Southern Hemisphere Prediction Experiment with a Nine-Level, Primitive-Equation Model

UDC 551.509.313(21613)

Southern Hemisphere Prediction Experiment With a Nine-Level, Primitive-Equation Model

REGINALD H. CLARKE-Division of Atmospheric Physics, Commonwealth Scientific and Industrial Research Organization, Aspendale, Victoria, Australia

ROBERT F. STRICKLER-GeophysicaI Fluid Dynamics Laboratory,' NOAA, Princeton, N.J.

ABSTRACT-A nine-level hemispheric primitive-equation data, the forecasts were generally of high quality, decreas- model is applied to make a 14-day experimental prediction ing in accuracy with height above the surface. After a for the Southern Hemisphere; this is the first numerical serious decline in skill on the fourth day, a recovery is prediction on the hemispheric scale for the Southern noted. Australian rainfall during the first 2 days was also Hemisphere and the first attempt to use the model for predicted with useful skill. summertime forecasting. Some properties of the model atmosphere are compared The predictions are verified against hemispheric data, with those of the observed Southern Hemisphere atmos- the inadequacy of which is clear. They are also verified sphere, and deficiencies in the performance of the model, in the Australasian sector where the data are more as well as in the analysis and initialization, are brought adequate. out by the comparison. However, on the whole, good For the first 2 days, especially over areas with good agreement is found.

1. INTRODUCTION 2. PREPARATION OF THE DATA

The nine-level primitive-equation hemispheric general The starting time (0000 QMT on Mar. 1, 1965) for the circulation model constructed by staff members of the experiments was chosen for the availability of satellite Geophysical Fluid Dynamics Laboratory (GFDL), data from TIROS 9, both for initialization and verifica- NOAA (Smagorinsky et al. 1965, Manabe et al. 1965), tion. We hoped to use these data partly to overcome was used by Smagorinsky et al. (1967) and Miyakoda et difEculties due to the sparseness of conventional data in al. (1969) for a series of Northern Hemisphere experi- the Southern Hemisphere. These dficulties are very real. ment a1 forecasts. In 1965, there were 85 daily radiosonde observations, less Following the success of these experiments, we decided than 20 percent of the daily total in the other hemisphere, to gather data to enable us to make global forecasts as and these were mainly concentrated in Australasia (35), well as to separate hemispheric forecasts for comparison. Antarctica (13), and South America (12). Figure 1, Results of the global experiments are reported by compiled from data available over the period Mar. 1-5, Miyakoda et al. (1971a). The results of the hemispheric 1965, shows the mean number of data items per day at forecasts for the Southern Hemisphere are reported in various heights and latitudes. (All available surface data this pa.per. were used, but only those from aerological stations are The present series of experiments, run in 1966, repre- included in the figure.) One sees that, on the generous sents the first prediction experiments to be made on the assumption that a daily sounding serves an area lOoX loo, hemispheric scale for the Southern Hemisphere and the in the troposphere the hemisphere is less than 30 percent first time the GFDL model has been applied to what is covered by conventional observations. In the stratosphere, virtually a summer situation. Earlier modeling work had the coverage falls off to about 10 percent at 20 mb and is been reported (e.g., Jenssen and Radok 1960) for a negligible at 10 mb. The data coverage is extremely poor restricted area. Other Southern Hemisphere forecasts of in the 50°-600S latitude belt, where the cyclonic centers shorter time period have been made recently with less most frequently occur (Taljaard 1967, Streten 1969). complex models (Baumhefner 1970, Staff, NMC 1970, Figure 3B shows the location of data available at 500 mb Gauntlett and Hincksman 1971). on the initial day. Such experiments are of interest for several reasons. Construction of geopotential and temperature isopleths for the 1000-, 850-, 700-, 500-, 300-, 200-, loo-, 50-, 20-, They are useful, for example, to the contemporary and 10-mb levels proceeded by standard methods (dif- Southern Hemisphere meteorologist when he considers ferential analysis from the surface upward) where the data the question of cost-benefit of improving data acquisition coverage is best. Over the blank areas, experience and systems. audacity gahed during years of Southern Ocean analysis (Clarke 1954) were used to project surface and 500-mb At Forrestal Campus of Princeton University. systems. Cloud photograph mosaics and nephanalyses

