Evidence of Mesoscale Lows off the West Coast of New Zealand

Transcription

1 369 Evidence of Mesoscale Lows off the West Coast of New Zealand ANDREW K. LAING AND STEVE J. REID National Institute of Water and Atmospheric Research, Wellington, New Zealand (Manuscript received 29 July 1998, in final form 26 January 1999) ABSTRACT Mesoscale lows are observed off the central west coast, South Island, New Zealand. These features are evidenced in pressure dips measured at a buoy moored about 50 km off the west coast. They do not appear in measurements from coastal stations. Wind data at the buoy show complex variations sometimes consistent with the passage of cyclonic vortices in the flow. Further, wind data from satellite scatterometers often indicate high spatial variability near the buoy at the time of these pressure dips, and satellite imagery depicts cloud movements consistent with small-scale cyclonic circulations near the coast. These features are often observed in the cool air after the passage of a trough or front. 1. Introduction The weather of west coast, South Island, New Zealand (for location see Fig. 1), has some of the most significant extremes in the country. The Southern Alps rise rapidly from the coast to m, although in some places there is a narrow coastal plain. The interaction of the mountain with the airstreams from the westerly quarter causes annual rainfalls of over 10 m in some places. On the plains adjoining the coast itself, annual rainfalls are about 2 3 m and are probably lower over the sea. The west coast region lies at latitudes south of continental Australia and has a wave climate that is directly influenced by the conditions over the Southern Ocean. Average wave heights are higher than in most other parts of New Zealand and, in storms, very high sea waves are sometimes observed. The present study is part of a project to examine past storms and the wave heights attained at the coast. Some historical wave measurement data are available for a number of locations on the west coast. These cover periods of about 5 years. Significant wave heights of over 6 m were recorded, and Laing et al. (1997a) have simulated many of the events in a numerical wave hindcast model, with considerable skill. However, for some events the hindcasts have overestimated the wave heights. Although examination of the weather charts has confirmed that high waves would be expected in these cases, wind data from a meteorological buoy and re- Corresponding author address: Dr. Andrew K. Laing, National Institute of Water and Atmospheric Research, 301 Evans Bay Parade, Greta Point, P.O. Box , Wellington, New Zealand. a.laing@niwa.cri.nz analysis of other data show that the wave height is affected by subtle changes in critical fetches close to the coast (Laing and Reid 1998). This work has led directly to the present paper in which we investigate the nature of mesoscale weather systems in the area. Carleton and Song (1997) have used satellite imagery for 1992 to study the occurrence of mesoscale lows, with diameters as small as 200 km, in the Australasian sector of the Southern Ocean. The lows frequently develop in cold airstreams behind larger frontal cyclones. The lows did occur near New Zealand, normally traveling toward the northeast with a speed averaging about 50 km h 1 and a duration of about 35 h. However, it will become apparent that this type of feature is different from the lows discussed in this paper, principally because of scale. In order to explain our observations the lows need to be much smaller in diameter and, probably, much shorter lived. They also are shown to have a strong relationship with the orography. Disturbances in which orography is a key factor have been discussed by a number of authors. Levinson and Banta (1998) have shown that the flow approaching the Oregon coast was significantly modified by the mountains and the land sea temperature gradient. Using lidar measurements, they showed that large changes of speed could occur over a spatial domain with dimensions of only a few kilometers. Sometimes these changes were relatively fixed in space and sometimes transitory. Buoy measurements (for example, Nuss 1998) along the U.S. west coast show propagating wind reversals. These are most readily apparent in time sequences from buoys. There is evidence that some of these features may be associated with mesoscale lows close to the coast (Thompson and Bane 1998). There is also evidence from previous analysis of sat American Meteorological Society

