The characterisation of orographic rainfall

Transcription

1 Meteorol. Appl. 7, (2000) The characterisation of orographic rainfall W R Gray, National Institute of Water and Atmosphere, P.O. Box , Kilbirnie, Wellington, New Zealand A W Seed, Co-operative Research Centre for Catchment Hydrology, Bureau of Meteorology, GPO Box 1289K, Melbourne, Victoria 3001, Australia The Otaki Precipitation Estimation by Radar (OPERA) programme was designed to investigate the processes that lead to enhancement of rainfall over the Tararua ranges of New Zealand. These ranges rise to 1500 m above the coastal plain and enhancement of rainfall by windflow over these hills leads to annual hill-top rainfall of over four times that upwind. The OPERA experimental campaigns aimed to characterise the enhancement processes by analysing data collected from a transect of high-resolution rain gauges and a locally deployed, high-resolution radar, supported by scanning radar and satellite observations. Measurements made during these experiments showed that orographic enhancement led to hill-top accumulations often twice that upwind, and up to as much as a factor of seven in one case. The data suggest that the most frequent occurring enhancement mechanism was triggered convection. This mechanism leads to an increase in rainfall over the hills of around a factor of two, primarily through an increase in the duration of rain. Seeder/feeder-type enhancement occurs less frequently but leads to larger enhancements. 1. Introduction The steep and rugged nature of New Zealand s topography can lead to the rapid response of river levels to rainfall. This gives little lead-time for flood forecasting. In order to increase this lead-time, flood-forecasting schemes need quantitative forecasts of precipitation that can be used as inputs into hydrological models. Much of New Zealand s rainfall is modulated by windflow over topography and it is important to understand the processes that lead to the enhancement of rainfall over hills if precipitation forecasts are to be accurate. Orographic enhancement can lead to hilltop storm totals of more than of four times those occurring upwind (Gray & Austin, 1993). The vertical depth through which the enhancement occurs is often shallow. This makes radar estimation of surface precipitation difficult as the radar often scans above this enhancement, due to the earth curvature effects or to avoid reflections off the hill itself. In addition, the enhancement occurs in areas that are frequently too remote for dense rain gauge deployment. Thus, generating rainfall estimates for these regions is complex and full of difficulties. A previous study of orographic enhancement in similar mid-latitude climates has found, for the hills of South Wales in Great Britain, orographic enhancements of up to an extra 2.5 mm h 1 (Hill et al., 1981). Kitchen & Blackall (1992) have viewed similar enhancements over 150 m high hills. However, it is not clear that these results are directly transferable to the more rugged orography and weather systems of New Zealand. The results described in this paper were generated from observations over the Otaki region of the North Island of New Zealand (Figure 1). The Otaki river catchment is in the Tararua ranges, in an area in which windflow over the ranges can lead to enhancements of a factor of over four, storm totals of over 300 mm, and river-level rises of over 5 m in 3 4 hours (Thompson et al., 1997; Gray & Austin 1993; WRC 1993). The steep terrain slope from the coastal plain to a maximum hilltop height of 1500 m results in rivers with a fast hydrological response to rainfall. The significant orographic enhancement and the short hydrological response time make flood forecasting for this river difficult. This area also acts as a primary water resource for much of the city of Wellington, so management of this resource is important. Management, both in terms of yields and extremes, will benefit from the improvement in rainfall forecasting expected to result from an improved understanding of the rainfall enhancement processes. The work reported on here investigated the properties of the rainfall seen over the hills with the aim of delineating the enhancement processes, as a first step in improving the forecasting of orographically enhanced rainfall. As moist air is lifted over hills, the resulting condensation produces cloud which can be converted to rainfall by a number of processes. 105