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~~ ~. I I I I I I1 interpolated to a uniform value, equal to the mean value at latitude 2OoS, in the equatorial zone. Collecting all available data for the 5 days from their various sources and checking, plotting, analyzing, and digitizing these data were laborious tasks, requiring much of the time (9 mo) available for this aspect of the joint project. Thus, the desideratum of at least 15 days of Hi analyzed data (the first for initial conditions and the E remainder for verification of a 14-day forecast) was not l achieved. Only 1000- and 500-mb height contours pre- G pared by the International Antarctic Analysis Centre, H T Melbourne, Australia, were available for verification of the last 10 days of the forecast. These analyses covered ( Km) the area poleward of latitude 30's and all longitudes with 92 8.4 19.4 18.6 e ee the exception of 80'-130'W. The analyses were extended 4.2 4.2 8.8 2p 18.6 a e to latitude 20's by reference to the available data, and the longitudinal gap in each case was filled subjectively by the customary forward and backward projection of systems from the analyzed areas.

70 90 3. THE MODEL AND THE COMPUTING PROCEDURE LATITUDE OS The model used was -the GFDL nine-level primitive- FIQURE1.-Mean number of radiosonde soundings per day in equation model described in essential detail by Sma- each 10' latitude belt of the Southern Hemisphere. The isopleths orinsky et al. (1965) and Manabe et al. (1965). Applica- show percentage coverage of the hemisphere as a function of latitude and height. tion of this model to prediction experiments has been described by Smagorinsky et al. (1967) and Miyakoda et al. (1969). The methods used for representing the made available by the National Environmental Satellite physical processes in the present model are described by Service aided in the analyses, in many cases enabling us Miyakoda et al. (1971~). to identify frontal systems, depressions, and an occasional Briefly, the model includes: moist processes, as in jet stream. The intensity of systems and the relationship Manabe et al. (1965), except that the moist convective between pressure and temperature fields could only be adjustment is applied when the relative humidity is 80 qualitatively estimated. Special difficulty was experienced percent or more; orography, smoothed to consistency with in the stratosphere, where data were both sparse and the grid ; long- and short-wave radiation (where humidity, spatially and temporally inconsistent. No satisfactory clouds, and ozone are based on Northern Hemisphere method was found for resolving inconsistencies, other than height-latitude mean values for October) ; constant conventional space- and time-smoothing techniques. (mean March) insolation; constant (mean March) sea- Attempts to use regression techniques based on Northern surface temperature; a water availability factor (over Hemisphere investigations, such as those used by Finger land), DW=E/EpOt,which is the ratio of evaporation to et al. (1965) for vertical extrapolation of geopotential potential evaporation computed from climatological and temperature, proved fruitless. It was impossible to precipitation data with the use of a procedure from attempt synoptic analysis at the 10-mb level, and latitude- Saltzman (1967) ; and prescribed albedo and snow cover. dependent mean values of geopotential and temperature The sea-ice surface temperature was held constant at had to be adopted. Mean humidity at the levels of low, 268.5'K. Grid spacing is characterized as N40 on a polar medium, and high clouds was estimated in the data-free sterographic projection (Le., 40 grid intervals from areas from the humidity-cloud relationships of Smagor- Equator to pole) with resolution varying from 320 km at insky (1960) applied to the nephanalyses. the pole to 160 km at the Equator. Horizontal diffusion All data in the area poleward of latitude 20's were then is modeled as in Smagorinsky et al. (1965), except that digitized on a polar stereographic grid of 1,977 points and Smagorinsky's horizontal mixing constant, ko, has been checked by computer for vertical consistency of tempera- reduced from 0.4 to 0.25 (Miyakoda et al. 1971b). tures and height increments; implied superadiabatic lapse The initialization procedures (Smagorinsky et al. 1967) rates were adjusted to adiabatic values where detected. were applied to each of the five data sets, 0000 GMT on Finally, the data were subjected to the initialization pro- Mar. 1-5, 1965; the diagnostic procedures were applied cedures described by Smagorinsky et al. (1967) to obtain to each day's data; and the results were averaged to initial fields of wind components, temperature, and give a 5-day mean picture of processes poleward of 20's. humidity and interpolated to the model grid of 5,025 These are expected to be valid within the limitations of points. In the course of these operations, as in the proce- the original analyses and the initialization procedures. dures mentioned above, temperature, geopotential, and According to Gauntlett (1971) , application of the humidity data equatorward of about 20's were smoothly balance equation results in some smoothing of the initial