2 370 WEATHER AND FORECASTING VOLUME 14 FIG. 1. Map showing the study area and measurement sites. ellite data. Cloud forms in satellite imagery indicate the possible development of atmospheric vortices in the present region of study, and one occurrence has been described by McGregor et al. (1992). Inspection of periods of satellite imagery suggests that these developments may be quite common. Also wind scatterometer data show that coastal areas host small but significant wind features. Where nearby orography influences wind patterns the scatterometer data have been shown to resolve detail (Laing and Brenstrum 1996) and have provided confirmation of wind features predicted with mesoscale models (Laing et al. 1997b). In this paper, we use wind data and imagery from satellites, and meteorological observations from Hokitika and a buoy moored off the west coast of South Island (Fig. 1), to observe and characterize mesoscale weather features in these coastal waters. We make particular use of data during the period in which both buoy data and National Aeronautics and Space Administration (NASA) scatterometer (NSCAT) winds (Bourassa et al. 1997) were available. In this way, we are able to match time sequences of data from the buoy and Hokitika with the spatial distributions of winds from NSCAT. The buoy observations are described in section 2 followed by the wind data from satellites in section 3. In section 4 we characterize conditions during events identified within the study period and in section 5 we look more closely at a number of specific cases. A discussion follows in section Buoy data The meteorological buoy is a 3-m diameter discus buoy of U.S. design that was stationed at S, E. This position is on the continental shelf off the west coast, South Island, about 50 km from the coast at its nearest point and about 70 km west of Hokitika, a major meteorological station. The buoy has been deployed at much the same position for substantial periods during the 4 yr During the buoy deployments, wind speeds and directions at 4 m above sea level, barometric pressure, solar radiation, sea temperature, and air temperature were obtained at 1-h intervals. Overall deployment time has been about 1.5 yr but sensor failures have meant that data records did not cover the full period. Wind was measured using two R. M. Young model wind monitors mounted at around 4 m above sea level. A small adjustment was made to obtain the speed at 10 m (an increase of about 8% and increasing slightly with wind speed) and another correction was applied to obtain a speed equivalent to a scalar horizontal wind speed. The pressure sensor was a Setra model 270. In the 1996 and 1997 deployments a systematic error appeared in the data from this instrument and in this paper the values have been increased by 2 hpa. The sea and air temperature data from the buoy are also used in this study. Calibration checks on the data showed that the sea temperatures during the deployment in 1997 were 0.4 C too high and accordingly the data have been adjusted downward by this amount. Mean speeds at the buoy position for each deployment are in the range 6 7 m s 1, approximately double those at Hokitika. A wind rose for the full deployment period of the buoy is given in Fig. 2. It shows that two directions occur more frequently than other directions: southwest and northeast (see also Stanton 1998), in alignment with the mountain barrier. The wind rose for the same period at Hokitika is also shown. The latter is markedly different from the buoy distribution with predominant wind directions from the east and west and many fewer high winds. The pronounced peak of wind directions from the east and southeast at Hokitika has most speeds below 4 m s 1 and can be attributed to density-driven cool flows off the mountains. The winds at the buoy and at the coast are thus both affected by the mountain barrier. In periods of strong winds, analyses of time series show that often there is coherence between the winds. A frequent feature in the buoy data, when it is compared with simultaneous data from Hokitika, is that from time to time the barometric pressure at the buoy departs from its trend over the previous few hours and falls below the pressure at Hokitika. One such case is shown in Fig. 3. A few other differences are apparent but they lack the magnitude and consistency of the major drop at 0400 UTC on 25 September 1994, which has a magnitude of about 5 hpa. There was no apparent malfunction of the buoy, sensor, or logger. At the sudden fall of pressure apparent in Fig. 3, the recorded temperature parameters showed no sudden change. However, during the hour of the pressure drop other major

3 371 FIG. 2. Wind roses showing the wind climate at (a) the buoy and (b) Hokitika, for the period of the buoy deployments off the west coast of South Island. The width and shading for each segment indicate the wind speed, and the length of each segment measures the percentage occurrence of each wind speed range. The number at the center of each rose is the percentage occurrence of calm and variable conditions. changes occurred, notably a large rotation of the buoy and a fall of the mean wind speed from about 8 m s 1 at the hour before the change to below 2 m s 1 at the hour after the change. Over the same time, the wind direction changed from the southeast to the northwest. As has been found in other events, the barometric pressure at Hokitika indicated the passage of a trough just before (about 4hinthis case) the pressure dip at the buoy. Wind speeds were generally light and variable over the period, especially at Hokitika. Full time series of the wind and pressure data during periods being discussed here are presented in Fig. 4. The pressure difference between the buoy and Hokitika is plotted, so as to emphasize the pressure dips. Wind speed and direction at the buoy and the Hokitika pressure are also plotted. There is a great deal of fluctuation in the pressure difference in Fig 4. In the cases of high wind the difference may be caused by the broad-scale pressure gradient. But, because the distance over which the observed

