Weather and Climate (1995) 15: 11-20

Campbell Island Upper Air Data Impact Study

James TraversMeteorological Service of New Zealand Limited.

Julian HemingU.K. Meteorological Office, Bracknell.

A study was performed to determine i f the removal of Campbell Island upper airdata would significantly affect the Bracknell Global Numerical Weather Prediction(NWP) model output.

Eight weather situations between September 1993 and July 1994 were re-run onthe Bracknell model wi th and without upper air data from Campbell Island. Thedifferences between the two model runs in the New Zealand area were assessed ineach case to determine the impact of the data on the model output. In the situationsstudied, the results indicated that the effect was minimal.

INTRODUCTION

New Zealand has maintained an upper airobservatory at Campbell Island since January1949. Due to the remoteness of the station(Fig. 1), main ta in ing these observationsplaced a heavy f inancial demand on theMeteorological Service, and since July 1992,the Meteorological Service of New ZealandLimited (MetService). Ear ly in 1994 i t wasdecided to determine i f MetService wasgetting "value for money" from the CampbellIsland upper air observations. In May 1994,a study was initiated, in co-operation with theU.K. Meteorological Office (U.K.M.0.), todetermine the impact of the upper air dataon the Bracknell Global NWP model. Thismodel operates on a global grid of 1.25 degreesby 0.833 degrees. F o r a more deta i leddescription see Cullen (1993).

I t was decided to use a case study approachand t o r u n t h e B r a c k n e l l m o d e lretrospectively for each situation selected.Campbell Island upper air observations wouldbe included in a control run of the model andexcluded in an experiment run of the model.I t was intended to ascertain if the removal of

ABSTRACT

SELECTION OF CASES

11

the Campbell Island data would significantlyaffect the output o f the Bracknel l Globalmodel in the New Zealand area, and thereforeimpact on the forecasts over the country. Inthe experiment, upper air data only would beremoved. Surface observations would remain.

The following criteria were used as a basisfor the selection of suitable cases for the study:

1. Significant changes should be apparentin Campbell Island upper wind observations(500 hPa or 250 hPa wind speed changes of30 knots or more or wind direction changesof 30 degrees or more between soundings).Also s ign i f i can t d i f fe rences shou ld beapparent between the Campbell Island andMacquarie Island upper wind observations(differences in wind speed and direction of atleast 30 knots and 30 degrees respectively at500 hPa and/or 250 hPa).

2. There should be a complete set o fobservations throughout the assimilationphase of the model runs.

12

30'

40"

150'

<06' 140' 150"

160'

160"

Fig. 1 Campbell Island Location Map

3. A significant or severe weather eventshould have occurred over New Zealand i.e.heavy rain, severe gales or signif icant o rheavy snow falls to low levels.

4. Preference should be given to situationswhere New Zealand is downstream fromCampbell Is land ( low index or southerlysituations).

These criteria were met in almost all cases.In Case 2 , one s e t o f upper da ta wasunavailable and therefore only two sets wereincluded in the control assimilation. In Case3, New Zealand was not downstream fromCampbell Island. The main interest in thiscase was the effect of the upper level trough,as part ly described by upper air data fromCampbell Is land, on the movement anddevelopment of the rainband over the SouthIsland.

The fol lowing weather situations wereselected, with corresponding MSLP (mean sealevel pressure) analyses (Fig. 2).

170'

170'

Campbell Island Upper Air Impact Study

E 180 W

E 180' W

170'

of New ZealandCase 7. 7-8 July 1994

160* , 56 .

c o l d southwestoutbreak

Case 8. 21-22 July 1994s l o w movinglowwest of South Island

Case 1. 4-5 September 1993h e a v y r a i n a n dsnow in eastern districts

Case 2. 21-23 December 1993K a i k o u r a floods

Case 3. 19-22 January 1994r e c o r d rainfal la tMilford Sound

Case 4. 19-21 February 1994p e r s i s t e n t south-east rain in southern New Zealand.

Case 5. 18-19 March 1994F a i r liefloods

Case 6. 25-26 June 1994trough stalled west

Campbell Island Upper Air Impact Study 1 3

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14 C a m p b e l l Island Upper Air Impact Study

METHOD

For each situation, a data time (11-1-0) wasselected. Usually, this was the point in timewhen t h e uppe r a i r observat ions f r o mCampbell Island were in a state of change orcould possibly have a pronounced effect on thesubsequent forecasts over New Zealand. Thistime was often 24-48 hours before the onsetof significant or severe weather over NewZealand. Hence, forecasts issued at this time(11-1-0) would be of vital importance.