626 1 Vol. 100, No. 8 Monthly Weather Review Unauthenticated | Downloaded 09/30/21 01:05 PM UTC DAY 0

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FIGURE2.-Observed (A, C, E) and predicted (D, F) 1000-mb contours (60-m intervals), South Pole to latitude 2OoS, for days 0-2. Nega- tive areas are shaded. B delineates the Australasian sector referred to in section 4. data, especially in the vicinity of the jet stream. Ex- The prediction procedures of the GFDL model were amination of the raw data suggests that flattening of the applied to the initialized data for 0000 GMT on Mar. 1, zonal wind profile in this way is minor in the present 1965, to produce forecasts out to 14 days. The predicted instance. fields were then subjected to statistical analysis by averag- The w-equation, used for computing vertical velocity ing various quantities at 12-hr intervals. A few of the and horizontal divergence, is based on the assumption of diagnostic results so obtained are shown in figures 5 adiabatic, frictionless flow. It is to be expected, therefore, and 6. that divergence magnitudes in the boundary layer would be underestimated, as will those of vertical velocity at all 4. VERIFICATION AND EVALUATION levels, together with dependent quantities such as vertical OF THE FORECASTS eddy fluxes. The magnitude of ageostrophic wind compon- The task of verification is complicated by the lack of ents will also be underestimated in the boundary layer. reliable verifying data over much of the hemisphere.

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180’

FIGURE3.-Observed (A, C, E) and predicted (D, F) 500-mb contours (60-ID intervals), South Pole to latitude 20°S, for days 0-2. B shows the location of wind (triangles), temperature (filled circles), and wind plus temperature (open squares) observations for day 0 at 500 mb.

Since the Australasian region is the only extensive area been truncated at latitude 2OoS, since few, if any, mean- with good data coverage, we decided to verify the fore- ingful isopleths lie equatorward of that latitude. The casts both on a hemispheric basis (2,453 gridpoints pole- 1000-mb height over Antarctica has been omitted because ward of lat. 20’s) and a regional one (292 gridpoints in of its fictitious nature; it lies far below the elevated the Australasian region south of Iat. 20’s) delineated in surface of the ice cap. fig. 2B). In general, the systems were predicted to progress a little too slowly eastward with the exception of the well- Hemispheric Verifications observed deep depression and trough to the east of New Figures 2 and 3 show initial 1000- and 500-mb geo- Zealand. The southeastward movement of this system potential fields and forecast and “observed” fields for was overpredicted by about 250 km/day. Some serious days 1 and 2 of the forecast. The hemispheric maps have discrepancies are to be seen in the southeastern Pacific,

628 / Val. 100, No. 8 1 Monthly Weather Review Unauthenticated | Downloaded 09/30/21 01:05 PM UTC the worst region, datawise, in the hemisphere. In the I I 1 I I 'n I 1 vicinity of the Australian continent, agreement is good I30 - at both levels, although the predicted 500-mb trough persistence south of the continent is lagging somewhat.