2 W R Gray and A W Seed Figure 1. Topography of the Otaki area showing contours (300 m) of height above msl. Also included are the locations of the campsite, scanning radar and the balloon site, as well as the locations (numbered) of the rain gauges. (a) Autoconversion is the growth of the droplets (through collision and coalescence) in the cloud formed by the windflow over the hill to produce rain without the need for any pre-existing rain. However, for a parcel of air moving over the hills, the duration spent ascending is likely to be too short for the formation of the heavier rainfall often seen. Indeed, for many hills, this mechanism is unlikely to produce any rainfall at all. (b) The seeder/feeder mechanism is that in which the low-level cap cloud (feeder cloud), formed by the windflow over the hill, provides a moisture source that is collected by drops falling from aloft from pre-existing seeder clouds, e.g., frontal rain bands (Bergeron, 1965). This process can lead to hills as low as 50 m producing significant orographic enhancements of 10 40% in the lowest 500 m above the ground. Larger hills can have enhancements leading to additional rainfall of over 6 mm h 1 (Robichaud & Austin, 1988). (c) Triggered convection occurs when windflow over the hills triggers potential instability through the lifting of the air column (Hill et al., 1981). The resulting convection leads to an increase in rainfall over the hills. 106 Each of these mechanisms is likely to have recognisable characteristics. The seeder/feeder mechanism should be characterised by the limited depth through which enhancement occurs, being restricted by the depth of the relatively shallow cap-cloud. Triggered convection should manifest itself as an increase in the variability of the rainfall in the hills as compared to that upwind. Autoconversion should have very steady characteristics, and be similar to seeder/feeder in the limited depth of enhancement. The experimental design aimed to distinguish between these processes through a combination of measurements at both the large and very small scale taken during three field campaigns. The instrumentation used included high-resolution rain gauges, a mobile X-band radar, a C-band radar, balloon soundings and satellite images. The first campaign was during June 1994 (winter), the second during January 1995 (summer) and the third during April 1996 (autumn). From these three experiments, seven raining periods from four storm events were studied. Analysis of the data collected shows that orographic enhancements of a factor of two are common for cases identified as triggered convection, and those cases described as seeder/feeder generally lead to enhancements exceeding a factor of four. Section 2 details the topography of the area, the equipment deployed and the data obtained. Section 3 outlines the approach to analysing the data. Section 4 tabulates the results. Section 5 presents a discussion of these results, which are then summarised in Section Data Figure 1 shows the location of the 16 high-resolution rain gauges, as well as that of the C-band scanning radar and balloon sounding site. These rain gauges convert the rainfall to equal-sized drops (Hosking et al.,