4 372 WEATHER AND FORECASTING VOLUME 14 FIG. 3. Example of pressure dip at buoy during 25 Sep (a) The pressure at Hokitika (solid line) and the buoy (dashed line). (b) Wind vectors at the buoy (upper) and Hokitika (lower). Each minor tick on the time axis indicates 6 h. pressure difference exists is only 70 km and surface winds are rarely more than 15 m s 1, it is difficult to explain differences of over 2 hpa as purely geostrophic effects. However, these effects may be explained by cyclonic curvature of the wind field. This mechanism increases the pressure difference needed for a dynamic balance and would admit pressure differences with the observed magnitudes. One possible cause of wind turning (curvature) that has been hypothesized in this region (see, e.g., Smith 1982) is a pressure ridge upstream of the Southern Alps (often accompanied by a barrier jet) and some of the longer-lasting events may be due to this. Nevertheless, most of the pressure dips in Fig. 4 appear to be quite short lived and suggest small-scale lows moving over the buoy. The largest observed magnitude was 8.1 hpa on 22 October 1996 and pressure differences over 5 hpa occurred 32 times in the period of record. Most of the pressure dips at the buoy occur around the same times as pressure troughs, usually a few hours after. This observation has been used to develop criteria with which to select events for further study. Specifically, events were identified in which the buoy pressure was more than 4 hpa below the Hokitika pressure and the latter parameter was more than 6 hpa below the pressures 36 h before and after. Of the 32 events with

5 373 FIG. 4. Pressure and wind data for the two study periods during which the buoy and NSCAT data overlapped: (a) Oct Nov 1996; (b) Apr Jun The time series show (from top to bottom) the surface pressure at Hokitika, the pressure difference between the buoy and Hokitika (Buoy Hokitika), the wind speed at the buoy, the wind direction at the buoy.

6 374 WEATHER AND FORECASTING VOLUME 14 pressure dips over 5 hpa, 18 met the last criterion. A further seven cases occurred in southwesterly conditions, but well after the main trough had passed, and the remaining cases occurred in northeasterly conditions. The only periods in which no large pressure dips are apparent tend to be periods without high winds or pressure troughs. The pressure dips usually last a few hours, but may persist for several days. 3. Satellite wind data The scatterometer (NSCAT) carried on the ADEOS satellite during the period September 1996 to June 1997 scanned the earth s surface in twin swaths, each 600 km wide. Although the satellite mission was short lived, the scatterometer covered 90% of the world s oceans in any 2-day period, providing good opportunities for capturing a particular wind feature near a given location. The European Remote-Sensing Satellite (ERS) scatterometers have been operating since However, coverage is in single 500-km swaths, only 40% of that from NSCAT. Hence, to optimize the coincidence of scatterometer coverage with deployments of the meteorological buoy off the west coast, South Island (see section 2), we have used the NSCAT data for this study. A swath from the NSCAT scatterometer is shown for a typical southwest airflow in Fig. 5. For the period overlapping the buoy deployments all NSCAT winds over waters immediately west of South Island were extracted from the high-resolution merged geophysical data products supplied by the Jet Propulsion Laboratory Physical Oceanography Distributed Active Archive Center (JPL PO DAAC). Up to two passes per day were available, normally at UTC and UTC, respectively. There were some gaps in data recovery and not all of these passes directly covered the area of the buoy, but nearly 75% of possible passes provided some data. We note that scatterometer data do not