In figure 48, the standard deviations of the errors in 100- the forecasts of height of the 1000- and 500-mb surfaces are compared with those for a simple persistence predic- penis tion. With the exception of day 6, the skill at 500 mb nX remains positive throughout the 14 days, but no claim is made that the skill so indicated is useful beyond the 1000 mb first 2 days. At 1000 mb, the skill becomes and remains 50 / Forecast virtually zero at and after day 4. 0.6'i Performance of Model in the Southern Hemisphere Versus Northern Hemisphere The model should perform almost equally well in the two hemispheres, given the same data density and accu- 0.4- racy, although one might expect minor differences due to orography and the hitherto unexplored effect of a summer, 0.2 - as against a winter, situation. Thus, the differences in V performance may generally be attributed to observational 0- deficiencies in the Southern Hemisphere; these deficiencies affect both the quality of the forecast and of the -02- verification. A comparison of the performances (figs. 4B,4C) reveals - 0.41 1 I I I I I I 2460 IO 12 14 16 at once the general superiority of the Northern Hemi- Days sphere forecasts. Exceptions are day 1 at 1000 mb and the FIQURE4.-(A) standard deviation of model forecast and persistence second week at 500 mb. Such a recovery in the second height errors (m) at 1000 and 500 mb for the hemisphere south of week ha3 not been observed in Northern Hemisphere 2OoS, (B) correlation between observed and predicted height forecasts. and its occurrence in the first Southern Hemi- changes at 100 and 500 mb for the hemisphere south of 20'5 com- sphere case must be regarded as fortuitous. pared with similar correlations for Northern Hemisphere forecasts (Miyakoda et al. 1969), and (C) skill score [V = (up- ur)/u.] The at 50 mb are not shown; these are very where upand u,are standard deviation of persistence and forecast Poor, as might be expected from the data availability* errors, respectively, for 1000 and 500 mb compared with the score Correlations range from 0.3 to 0.6, and the skill score, v, for Northern Hemisphere forecasts. is negative throughout the 4 days available for verification. (For definition of V, see caption to fig. 4.) Radok (1967) pointed out that correlation between- predicted and ob- statistical stationarity, may serve as observed values with served chmges will have a value of 0.5 if the covariances which to compare similarly computed forecast values over between observed and initial, observed and forecast, and the 14 days of the experiment. For this purpose, results initial and forecast fields are all equal (Le., there is no from days 4 to 14 have been averaged. To limit the averaging skill). The value 0.5 broadly corresponds to the point to nearly stationary conditions, we have omitted the where forecast errors are comparable with persistence. first 3 days when some of the space-mean properties, for Thus, one should not accept correlation values as low as example, meridional circulation, were changing rapidly. 0.5 as evidence of skill. Clearly, only the statistics of long-period forecasts, such as in the present case, can be compared with those for the Some Statistical Properties of the Predictions observed atmosphere in this manner. Some of the dif- It is desirable to apply some kind of hemispheric check ferences between the observed and predicted statistics on the statistical properties of the forecasts to learn may be related to the fact that different periods have been whether the model atmosphere behaves rea,listically in treated, but in most cases this is not true. Something of a statistical sense. In the present case, an overall check value can be learned from the comparison. on the properties of the observed atmosphere is also The integral properties chosen for comparison are needed. shown in figures 5 and 6. As was expected, observed and An extensive range of diagnostic integral computations predicted streamlines (fig. 5A) differ considerably, due, has been devised by GFDL. In the present case, we have at least in part, to the deficiencies of the w-equation that the mean values of these integrals derived from the affect the observed but not the predicted field of flow. initialization procedures for each of the 5 days, together Nevertheless, there is qualitative agreement on the two with averages estimated from the raw data, equatorward main circulation cells and on the equatorward flow in the of 20's. These 5 days of data, displaying fairly satisfactory middle stratosphere (highest model level) , although the