3 The characterisation of orographic rainfall 1986; Stow et al., 1998). The total drop numbers over either 15- or 120-second intervals are electronically recorded. Gauges in the most remote locations were set to 120-second sampling to enable them to be left unattended for up to three months. In the statistics outlined in the tables of Section 4.2, gauge data were analysed at 120 s resolution. Note that not all of the gauge sites were populated for all the experiments. The high-resolution X-band radar (Seed et al., 1996) was located within the catchment at the Camp site (see Figure 1) for the 1994 and 1995 experiments, and operated in a vertical pointing mode. In this mode, the radar continuously recorded the vertical profile of reflectivity up to a height of 7 km at 30 m resolution. For the 1996 experiment, the X-band radar was located on the coast (near Gauge 1) to take advantage of the high time and space resolution of its scanning mode. The scans were made at an elevation of 6, with one scan being completed every 6 seconds. The range resolution is 350 m, and reflectivity is recorded out to a range of 22.5 km. The Ericsson C-band radar is operated by MetService New Zealand Ltd and records the reflectivity at 1 km resolution out to 120 km and at 2 km resolution out to a range of 240 km. The distance from the radar to the catchment is about 60 km. The radar has a 0.86 beam width. Scans at 10 elevations (0.5, 0.9, 1.3, 2.4, 3.5, etc.) were made over a two-minute period, once every 15 minutes. The data were quality controlled to minimise the effects of ground clutter, attenuation and the vertical profile of reflectivity, by using the approaches outlined in Uddstrom & Gray (1996). The radar has a clear field of view over the ocean upwind of the mountains. However, ground clutter and beam blocking prevent the use of the radar data in estimating rainfall at low levels over the ranges. To minimise the impact of beam blocking and ground clutter, the radar images shown were formed from the 1.3 elevation scan, a beam-centre height over the catchment of about 2.5 km. Balloon soundings were made once every 12 hours from the coastal site shown in Figure Method The general approach was to move from the large-scale measurements down to the small scale. This was done in order to fix the broad-scale context within which the high-resolution measurements were made. Thus the satellite and C-band scanning radar data were used to determine the area over which the rain gauge data and high-resolution radar data were representative and to characterise the nature of the precipitation impinging upon the orography. The upwind rainfall fields were used to determine the direction of the rain cell motion thereby showing any misalignment between the rain gauge transect and the direction of the rain cell motion which would result in misleading interpretations of the rain gauge data. The rain gauge data were analysed to give simple statistics on the fraction of time spent raining and the average rain rate while raining. These two statistics were most useful in characterising the processes involved in the enhancement of the rain. The characteristics of triggered convection were assumed to be a substantial increase in the fraction of time spent raining as the rain moved from the coast to the hills, with little or no increase in the rain rate. The seeder/feeder mechanism was assumed to result in an increase in the rain rate while raining but not necessarily an increase in rainfall duration. The balloon sounding data were used to determine if there were layers of potential instability in the atmosphere. The moisture and wind information were also inspected to ensure there was capacity for orographic enhancement. Finally, the X-band radar data were scrutinised for evidence, in the scanning data, of the triggering of convection in the foothills of the ranges, or, for the vertical pointing data, of the characteristic low-level increase in rainfall of the seeder/feeder mechanism. 4. Results From the three experimental periods, seven shorter periods (cases) were chosen for investigation (Table 1). These cases were selected with criteria based on the availability of data, the length and intensity of the rain that fell and the uniformity of the rainfall enhancement process. From the first experiment, two events were chosen (Cases 1 and 2) from rain on 10 and 11 June 1994, each with gauge totals of up to 70 mm in the ranges. The second experiment gave three significant periods of rain (Cases 3, 4 and 5), from data collected around 25 January 1995, with totals up to 16, 12 and 41 mm respectively. From the final experiment, two periods were investigated, (Cases 6 and 7), on 4 April and 9 April 1996, with totals up to 38 and 25 mm respectively. For each case we have examined synoptic maps, satellite images, balloon sounding data, accumulated C- band scanning radar data and rain gauge data Cases 1 and 2 The synoptic map for Cases 1 and 2 (Figure 2) shows a slow moving depression lying to the west of the area. This depression had two major rain bands whose passage across the domain led to the two enhancement case studies. Wind flow (Figure 3) at 1.5 km asl (hill top) was from the north-north-west at 30 kn. Flow below 107

4 W R Gray and A W Seed Table 1. Event start and finish times, including the duration and the maximum rainfall accumulation observed. T/C = triggered convection, S/F = seeder/feeder Case Start time End time Duration Max. rainfall Enhancement (NZST) (NZST) (mins) (mm) type T/C T/C T/C T/C S/F T/C S/F each of the case studies. These images were then inspected to check that the rain gauge data were representative of the enhancement across the ranges. The image for Case 1 (Figure 5(a)) reveals that the area along the transect was a locally preferred area for precipitation, even before the rain traversed the hill. This implied that care was needed in the interpretation of the gauge data to avoid the variations parallel to the mountain ridge being misinterpreted as enhancement. The accumulation for Case 2 (Figure 5(b)) shows that no area was preferred. Note that for Cases 1 and 2 only the lower resolution radar data (2 km in range) were available. Figure 2. MSL analysis for Cases 1 and 2, 0000 UTC for 10 June this level was from a more northerly quadrant. Inspection of the temperature and humidity profiles shows the atmosphere to be neutrally stable, with no clear indication of potential instability. Satellite pictures show that the first cloud band resulted from the passage of a front while the second, started 22 hours later, produced rain from a cold-air vorticity maximum. The cloud for Case 1 was generally broad scale in nature, although there were distinct clumps (Figure 4(a)). For Case 2, the cloud was clearly formed of convective clusters (Figure 4(b)). It is important to establish that the observed changes in rainfall are due to orographic enhancement and not the variability of the large-scale system. To do this we have averaged the scanning C-band radar data in time for 108 Table 2 shows the rainfall characteristics for Case 1 as measured by the gauges at the coast, on the hills and at the peak. Gauge 3 was used as the coastal gauge (20 m amsl), gauge 9 as the hill gauge (900 m amsl), and gauge 11 as the ridge top gauge (1500 m amsl). The statistics were formed from 120 s data. The mean rain rate, standard error of the mean, the median and the standard deviation formed from the time-series excluding the 0 mm h 1 samples show that, in this case, the rainfall intensity distribution varies only slightly over the ranges. The column listed hillside/coast is the ratio of the value at the hillside to that at the coast. Similarly, ridge/coast is the ratio of the values at the ridge to that at the coast. Here the highest intensities occurred upwind of the ridge top, as did the greatest accumulation. For example, the ratio of the coastal to hill rain rates while raining was 1.26, yet the ratio for the coast to ridge was Similarly, the ratios of the total accumulation were 1.6 for hillside/coast and 1.25 for ridge/coast. Also displayed is the duration, which is the amount of time actually spent raining, expressed both in minutes and as a fraction of the total period. The ridge-top gauge recorded the highest fraction of time spent raining (84%).

