's real-time data base (ie rtdb) at a coarse horizontal resolution of 160x110 and only a small number of other single-level fields are put into rtdb at the full horizontal resolution of 320x220 (see Figure 10). Table 3 shows the various LAPS_PT375 fields available through rtdb.

FUTURE DEVELOPMENTS

Immediate plans include the operational implementation of MESO_LAPS_PT125 and MESO_LAPS_PT050, which are based on LAPS_PT375. 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.

REFERENCES

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.