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FIGURE5.-(A) observed and predicted streamlines (10'2 g/s) of the mCdionwJ circulation, (B) observed and predicted mean zonal wind speed (m/s), and (C) observed and predicted eddy kinetic energy (u'2 + d2)/2in m2 . s-~where u' and u' are departures from zonal means and the bar indicates a space-mean. The vertical line at latitude 20's indicates the equatorward limit of the observed data used in initialization. Equatorward of this line, in (B) and (C), observed quantities are derived from raw data rather than from the initialization procedure. polar meridional circulation cell is almost missing from The main features of the mean field of zonal wind the observed streamlines. The three cells computed by the (fig. 5B) were reproduced well. However, the westerly model are in good qualitative agreement with those of jet is predicted slightly poleward of its observed position, Newel1 et al. (1970) for autumn, although they are some- and the predicted easterly jet in the tropical stratosphere what stronger. Central values of the stream function are is much weaker than observed. The behavior of the model about 95 and --43X1012 g/s in the predicted, compared in low latitudes may be seriously affected by the equatorial with 50 and -10X1012 g/s in Newell's analysis, for the wall, while the observed wind must be treated with reserve. Hadley and Ferrel cells, respectively. Sadler's analysis (Miyakoda et al. 1971a, fig. 2) of the

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LATITUDE 'SOUTH- FIQURE&-(A) observed and predicted values of the quantity 2rE%s2+ v'u' (loz2cm4 s-2) where R is earth radius and 41 is latitude, (B) observed and predicted values of the quantity ZTR cos + cpu'T' (1018 cm* . s-a) where T' is departure from zonal mean temperature and c, is specific heat, (C) observed and predicted mean temperature (OK) [note that, equatorward of ZOOS, quantities in (A)-(C) are estimated from raw radiosonde data], and (D) observed and predicted values of mean relative humidity (percent). Full lines for observed humidity are derived from initialization; broken lines are from the raw radiosonde data. stratospheric easterlies differs from the present one in southern summer. Another feature worthy of comment is locating the maximum at the Equator. Ours, however, the previously noted tendency for energy from the agrees well with that of Newel1 et al. (1970) for the westerly jet to "leak" upward into the stratosphere; the

August 1972 / Clarke and Strickler / 63 1 Unauthenticated | Downloaded 09/30/21 01:05 PM UTC 160 I 1 I I ' P--d I Figure 6C compares observed and predicted tempera- 500mb yt 1 \ 140 A persistence \ f/ \ tures. Prediction errors are greatest near the pole (i.e., I/ '. predicted temperatures are too low at all levels, reaching -8.3'K at = 189 mb). In general, levels at and above 500 mb are too cold (areal mean temperature deficit reaching 80 2.1OE at =189 mb), and the lower troposphere is too warm (max. error 0.9'E at = 926 mb) . 60 The relative humidity comparison of figure 6D reveals 40 some rather striking dissimilarities. Observed humidity is represented by two sets of isopleths, one (dashed lines) 20 based entirely on raw station data, averaged by 10' latitude belts, and the other (solid line), from the initialization procedures. Miyakoda et al. (1971a) suggest that insufficient moisture is predicted in the Tropics because I the equatorial wall prevents the cross-equatorial flow of moisture from the winter hemisphere to the summer hemisphere, which is a feature of the real Tropics.