5 (a) The characterisation of orographic rainfall (b) Figure 3. Tephigrams for (a) Case 1, 0000 UTC, 10 June 1994 and (b) Case 2, 1200 UTC, 11 June Figure 4. NOAA AVHRR IR satellite images for (a) Case 1, 1600 UTC, 10 June 1994 and (b) Case 2, 0600 UTC, 11 June Table 3 shows that Case 2 was characterised by an increase in duration closer to the ridge top, but with no statistically significant difference between the mean rain rates. Interestingly the ridge-top gauge recorded the greatest accumulations with the lowest average rain rate for the periods during which it was actually raining Cases 3, 4 and 5 The synoptic map for Case 3 (Figure 6(a)) shows a depression to the west of the area, as was the situation for Cases 1 and 2, but this time it was faster moving. The synoptic map 24 hours later (Figure 6(b)) for Cases 4 and 5 shows the passage of a depression across central 109

6 W R Gray and A W Seed Table 2. Rain gauge summary statistics for Case 1. Duration 900 minutes Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 502 (56) 638 (71) (84) 1.50 Total (mm) Table 3. Rain gauge summary statistics for Case 2. Duration 1072 minutes Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 256 (24) 648 (60) (74) 3.08 Total (mm) Table 4. Rain gauge summary statistics for Case 3. Duration 600 minutes Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 352 (59) 458 (76) (83) 1.41 Total (mm) Table 5. Rain gauge summary statistics for Case 4. Duration 360 minutes Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 244 (67) 288 (80) (92) 1.36 Total (mm) Table 6. Rain gauge summary statistics for Case 5. Duration 212 minutes Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 184 (87) 210 (100) (100) 1.15 Total (mm)

7 (a) The characterisation of orographic rainfall (b) Figure 5. Average rain rates (mm h 1 ) for (a) Case 1, as estimated from the C-band for the 900 minutes up to 1945 UTC, 10 June 1994, and (b) Case 2, as estimated from the C-band for the 1072 minutes up to 2200 UTC, 11 June The data was observed at 2 km resolution in range, with a maximum range of 240 km. Figure 6. MSL analyses for (a) Case 3, 0000 UTC 26 January 1995 and (b) Cases 4 and 5, 0000 UTC 27 January New Zealand. Case 3 covered the passage of an occluded front, with the tephigram showing a moist stable profile (Figure 7). The rain was associated with the embedded convective clouds seen in the satellite image (Figure 8(a)). Rain in Case 4 came from bands ahead of the cold front (Figure 8(b)), while the more stratiform rain of Case 5 came with the passage of the cold front itself (Figure 8(c)). Balloon sounding data for Case 5 show the atmosphere to be moist and not strongly unstable, with low-level winds up to 40 kn, backing with height (Figure 7). Rain gauge data in Tables 4 and 5 show small increases in both duration and intensity for both Cases 3 and 4. Despite this, the peak rainfall accumulations are only 16 and 12 mm respectively. In contrast, Table 6 for Case 5 shows a statistically significant increase in rain rate for the hill and mountain gauges. Indeed, the ridge-top rain rate is over six-fold that of the coastal gauge. Simultaneous with this increase in intensity came a small increase in the fraction of time spent raining. Note that it was raining at the coastal gauge for over 85% of the time, in marked contrast with the previous cases Case 6 Precipitation for Case 6 occurred with a 30 kn northwesterly flow ahead of a broad trough (Figure 9) which produced predominantly showery rain bands (Figure 10). Analysis of the tephigram suggests that the 111