return winds immediately adjacent to the coast. The scatterometer receiver was gated so as to sample resolution cells with 25-km cross-track spacing. Since the instrument s fore and aft beams were oriented at 45 and 135 azimuth relative to the swath direction, the effective cell length along beam was 37 km. Further, to avoid land contamination through antenna side lobes, cell dimensions are expanded 30% before applying the land-flagging algorithm (S. Dunbar 1998, personal communication). Hence, there are no scatterometer data within the 50 km between the buoy and the coast. All NSCAT data are presented as neutral-equivalent 10-m winds. Comparisons with buoy winds Consistency between buoy and scatterometer winds was first checked. The process of wind retrieval from the scatterometer identifies a number of possible wind vector solutions (ambiguities) for each resolution cell. These are ranked according to the maximum likelihood estimate (MLE) probability. An ambiguity removal algorithm is then applied that selects a solution based on geophysical consistency over the swath using a vector median filter. However, in coastal waters with smallscale variations in wind direction these solutions may not be the most appropriate. Thus, we have considered three options in selecting the NSCAT wind solution for each resolution cell: the highest-ranked MLE, the solution selected by the ambiguity removal algorithm, and the solution with the wind direction that matches best the direction measured at the buoy. Comparing the wind speeds at the buoy with those at the nearest NSCAT cell results in good agreement, both with the algorithm-selected solution and the top-ranked MLE solution. In 95 out of 167 cases these solutions were the same anyway. The correlation coefficients for the comparisons were 0.89 and 0.90, respectively, with a slightly low bias in the NSCAT winds of 0.07 m s 1 and 0.15 m s 1, respectively. The average distance of the center of a resolution cell from the buoy was 16.5 km with a maximum of 50 km. In 30 of the cases the distance was greater than 25 km. The directions provide more interesting comparisons, as it is here that the ambiguities differ most. It was expected that the vector median filter might override small-scale variations in wind direction associated with mesoscale features. The differences between directions from buoy measurements and NSCAT for both the algorithm-selected and best-matching wind solutions are plotted in Fig. 6. It is evident that the NSCAT ambiguity removal algorithm does a reasonable job (Fig. 6a). However, it is also clear that there are valid NSCAT solutions that are closer to the buoy wind direction (Fig. 6b), especially for northeasterly winds. The implication is that winds from this sector are spatially less consistent in direction than winds from the southwest, hence providing more problems for the ambiguity removal algorithm. Considering the directional differences for the best matching NSCAT solution as a function of wind speed reveals that above 7.5 m s 1 there are few directional differences greater than 20. Thus, the discrepancies are mostly for light winds. 4. Wind characteristics during pressure events In section 2 we introduced the phenomenon of pressure dips at the meteorological buoy relative to Hokitika on the coast (Fig. 3). The point data at the two sites describe this meteorological condition but give limited insight into spatial characteristics of the associated meteorological features. Here the NSCAT data provide another perspective, notwithstanding two limiting factors. The first is the lack of scatterometer winds close to the coast, especially between the buoy and Hokitika. The second is that although the NSCAT data provide estimates of wind characteristics in 25-km resolution cells, rapid directional changes in wind will challenge the res-