01 I I I I I 1 I 2 4 6 8 IO 12 14 Verification in the Australasian Sector Days Regional verification is based on too small areal FIGURE7.-(A) standard deviation of height errors (m) at 1000 an and 500 mb for Australasia south of 2OoS, with persistence errors, sample to be used as a test of general capability of the and (B) correlations between observed and predicted height model; but it is a much more satisfactory exercise in that changes at 1000 and 500 mb for Australasia south of 20's. one may have confidence in the observed data. For rainfall verification, there is at present little alternative but to observed mean westerly wind maximum of 5.8 m/s at check on a regional basis. level 1 (=9 mb) is magnified in the forecasts to 18.1 m/s. Figures 7A and 7B show the standard deviation of the Correspondence between predicted and observed values geopotential error and correlations, respectively, between of meridiorial transport of angular momentum (fig. 6A) observed and predicted height changes for the Australasian is not good, but, in view of the observational difficulties, sector. The performance of the model in this series is it may be classified as acceptable. Both are in reasonable somewhat better over the Australasian sector than else- accord with Newell et al. (1970) and show a middle- where at both 1000 and 500 mb despite the fact that latitude maximum of poleward transport of similar activity, as judged by surface pressure vaxiance and rain- magnitude, although the predicted maximum is displaced fall, was above normal during the period. Also, for the somewhat toward the south. Both the observed and pre- first 2 days, the 1000-mb level, which in general is more dicted patterns show a high-latitude equatorward flux, dacult to predict, appears to verify better than 500 mb, but the predicted maximum is larger than the observed possibly reflecting the effect of a better data network at (3.2X1022compared with 1.9X1022C~"S-~) and is shifted the lower level. equatorward. Both show a region of equatorward flux Figure 8 shows observed and predicted maps of the (much larger in the observed case) at low levels beneath 1000- and 500-mb heights on day 7 for the Australasian the poleward maximum. Such an equatorward flux was sector. One sees at once that the high skill score results actually observed in the boundary layer during the only from a correct placement of the main trough; the Wangara experiment (Clarke et al. 1971), extending over details are not correctly predicted, nor is the rain well 40 days at latitude 34.5's. If the value found is repre- foreshadowed. sentative of its latitude, the flux at 350 m above the Figure 9 presents Hovmoller diagrams of the 1000- and ground woultl be 1.7X loB cm4.s-2 toward the Equator. 500-mb predicted and observed geopotential, averaged Figure 6B presents observed and predicted meridional between latitudes 35' and 45'S, for the Australasian heat fluxes that may be compared with Robinson's (1970) sector (115'E-170'W). After the first few days, during annual distribution and the summer distributions of which the model failed to predict the lack of movement of Newell et al. (1970). Because of the difficulty of matching the depression east of New Zealand, the course of the temperature and geopotential in a realistic way in the trough and ridge pattern is fairly well reproduced, albeit course of estimating fields from satellite data, there are with diminished amplitude. A noteworthy feature is the significant discrepancies. In both patterns, however, there failure to predict the rearward redevelopment of the is a broad midlatitude band of poleward flux with a trough over southern Australia on day 9. maximum of 12-16X10'8 (cf. Robinson's mean Since the model computes rainfall, it is of considerable annual value of 24X1Ols) and a high-latitude and a low- interest to check how well the daily accumulation of rain- latitude band of equatorward flux. The observed fluxes fall compares with that predicted. For this purpose, it is in the stratosphere bear little resemblance to the pre- desirable to average many rainfall recordings in the area dicted ones, but observational difficulties here are represented by each gridpoint of the computational mesh. overwhelming. The 1,660 regularly reporting precipitation stations in

632 J Vol. 100, No. 8 J Monthly Weather Review Unauthenticated | Downloaded 09/30/21 01:05 PM UTC FIGURE&-Observed and predicted constant pressure [contours in dekameters (dam)] for day 7 in the Australasian sector.

115"E 15O'E 17W 1159 17o"w FIGURE9.-Hovmoller diagrams showing observed and predicted mean height of the 1000-mb surface (m) and the 500-mb surface (dam) between latitudes 35' and 45's as a function of longitude and time in the Australasian sector (ll5'E-17OoW).

the mainland network of the Commonwealth Bureau of one. Only 53 had more than five rain stations. We decided Meteorology are very unevenly distributed; of the 178 to use all gridpoints having at least one rain station, aver- gridpoints falling on the Australian mainland, 37 had no aging stations where there were more than one, for com- rain stations at all, while 32 of the remainder had only parison with predicted precipitation values. It should also

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FIGURE1 1.-Rainfall verification over the Australian continent : (I) correlations between cube root of predicted and observed rainfall, (11) Wallen's skill scorc calculated on a rain-no rain basis, (111) mean predicted rainfall in mm/day (Australia), (IV) mean observed rainfall (Australia), (V) mean predicted rainfall (Australia south of 20'59, and (VI) mean observed rainfall (Australia south of 1st. 20's).