8 W R Gray and A W Seed Figure 7. Tephigrams for 1200 UTC 26 January, 0000 UTC and 1200 UTC, 27 January atmosphere was unstable in the lowest 4 km (below 13 C). The satellite image (Figure 11) shows a band of showers approaching the Otaki region. The accumulated C-band scanning radar data (not shown) reveal that, in this case, there was little rainfall upstream 112 of the ranges, but considerably more over the hills. Rain gauge data from Case 6 (Table 7) show again a marked increase in the fraction of time spent raining as the rain traverses the hills.

9 The characterisation of orographic rainfall (a) Figure 9. MSL analysis for Case 6, 1200 UTC, 3 April (b) (c) Figure 10. Tephigram for Case 6, 0000 UTC, 5 April Figure 8. GMS IR satellite images for (a) Case 3, 0426 UTC, 26 January 1995, (b) Case 4, 1230 UTC, 26 January 1995 and (c) Case 5, 1930 UTC, 26 January Figure 11. GMS IR image for Case 6, 2332 UTC, 3 April

10 W R Gray and A W Seed Figure 12. MSL analysis for Case 7, 1200 UTC, 8 April Figure 14. GMS IR satellite image for Case 7, 2330 UTC, 8 April Table 8 for Case 7 shows rain gauge data more similar to that of Table 6 for Case 5. Noticeable here is the fact that it was raining only 54% of the time at the coast, increasing to 82% at the mountain gauge (a factor of 1.5) and that the rain rate increased by a factor of three High-resolution X-band radar data For Cases 1 5, the high-resolution X-band radar was operated in the vertical pointing mode and located amongst the hills. The surrounding hills were around 450 m in height, but the height of the radar was only 140 m asl as it was in a steep valley (1 km wide). For Cases 6 and 7, the radar was located on the coast, and generally scanned in a PPI mode over the hills. Figure 13. Tephigrams for Case 7, 1200 UTC, 8 April 1996 and 0000 UTC, 9 April Case 7 Rainfall for Case 7 occurred with the passage of a broad-scale frontal rainband (Figure 12). The atmosphere was moist and stable, with light 20 kn winds at low levels, but 40 kn north-westerlies at higher levels (Figure 13). The satellite image (Figure 14) shows a broad uniform cloud sheet covering the region of interest. The accumulated C-band scanning radar image (not shown) also reveals broad uniform rainfall, with no region being preferred. 114 Figure 15(a), from Case 1, shows the vertical profile of reflectivity during the convective showers that passed overhead. The average over the one-hour period shows little or no increase in reflectivity at the lowest levels. Similar profiles are seen for Cases 2, 3 and 4. In contrast, Figure 15(b) from Case 5 shows an increase in reflectivity down to a height of around 1 km, then little change below this height. For Case 6, the X-band high-resolution scanning data show the triggering of convection over the hills. Sequences of the 6-second images show that, as a few showers move from over the coast onto the hills, extra showers develop and propagate inland (Figure 16). Case 7 was characterised by widespread precipitation. Figure 17 shows the very uniform nature of the rainfall, with no preferred areas not even over the ranges. This is consistent with the hypothesis that the near threefold enhancement in rain rate is occurring at low levels and the radar scanning at 6 overshoots this enhancement. This is consistent with this case being classified as a seeder/feeder event.