7 375 FIG. 5. Typical south-southwest flow pattern over study area for ADEOS pass at 2300 UTC 31 May 1997 (small wind barbs). European Centre for Medium-Range Weather Forecasts (ECMWF) surface pressure isobars and 10-m wind fields (large wind barbs) for 0000 UTC 1 Jun 1997 are also shown. Crosses mark positions of the buoy and Hokitika. olution of the system and the ability of the wind retrieval algorithms to unambiguously select the most appropriate wind solution. It is important to check that the agreement between NSCAT and buoy winds established in section 3 is consistent during these pressure phenomena. Hence, the pressure difference between Hokitika and the buoy is introduced as a discriminator in the comparison. In general the spread in directional discrepancy between buoy and NSCAT decreases as the pressure difference increases. If the comparisons are limited to those where the pressure difference exceeds 4 hpa and suitable NSCAT data are available (27 cases), there is at least as much agreement between NSCAT solutions and buoy winds as in the more general comparisons shown in Figs. 6a and 6b. With southwesterly winds at the buoy the algorithm-selected solutions show good agreement with the buoy, but this is not always the case for easterly

8 376 WEATHER AND FORECASTING VOLUME 14 FIG. 7. Spatial variability of the wind during the selected events, showing the standard deviation of the NSCAT wind direction within 100 km of the buoy plotted against the buoy wind direction and the pressure drop between Hokitika and the buoy. FIG. 6. Differences between buoy and NSCAT wind directions as a function of wind direction at the buoy. The satellite-derived wind is for the nearest NSCAT cell to buoy during each coincident pass using (a) the wind direction from NSCAT selected by the ambiguity removal algorithm and (b) the ambiguity that is closest to the wind direction measured at the buoy. events. However, for these latter events, where the difference exceeds 45 there is usually another, closer NSCAT solution (the best-matching solution). This typically shows only small deviation from the buoy wind direction, rarely exceeding 20. Calculating the local variance of the NSCAT directions provides a useful measure of the spatial variability. For this purpose all NSCAT data within 100 km of the buoy were used. The mean direction for these NSCAT data was found and the standard deviation was calculated from the departures within the range 180 to 180. The selection of events was now limited to those in which the peak pressure difference exceeded 4 hpa, a significant pressure trough had just passed through, and the scatterometer pass was within 3hofthepeak of the pressure difference. A couple of exceptions were made where large pressure differences occurred in northeasterly conditions. The resulting 22 events are plotted in Fig. 7 where, for each NSCAT wind, the vector selected by the ambiguity removal algorithm is used. It is evident from Fig. 7 that the southwesterly cases occur in quite uniform wind flows. The variations are low, all below 9. The northeasterly cases, however, exhibit large variations, between 8 and 36. If a mesoscale circulation is centered between the buoy and the coast in a southwesterly flow, the NSCAT data would not register any wind variation. The local variability would be low and the wind at the buoy would remain southwesterly. If such a feature were north of the buoy, then the wind at the buoy would be easterly, even though the general synoptic flow could be southerly or southwesterly. Here the feature would register with high local variability in the NSCAT data. The spatial variance may therefore have potential as a detector of mesoscale weather systems. 5. Case studies The event selection in section 2 identified a number of events in which dips in pressure occurred at the buoy and section 4 established some characteristics of the local wind variability during these events. We now describe some of these cases in more detail. The cases have been selected on the basis of availability of NSCAT imagery coincident with the pressure dips. However, the case with the largest pressure dip has been selected because of the clear evidence of a circulation available in satellite cloud imagery.

9 377 FIG. 8. Data for case a, 15 Oct 1996: (a) pressures at the buoy (dashed line) and Hokitika (solid line), (b) winds at the buoy (upper) and Hokitika (lower), (c) sea air temperature difference at the buoy, (d) GMS infrared imagery for 1600 UTC 15 Oct with positions of buoy and Hokitika marked (black crosses), (e) NSCAT wind data for 1200 UTC 15 Oct with surface pressure isobars from ECMWF analysis at 1200 UTC. The shaft of the wind barbs indicate the wind direction, each full barb equals 10 kt (5 m s 1 ) and each half barb is 5 kt (2.5 m s 1 ). The vertical lines in (a) (c) mark the times of the image in (d) and NSCAT pass in (e). a. 15 October 1996 A major low passed over the South Island during 13 October. Following this, a southwesterly flow became established over the area on 14 October. After the cool change, convective cloud cleared from the area near the west coast but persisted offshore. A cloud band moved northeast along the west coast passing Hokitika at about 0800 UTC on 15 October. A pressure dip developed at the buoy at about 0900 UTC on 15 October (Fig. 8a) in the air behind the cloud band, and reached its maximum pressure difference of 5.0 hpa at 1100 UTC. The Hokitika measurement of the 800-hPa wind at 1200 UTC was 13 m s 1 from 225 true. Figure 8a shows that the pressure dip on 15 October was preceded by dips of shorter duration on 14 October and there was an upward perturbation of pressure late on 14 October at the buoy. Winds at the buoy were from the southwest for most of the period, generally decreasing in strength (Fig. 8b). However, the period of the pressure dip was marked by an increase in wind strength and counterclockwise turning of wind direction, consistent with a low moving northward to the east of the buoy. At Hokitika a period of winds from the north about 0000 UTC on 16 October may also be related to the passage of a small low. The maximum in sea air temperature difference (Fig. 8c) coincided with a drop in air temperature associated with a clearing of cloud and easing of winds in the cool southwesterly flow. Figure 8d shows imagery from the 12- m infrared channel of the Geostationary Meteorological Satellite (GMS) during this event. The arrow marks the position of what appears to be a small feature between Hokitika and the buoy. Hourly images were available, and looping through these images indicates northeasterly winds