1971a), which from most points of view verified lower than the hemispheric model, yielded higher overall rainfall correlations over Australia (0.35 vs. 0.19 for the 14-day average, with 0.66 and 0.57 on the first 2 days). This might be expected from the more realistic initialization in the Tropics. 2. If the precipitation forecasts are regarded as hit or miss prediction of rain or no rain, Wallen's method (Bleeker 1946) of verification can be applied. This has been used successfully to test the DAY I - PREDICTED value of long-range forecasting systems. To allow for the tendency of the model to produce small values of precipitation ovei large areas, 9 wc regardcd only predicted precipitation of more than 1 mm as a FIGURE10.-Observed and predicted accumulated rainfall (in.) forecast of rain, while observed rainfall of 0.25 mm or more was at day 1 over the Australian continent. The grid squares with regarded as constituting a rain day. The skill score achieved by this no data are indicated by shading. system is shown in curve 11 of figure 11 and is seen to agree closely with that indicated by the correlations. For most practical purposes, the useful skill is confined to the first 2 days. be noted that Australian rain gages are read at 0900 LST, which implies a discrepancy of from 1 to 3 hr in various To illustrate the performance of the model in parts of the continent from the time of the predicted Australia's rather extreme summer conditions (the grid- accumulations at 0000 GJIT. points for testing rainfall range from lat. 11' to 41'S), we Figure 10 shows accumulated predicted and average add curves 111-VI in figure 11. They represent areal mean observed rainfall at the 141 gridpoints for which data are predicted and observed precipitation over (1) the Austra- available on day 1. One can see that a fair measure of lian continent (141 points) and (2) that portion of the agreement has been achieved despite the unsatisfactory continent (100 points) south of latitude 20's. Curves I11 method of initialization north of latitude 20's where the and IV (14-day means are 4.24 and 4.36 mm/day, re- bulk of the precipitation occurs. On subsequent days, the spectively) show that, overall, the predictions yielded agreement is much less marked, and the application of results very close to those observed. On the other hand, statistical methods of detecting skill is required. The curves V and VI show that too much rain is predicted for following two methods have been applied : the dry southern part of the continent (14-day predicted and observed means are 2.70 and 1.63 mm/day). Areal 1. The first method consists of correlating predicted and observed averaging showed that, for that part of the continent rainfalls at each of the 141 gridpoints. To reduce the marked skew- covered by our data, March normal rainfall is much less ness of the distributions, we correlated the cube root of the rainfall 14 amounts after rounding to the nearest 0.01 in. (0.254 mm). The than that measured over this particular days (in the results for each of the 14 days of the forecast are shown in curve I of ratio 1.73:4.36 for the whole continent and 1.42:1.63 for figure 11. Predictions made with the global model (Miyakoda et al. that part south of lat. 20's).