In each of the cases, the Bracknell globalmodel w a s r u n w i t h 3 0 h o u r s d a t aassimilation, followed by a 60 hour forecaststart ing a t H+0. Since Campbel l Is landcarried out combined radar/radiosonde flightsat 00Z and 12Z daily, this would result in theomission of three consecutive ascents from theexperiment run of the model.

A total o f 16 model runs were thereforeperformed in this study (8 cases with controland experiment for each case). Th is wascarried out over a period of about one month,as c o m p u t i n g t i m e o n t h e C r a y C 9 0supercomputer at Bracknell allowed.

Model diagnostics were available on aworkstation at the UK MetOffice in Bracknell.Control, experiment and difference charts at12 hourly intervals in the forecast period from11-1-0 to 11-1-60 were analysed on a case by casebasis. A l l difference charts produced werecontrol values minus experiment.

The following model fields and differencecharts were selected for analysis:

1. M e a n sea level pressure2. 1 0 metre and 850 hPa winds.3. 8 5 0 hPa temperatures4. 8 5 0 hPa wet bulb potential temperature5. 8 5 0 hPa and 700 hPa relative humidity6. 5 0 0 hPa geopotential height and 500

hPa wind speed7. 2 5 0 hPa geopotential height and wind

speed8. 1000-500 hPa thickness9. 7 0 0 hPa vertical motion.The study was not intended to determine

the accuracy or otherwise of the control orexperiment forecasts, but to highl ight thedifferences between the two.

RESULTS

The results are presented in no particularorder. The examples shown are considered to

Fig. 3 850 hPa temperature differences (control minusexperiment) at 1-11-12 for Case 7. Contours in degreescelsius. Shading indicates positive values, ie controlpredicted 850 hPa temperature higher than experimentprediction.

be a representative selection from the study.By fa r the ma jo r i t y o f charts examinedindicated minimal or negligible differencebetween control and experiment fields overNew Zealand.

A typical example is shown at Fig. 3. Here,the 850 hPa temperature differences for Case7 at 11-1-12 are one degree or less over NewZealand. The control was between 0 and 1°Cwarmer than the experiment over the SouthIsland and parts of the North Island. Positivedifferences (control minus experiment) areshaded. I n this case, low level temperaturewas one o f the most important elements inthe forecast over the country.

Negligible differences were produced inMSLP fields in all cases as far ahead as 11+60.The largest difference observed is shown atFig. 4. In this example from Case 1, the meansea level pressure west of the South Islandwas o f the order o f 2 hPa lower i n theexperiment than in the control. Given thatmost of the situations involved large surfacepressure gradients over the country, thecontrol and experiment runs of the modelproduced remarkably consistent results i nMSLP and low level wind fields.

Similarly in the 500 hPa height field, wherelarge amplitude troughs or even cut-off lows

Campbell Island Upper Air Impact Study

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were present, there were on ly smal l o rnegligible differences observed. A n examplefrom Case 5 is shown at Fig. 5(a). This wastypical and shows less than ldam differencein the 500 hPa geopotential height fields overNew Zealand at 11+24. In this situation, a cut-off low at 500 hPa was present over centralNew Zealand, shown at Fig. 5(b).

The 850 hPa and 700 hPa relative humiditycontrol and experiment fields were different,to varying degrees, in almost all cases. Mostlythe differences were sufficiently far from NewZealand or sufficiently small that the impacton the forecasts over New Zealand would havebeen small. However in Case 1, Case 5 andCase 8, there may have been a change inforecast emphasis as a r esu l t o f thesevariations. F o r example, i n Case 5, theexperiment run produced 850 hPa relativehumidity differences 12% to 18% higher thanthe control run in the vicinity, and to thesoutheast of Stewart Island at 11+12. Theeffect diminished rapidly after 11+24. Thiscould have lead to more emphasis being placedon precipitation in that area. However thiswas infact well to the south of the belt of heavyrain.

Often, relative humidity differences weredue to small movements i n areas of large

15

Fig. 5(a) 500 hPa geopotential height differences (dam) atH+24 for Case 5. Shading indicates positive values, iecontrol heights greater than experiment. Contour interval1 dam.