11 Table 7. Rain gauge summary statistics for Case 6. Duration 720 minutes The characterisation of orographic rainfall Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 170 (24) 358 (51) (84) 3.42 Total (mm) Table 8. Rain gauge summary statistics for Case 7. Duration 600 minutes Measure Coast Hillside Ratio of hillside Ridge Ratio of ridge Mean (mm h 1 ) Standard error (mm h 1 ) Median (mm h 1 ) Standard deviation (mm h 1 ) Duration (min (%)) 326 (54) 442 (75) (82) 1.52 Total (mm) Discussion From the above results, some conclusions can be drawn as to the likely enhancement process occurring during these events. Cases 1 and 2 were dominated by an increase in duration, but with no significant change in the mean rain rate from the coast to the hills. This leads to the conclusion that the enhancement resulted from triggered convection. Indeed, there was no evidence of the characteristic seeder/feeder vertical profile in the vertical pointing radar data (e.g., Figure 15). The tephigrams (Figure 3) showed no strong instability that could have been triggered by the airflow over the hills. However, the extensive areas of convection seen on the satellite imagery (Figure 4) indicate that the tephigrams may not be representative of that case. Cases 3 and 4 were characterised by small increases in both the duration and rain rate. No clear conclusion was drawn as to the enhancement mechanism from the gauge data alone. However, the vertical pointing radar data show no evidence of the characteristic profile for the seeder/feeder mechanism and therefore it is likely that the enhancement occurred through triggered convection. The moist neutral profiles shown in the tephigrams (Figure 7) would not be inconsistent with this enhancement process. Case 5 had enhancement of a factor of seven, primarily through an increase in the rain rate. The C-band scanning radar observed only around 25 30% of the accumulations observed by gauges. This evidence, plus the seeder/feeder-type vertical profile (Figure 15(b)) suggest that seeder/feeder was the operative mechanism. Note that the seeder/feeder enhancement appeared not to continue all the way down to the ground. This is assumed to be a result of the subsaturation at low levels (Figure 7). Case 6 was similar to that of Cases 1 and 2 and was dominated by a two-fold increase in duration. Hence, the mechanism is likely to be triggered convection. This is most clearly shown in the high resolution scanning X-band radar data (Figure 16). The tephigram for this case is difficult to interpret (Figure 10), but it is likely that any increase in humidity in the lowest layers would have lead to convection up to around 15 ºC. Case 7 had a three-fold increase in rain rate from a broad-scale rain band. The tephigram shows a moist but stable atmosphere (Figure 13). Enhancement here was most likely from seeder/feeder. One of the assumptions we have made in the analysis presented here is that triggered convection primarily increases the fraction of time spent raining, and seeder/feeder primarily acts to increase the rain rate. However, this does not preclude the possibility of both mechanisms operating at the same time thus increasing both intensity and duration. In Case 4 there were significant increases in both intensity and duration. Obviously, the fraction of time spent raining can only increase till it reaches 100%. Further increases in total rainfall must then come from increases in mean rain rate. A more sophisticated assessment of the process leading to enhancement may be needed at this point. If there were an increase in general rain rate from near 0 mm h 1 to, say, 1 mm h 1, but also a triggering of embedded convection, then the simple summary 115

12 W R Gray and A W Seed Figure 15. Data from the vertical pointing radar for (a) Case 1, the period around 0000 NZST, 11 June 1994 and (b) Case 5, the period around 0700 NZST, 27 January Left: reflectivity as a function of time and height; right: average reflectivity as a function of height. 116

13 The characterisation of orographic rainfall Figure 16. PPI maps of X-band radar reflectivity at 15-minute intervals for Case 6, the period from 1330 UTC, 4 April The elevation angle was 6. The maximum range that reflectivity is recorded is 22.5 km. Figure 17. PPI map of X-band radar reflectivity for Case 7, 0545 UTC, 9 April The elevation angle was 6. The maximum range that reflectivity is recorded is 22.5 km. 117