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Unauthenticated | Downloaded 09/30/21 01:05 PM UTC As another test of the summertime performance of the with an observed spring vuliic of necirly :%OOO ni. Thils, model, the forecast results wre checked for evidence of the the kutabntic outflow with Iiiiticyclonic circiiltition is monsoon, which manifests itself in anomalously high reprotlucetl in the motlcl, but its depth is probably temperatures and low pressures in the lower levels of the undcrcstiniated. troposphere, and a low-level mean cyclonic circulation with inflow. KOsuch evitlencc was found in the foreccist fields. 5. CONCLUSIONS For example, mean inflow at the coast was 22 and 8 cm/s Tlie results of this esperiment suggest that, despite the at levels 8 ( =926 mb) and 9 (=991 nib) compared to an present serious luck of suitiible (Ititti in the Southern observed value of 120 cm/s at 300-500 m. The mean tem- Hemispliere, hemisplieric forecasts useful for 2 or 3 diiys perature excess over the continent at level 9 \vas 0.5’1<, can be made with the GFDI, model. During this period, compared with 3.5’K for the mean ,\larch screen tem- skill was found to decline with height, probubly reflecting perature. These defects, and possibly the excessive rain- the superior surluce clutu coverage. fall computed south of latitude 20’5 are believed to be From ttie first few dugs, the model performed generally due largely to the unrealistic values adopted for E/ Dw= worse in the Southern Hemisphere tlian in the Sorthern E,,, over the dry Australian interior. South of latitude Hemisphere, as might be espected from the datu uvail- 2OoS, the water availability factor ranged from 0.3 to 0.8. ability. Severtheless, ut 1000 nib where datu ure more The resulting evaporation computed over the driest part plentiful, the first 2 tlnys’ results were comparable \\-it11 of the continent mas little less than that over the neigh- those for the Sortliern Hemisphere. boring oceans. Over the Austrnlasiun sector, the largest region repre- With the use of Budyko’s (1960) values for mean net sented by anything upproaching an acceptable datu radiation in March, and with E, the evaporation rate, coverage, the model performance in the first 2 days was assumed equal to the mean rninfiill for the area south of superior in almost every respect to that in the rest of the latitude 2OoS, we found that the Bowen ratio, H/YE hemisphere. Its performunce was suficient to ruise hopes (H is sensible heat flus at the surface, Y’is the latent heat thut \venther forecasting (including ruinfull) in this region, of vaporization for water), should have a mean value of for 2 days and perhaps more, can be markedly improved. about 2, as opposed to that of 0.08 computed from the This conclusion is supported by recent results with u forecasts. Thus, there appears to be a need for a formula- modified version of the same niodel (Gnuntlett and tion of E/EDotin terms of water substance in the soil; that Hincksmnn 1971). is, one that will not imply an evaporation of nonexistent A disappointing feature of the present series was the soil moisture. failure to predict the virtual lack of movement of the The Antarctic Circulation deep Sew Zealand depression for the first 4 days. But for this failure, skill scores may well have been higher on the A further test of the model is its ability to reproduce the third and fourth days in the Australian area. well-known vertical circulation over Antarctica (Rubin One should not read too milch into the high skill scores and Weyant 1963, Berson 1966) and its katabatic winds indicated for the second week of the forecast at 500 mb (Ball 1957). Rubin and Weyant calculated the mean for the hemispheric prediction and at both 500 and 1000 outflow at standard pressure levels. At the lowest level mb over Australia. It may be sipnificunt that there is used (850 mb), the mean outflow for Jlarch [vas 1.4 m/s, little, if any, response to this skill in the Australian rainfall decreasing to zero a little above 700 mb. Berson computed predictions. The evidence of skill in the Australasian the mean outflow for a 40-day period in spring to be 3.3 region, nevertheless, escites speculation that 11 “data m/s at 980 mb and 2.6 m/s at 940 mb, decaying to nearly oasis” may have some advantages in medium-range zero at 700 mb. predictability. This outflow and the corresponding higher level inflow The summer monsoon over Australia is not well repro- and interior descent are modeled with some success, as duced by the model, due apparently to a weakness in may be seen from the predicted streamlines (fig. SA). modeling evaporation. Greater success in modeling the ,Mean vertical velocities were predicted to be about- 0.3 circulation was achieved over Antarctica. cm/s in areas poleward of latitude 80’s at the 500- to 600- mb level decreasing with height and becoming positive ACKNOWLEDGMENTS above 100 mb. In the lower levels, this agrees well with The authors owe a great debt of gratitude to other members of the vertical velocities deduced by Berson and by Rubin GFIIL, especially J. Smagorinsky, who made these experiments and Weyant. possible; K. lliyakoda, who contributed mnch help and guidance; and G. D. Hembree, whose help in collection, preparation, and An examination of the model outflow values, referred initialization of the data was indispensable. 11. L. Scff, L. W. Reed, to Antarctic topography, shows that at level 9 (repre- I. Shulman, and T. L. l1attk also provided much skilled assistance senting roughly the lowest 300 m) the mean outflow is in programming and data handling problems. 3.5 m/s at the coast, decreasing to 1.8 m/s at the 1800-m contour, in good agreement with Berson’s value. However, REFERENCES no mean outflow is predicted at higher levels around the Ball, F. K., “The Katabatic Winds of Adelie Land and King George coast. The computed depth of the anticyclonic circulation V Land,” Tellus, Vol. 9, No. 2, Stockholm, Sweden, ?thy 1957, around the continent is a little above 1000 m, compared ~1).201-208.

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[Received October 2’6, 1971; rewised -44arch 3, 19721

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