5(b) H+24 500 hPa geopotential height (dam) forCase 5 (control).

16

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Campbell Island Upper Air Impact Study

Fig. 6(a) 700 hPa relative humidity differences (percent) at 11+0 for Case 6. Shading indicates positive differences, lecontrol 700 hPa RH greater than experiment. Contour interval is 6 percent.

Campbell Island Upper Air Impact Study

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17

18

relative humidity gradient. Fig.6(a) illustratesthe 700 hPa relative humidi ty differencesfrom case 6 at H+0 of up to 18% in the areanear and to the east of Campbell Island andto t h e n o r t h o f N e w Zea land. T h e s edifferences occurred in an area of strong 700hPa humidity gradient, shown at Fig. 6(b),and represent only a small displacement ofthis gradient.

In severa l cases s tud ied the re werevariations in the 500 hPa wind speed field.These w e r e p r i m a r i l y d u e t o s m a l ldisplacements of areas of large wind speedgradient, typically brought about by smallchanges in the position of upper trough axeswhere wind speed gradients were large.

Differences in 10 metre and 850 hPa windspeeds, 250 hPa geopotential heights andwind speeds, 1000-500 hPa thickness and 700hPa vertical motion, were all minimal in theNew Zealand area.

Finally, output was analysed on a globalscale to determine if significant impacts wereproduced elsewhere in either hemisphere, andagain the effects were minimal or negligible.

OBJECTIVE VERIFICATION

A root mean square (RMS) forecast erroranalysis was carried out over the area 20S to70S, 150E to 160W. Forecasts at 11+24 and11+48 of mean sea level pressure and 250 hPageopotential height were verified against anoperational analysis or "best" analysis. Whileverifying against an operational analysis isnot ideal, it was not possible to verify againstactual observations. The Case 3 veri fyinganalysis was unobtainable.

Table 1 contains the actual RMS forecasterror values obtained between the operationalanalysis, and the control and experiment runsat 11+24 and 11+48. From the table it is clearthe differences between the two runs arenegligible.

A t- test was applied to the differencesbetween the RMS forecast errors of the controland experiment. Using the hypothesis thatthe mean difference was zero, wi th 6 (or 7)degrees of freedom, the t values obtained allfell well within the acceptance region of thishypothesis.

Campbell Island Upper Air Impact Study

CONCLUSIONS AND DISCUSSION

The impact of the Campbell Island upperair data on the Bracknell global NWP modelin the cases examined in th is study, wasminimal, o r even negl igible i n the NewZealand area.

In most cases, the differences produced bythe removal of Campbell Island upper air datawould h a v e m e a n t n o change t o t h esubsequent forecasts over New Zealand i fthe B rackne l l mode l o u t p u t was usedexclusively to produce forecasts.

The most pronounced impact was in the 850hPa and 700 hPa relative humidity fields, andthis effect was most often greatest near, andto the east of Campbell Island.

I t is possible that the effects of previousascents, beyond the 30 hours assimilationperiod chosen here, may have been presentin some of the cases. Further assessment ofthe appropriate assimilation period for a studyof this kind could be carried out, using forexample, parallel model runs over a two orthree week period. This was not attempted.

Other data impact studies have shownsignificant impact on NWP model forecastswhen observations were removed from theassimilation process. Graham (1990) removedNorth Atlantic tempship observations over abroad spatial domain and Barwell and Lorene(1985) removed aircraft winds, also over abroad spatial domain, and covering one 72hour period. Heming (1990) found that twoNorth Atlantic tempships made a significantdifference to the forecast of a severe storm tocross the U K in January 1990. The shipshappened to be positioned in a critical positionnear the depression during the early stagesof its development.

However in this data impact study, a singlerawinsonde was removed in eight individualcase studies, and the result was only a smallimpact.

The set of cases presented here is by nomeans complete or exhaustive.