14 W R Gray and A W Seed statistics would show an increase in both the intensity and duration. An analysis such as that presented by Harris et al. (1996), in which the intermittency of the rainfall is prescribed through fractal analysis may be able to show up this embedded convection. Note that convective cells will not occur back-to-back as the occurrence of convection increases there will come a point at which the cells start to interact, and hence begin to limit the growth of each other. Using the rain gauge data to indicate the enhancement process was only possible because the time resolution of the gauges was less than the time-scale of the passage of convective showers. If only hourly totals were available, then much of the increase in the fraction of time spent raining would have been translated into rainfall intensity. For example, in Case 2 the 120 s data showed that the mean rain rate in the hills was 83% of the coastal rate, while the time spent raining was 2.5 times that at the coast. If the data were degraded to hourly resolution, the rain rate in the hills would have been 340% of the coastal rate while the duration would have been identical at the coast and in the hills. Without the high-resolution data, the distinction between seeder/feeder and triggered convection could not be made. The summary statistics were quite a useful way of interpreting the gauge data, especially as it was often difficult to interpret the raw gauge time-series records. This was particularly so for the convective events, as the small-scale nature of the convection meant that the same rain cell did not necessarily pass over each gauge in the transect. However, summarising the data in the fashion of Section 4 enabled a distinction to be made between the likely enhancement processes. In the analysis of the gauge data, a rain/no rain threshold of 0 mm h 1 was used to define the duration of the rain. Another threshold could have been chosen to give the duration of heavy rain, and this could then have been used as an indicator of convection. Certainly, choosing another threshold, e.g., 1 mm h 1, would not be unreasonable, but would not alter the conclusions significantly. From the contrast between Cases 1 and 2, it is clearly necessary to have measured the distribution of the rain parallel to the mountain range in order to be able to assess the representativeness of the gauge transect. In Case 2, the rainfall accumulation had modest gradients parallel to the mountains, so the change in rainfall characteristics normal to the range can be estimated with confidence. In Case 1, however, the variability parallel to the range was of the same order of magnitude as that normal to the range confounding the interpretation of the gauge data. This demonstrates the difficulty in using rain gauges alone to monitor the rainfall variability in both the transverse and along transect directions with sufficient resolution. This is particularly true 118 when attempting an event-by-event analysis of the rainfall. It is clear that the accumulated scanning radar data was effective in determining that the gauge array was representative of the rainfall distribution. One of the features of the triggered convection was that it tended to occur only during the passage of previously existing showers. Figure 16 shows that before and after the passage of the intense convective shower no rainfall was occurring over the hills. As showers pass, convection is triggered over the hills, which then moves off, with no more developing behind it. This would be consistent with the concept that the showers occurring upwind were marking areas of instability, and that this was further released with the passage of this air over the hill. The combined rain gauge and radar data sets showed that the patches of rain were not enhanced in any simple fashion, but that existing areas were enlarged, rain rates within patches could increase, and new areas were formed over the hills. These phenomena highlight the fact that understanding orographic enhancement is not simple. Indeed, if radar were to be used to increase the lead-time for flood forecasting, then it would be necessary to forecast, in advance, the manner in which the enhancement will be manifested. The information from balloon soundings may be useful in delineating the likely enhancement process. Figure 10 shows that the atmosphere was unstable in Case 6, and hence consistent with triggered convection being the preferred enhancement mechanism. In contrast, Figure 7 for the seeder/feeder case showed a stable but moist atmosphere. However, care must be taken in the interpretation of the profiles. The drier layer in the lowest levels of the Case 5 sounding appeared to result in less enhancement than might have resulted from a saturated sounding. Cases 1 and 2, however, had soundings that appeared stable while triggered convection was enhancing rain. Obviously, soundings will not provide a fail-proof method of forecasting enhancement. Where soundings are not available, numerical model output may be useful, through inspection of the temperature and humidity profiles, the model vertical motion fields and from the triggering of the model s convective parameterisation schemes. One clue that may also aid in the forecasting of the enhancement process is the horizontal scale of the precipitation upwind of the hills. It was seen that the enhancement for the seeder/feeder mechanism tended to come from broad-scale rainfall (durations near 100%). Showers upwind were matched by triggered convection as the enhancement process. This could be a very useful tool in developing a flood-forecasting scheme. If it was possible to forecast the enhancement process from the horizontal structure of the precipita-

15 The characterisation of orographic rainfall tion upwind of the hills, then a scheme could be developed that used just one source of data, without the overheads and complexities of combining data of different types. Care must be taken when generalising these results. The events resulted in totals that were far from exceptional. For example, the events analysed by Austin & Gray (1993) had hill-top totals of 300 mm. Intense events leading to hill-top totals of 500 mm have been recorded (WRC, 1997, 1998). Inspection of 15 years of river flow data (not presented here) suggests that the events examined in this study represent the commonly occurring events, but may not be representative of the flood-producing events. On the basis of the data presented here, it is likely, but not proven that, as seeder/feeder gave the highest accumulations here, the flooding events would also result from the seeder/feeder mechanism. 6. Conclusions Seven cases from three experiments at three different times of the year were studied. High-resolution rain gauge and radar data were analysed and the orographic enhancement mechanism determined. From the results of these experiments, it is evident that the most commonly occurring enhancement mechanism was triggered convection. It occurred in five of the seven cases and was the mechanism operating for the longest periods. Triggered convection manifested itself as an increase in the fraction of time spent raining. Seeder/feeder processes occurred only for short periods of time but led to the highest enhancements, of up to a factor of seven. This enhancement was manifested primarily through an increase in the rain rate. Small increases were made in the fraction of time spent raining, but this was limited as the threshold of 100% was reached. It was notable that the processes could change abruptly. The seeder/feeder events followed periods of triggered convection, but lasted for only short periods. This implies that it is necessary to forecast the enhancement process in a radar-based flood-forecasting scheme. Although seven events from three experimental campaigns were studied, they were not extreme events. Future work aiming at flood forecasting will need to explicitly target extreme events. Future work may also focus on the utility of the horizontal scale as a predictor of the enhancement mechanism. Acknowledgements The authors would like to acknowledge the support of MetService (NZ) Ltd in providing the radar and satellite data. We would like to thank the Wellington Regional Council for their support, particularly during the field experiments. We would also like to thank Leif, Conrad, Joanne, Roger and the Seed family for their work in the field. Thanks to Professor Austin, Dr David Wratt and Dr Mike Uddstrom for the supervisory support. Thanks also to Dr Howard Larsen, for the many discussions and the extensive proof-reading of the script. The work undertaken by Dr Gray was funded under the Foundation for Research Science and Technology Grant no. CO1227. References Bergeron, T. (1965). On the low-level redistribution of atmospheric water caused by orography. Supp. Proc. Int. Conf. Cloud Physics, IAMAP/WMO Tokyo, Gray, W. R. & Austin, G. L. (1993). Rainfall estimation by radar for the Otaki Catchment: The OPERA pilot study. J. Hydrol., 31: Harris, D., Menabde, M., Seed, A.W. & Austin, G. L. (1996). Multifractal characterization of rainfields with a strong orographic influence. J. Geophys. Res., 101: Hill, F. F., Browning, K. A. & Bader, M. J. (1981). Radar and rain gauge observations of orographic rain over South Wales. Q. J. R.. Meteorol. Soc., 107: Hosking, J. G., C. D., Stow, S. G., Bradley & Gray, W. R. (1986). An improved high-resolution rain gauge. J. Atmos. Oceanic Technol., 3: Kitchen, M. & Blackall, R. M. (1992). Orographic rainfall over low hills and associated corrections to radar measurements. J. Hydrol., 139: Robichaud, A. J. & Austin, G. L. (1988). On the modelling of warm orographic rain by the seeder-feeder mechanism. Q. J. R.. Meteorol. Soc., 114: Seed, A. W, Nicol, J., Austin, G. L., Stow, C. D. & Bradley, S. G. (1996), The impact of radar and raingauge sampling errors when calibrating a weather radar. Meteorol. Appl., 3: Stow, C. D., Bradley, S. G., Farrington, K. E., Dirks, K. N. & Gray, W. R. (1998). A rain gauge for the measurement of finescale temporal variations. J. Atmos. Oceanic Technol., 15: Part 1, Thompson, C. S., Sinclair, M. R. & Gray, W. R. (1997). Estimating long-term annual precipitation in a mountainous region from a diagnostic model. Int. J. Climatol., 17: Uddstrom, M. J & Gray, W. R. (1996). Satellite cloud classification and rain rate estimation using multispectral radiances and measures of spatial texture. J. Appl. Meteorol., 35: WRC. (1993). Meteorology and hydrology of the November 1993 flood report. Report Prepared by the Wellington Regional Council, WRC/CI/t-93/79. WRC. (1997). Meteorology and hydrology of the 4-5 October 1997 flood. Report Prepared by the Wellington Regional Council, WRC/RINV-T-97/56. WRC. (1998). Meteorology and hydrology of the October 1998 and October 1998 floods. Report Prepared by the Wellington Regional Council, WRC/RINV-T-98/

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