While the cases involved were selectedusing the criteria set out above, i t is quitepossible there is a situation either in the pastor in the future in which Campbell Islandupper air data would have been or would bevital to the Bracknell Model analysis in theNew Zealand area. A situation involving asoutherly jet over Campbell Island with littleor no evidence o f such a fea ture ove r

Campbell Island Upper Air Impact Study 1 9

CNTL H+24 EXPT H+24 Duff

case 1 3.52 3.76 0.24case 2 1.73 1.68 -0.05case 4 2.12 2.14 0.02case 5 1.82 1.85 0.03case 6 2.94 2.92 -0.02case 7 2.30 2.25 -0.05case 8 3.40 3.27 -0.13

mean 2.5471 2.5529 0.0057stdev 0.7380 0.7772 0.1162

0.1205tiny (0.05) 2.4469

4.61 4.50 -0.113.72 3.66 -0.063.96 3.95 -0.014.19 4.15 -0.045.41 5.48 0.079.29 9.30 0.015.19 5.24 0.055.94 5.41 -0.53

5.2888 5.2113 -0.07751.7870 1.7912 0.1919

-1.06862.3646

3.42 3.76 0.343.83 3.84 0.014.98 4.89 -0.092.64 2.65 0.013.90 3.99 0.096.80 6.66 -0.144.68 4.62 -0.065.47 5.27 -0.20

4.4650 4.4600 -0.00501.3049 1.1999 0.1669

-0.07932.3646

3.10 3.21 0.112.22 2.14 -0.082.65 2.60 -0.053.14 3.13 -0.013.27 3.32 0.053.18 3.11 -0.073.07 2.95 -0.12

2.9471 2.9229 -0.02430.3766 0.4155 0.0804

-0.74002.4469

MSLP (hPa) 2 5 0 hPa Geopotential height

CNTL H+24 E X P T 11+24 D u f f

case 1case 2case 3case 4case 5case 6case 7case 8

case 1case 2case 4case 5case 6case 7case 8

MSLP (hPa) 2 5 0 hPa Geopotential height

CNTL H+48 E X P T 11+48 D i f f C N T L H+48 E X P T 11+48 D i f f

case 1case 2case 3case 4case 5case 6case 7case 8

meanstdev

tiny (0.05)

Table 1 Area mean RMS forecast errors against analysis for mean sea level pressure and 250 hPa geopotential height overarea 20s to 70s from 150e to 160w. Figures in hectopascals (MSLP) and dam (250 hPa height). Values oft and tinv(0.05)obtained using Student's t distribution with 6 (or 7) degrees of freedom.

20 C a m p b e l l Island Upper Air Impact Study

Macquarie Island, and with a marked polaroutbreak over New Zealand, might be such acase. I n this situation, the exact position ofthe southerly jet core would be crucial to theoutcome of the forecasts, particularly thosefor the South Island. Such a case was notanalysed in this study as a suitable situationcould no t be found w i t h i n the per iod o favailable data.

I t is possible the existence of the MacquarieIsland observation, 380 miles westsouthwestof Campbell Island, significantly reduces theimpact of the removal of Campbell Island data,especially in a situation where MacquarieIsland is upstream from Campbell Island. I fa study were carried out with both CampbellIsland and Macquarie Island upper air dataremoved from the global model database, thenthe impact may be significantly greater thanwas shown here.

This study was intended to highlight theeffects o f the Campbell Is land upper a i robservations on the Bracknell global model.I t did not examine the impact of the data onother aspects of the forecasting process overNew Zealand e.g. the impact on other NWPmodels, objective verification of model fields,manual interpretation or analysis.

ACKNOWLEDGMENTS

This s tudy was a co-operative e f f o r tbetween the Meteorological Service of NewZealand Limi ted and the U.K. MetOffice.Thanks are due to :U K M 0Messers Co l in Flood, Dave Shaw, A l a nRadford and Mike LongworthMetService Nz LtdDr Neil Gordon and Prof. Augrie AuerMessers Rod Stainer, Mike Pointer, Bob Lakeand Ian Miller.

REFERENCES:

Barwell B.R. and LorencA.C. 1985 "The Impact OfAircraftWind Observations" Quart. J. R. Met. Soc., (1985) ,111,103-129

Cullen M.J.P. 1993 The Unified Forecast/Climate Model.Meteorological Magazine, 122, 81-122

Graham R.J. 1990 "The Impact of NorthAtlantic TempshipObservations on Global ModelAnalyses and Forecastsdur ing a Case o f Cyclogenesis". S h o r t RangeForecasting Research Technical Note No 46. U.K.M.O.(National Meteorological Library, Bracknell)

Heming J.T. 1990 "The Impact of Surface and RadiosondeObservations from two Atlantic Ships on a NumericalWeather Prediction Model Forecasts for the Storm of25 January 1990. Meteorological Magazine, 119, 249-259.