10 378 WEATHER AND FORECASTING VOLUME 14 over the coast to the east of the arrow and south to southwest winds to the west. Figure 8e shows the spatial distribution of the southwesterly flow in NSCAT data. In this case there was only a low standard deviation in wind direction near the buoy (4.0 ). A cyclonic feature east of the buoy at the time of the NSCAT pass is possible. b. 22 October 1996 A trough crossed South Island late on 21 October and early on 22 October. A cold surge up the west coast was again identified by open cell convection in GMS imagery. The surge reached Hokitika at 2200 UTC on 21 October. Subsequently the area to the south cleared while convective cloud persisted offshore. The development of a small protuberance of cloud extending offshore from Haast (for location see Fig. 1) is evident in the GMS visible imagery (Fig. 9d). The subsequent evolution of this cloud feature can be traced as it moved into a definite cyclonic circulation and extended north over the buoy. At this time (about 0400 UTC on 22 October) a pressure dip developed at the buoy (Fig. 9a) and reached its maximum pressure difference of 8.1 hpa at 0700 UTC. The 800-hPa wind at Hokitika for 0000 UTC was from 245 true at 12.4 m s 1 and at the buoy was 15 m s 1. However, by 0800 UTC the buoy wind had swung around to a 15 m s 1 easterly (Fig. 9b). At 1200 UTC both the surface wind at the buoy and the 800-hPa wind at Hokitika were about 5ms 1 from the northeast. The sea air temperature at the buoy (Fig. 9c) also shows a strong maximum within 2hofthemaximum pressure dip at the buoy. This feature was the most pronounced in the data used. It displayed the largest pressure difference and clearest satellite imagery of a vortex. The GMS cloud imagery in Fig. 9d shows clear evidence of the developing system at 0300 UTC. A loop over subsequent hours showed a clockwise-rotating system, and in Fig. 9e, at 0700 UTC, the National Oceanic and Atmospheric Administration-12 (NOAA-12) 12- m infrared imagery provides a clear picture of the low cloud system centered over the buoy position. Unfortunately there were no scatterometer wind data available. Subsequently, quite a significant, small, low developed and moved north into the eastern Tasman Sea off central New Zealand. c. 31 October 1996 This case is typical of those in which the wind at the buoy becomes variable after a front but the general flow offshore remains southwesterly. The wind and pressure data from the buoy and Hokitika are shown in Figs. 10a,b. The main front went through just before 1800 UTC on 30 October and there was a maximum difference in pressure between the buoy and Hokitika of 5.7 hpa at about 1300 UTC on the 31 October. The 1200 UTC 800-hPa wind at Hokitika was from 230 true at 10.8 m s 1. The strong surface southwesterlies following the front suddenly eased to light southeasterlies at the time of the dip in the pressure at the buoy. In this case surface wind data were also available from Haast (see Fig. 1) and show a southeasterly of up to 6 m s 1 developing at 1500 UTC on 31 October. A further characteristic of this event is, again, a substantial sea air temperature difference at the buoy (Fig. 10c). A loop of hourly imagery from GMS clearly shows a small cloud feature developing near the coast, to the east of the buoy (marked with the arrow in Fig. 10d). Although the NSCAT data for 1100 UTC (Fig. 10e) exhibit a high standard deviation in direction (16.3 ) near the buoy they do not clearly show any wind feature. However, the scatter in wind direction indicates some uncertainty in the wind solution chosen by the ambiguity removal algorithm, and closer inspection of the data indicates that other ambiguities exist, which would be consistent with the imagery. Figure 10d shows the NSCAT swath with an alternative selection of wind ambiguities near the buoy, agreeing better with the buoy winds and indicating curvature in the low-level wind field. d. 21 May 1997 A trough crossed South Island on 20 May Before the passage of the weather system, northerlies of up to 14 m s 1 were observed at the buoy with northeasterlies at Hokitika. After the passage of the trough there was a period of southwesterlies with generally rising pressures. At this time, a slow-moving low became established about 1000 km northwest of Hokitika. It persisted with about the same central pressure for the three days May, bringing a northeast flow over the west coast. Also during this period there was a strong flow forced through Cook Strait between North Island and South Island. This flow extended around the northwest corner of South Island where it reoriented to northeasterly, flowing approximately in alignment with the coast. The area was under heavy cloud cover for the duration of this event, and so satellite imagery was not useful. During 21 May the pressure dipped at the buoy, reaching a maximum difference of 7.5 hpa at 0900 UTC (the second largest pressure difference in the data used). Figs 11a,b show the wind and pressure data for the buoy and Hokitika. A longer wind record (not shown) displays a hint of periodicity in the wind direction, with occasional directional shifts to south of east. Speeds at the buoy were about 14 m s 1 at the time of the maximum pressure difference. At Hokitika, the 800-hPa wind at 0000 UTC on 21 October was 4.6 m s 1 from 15 and at 1200 UTC was 8.8 m s 1 from much the same direction. Again the positive sea air temperature difference in Fig. 11c suggests some role of heat fluxes and convection in this case. The scatterometer data for 1100 and 2200 UTC 21

11 379 FIG. 9. Data for case b, 22 Oct 1996: (a) (c) as for Fig. 8; (d) GMS visible imagery for 0300 UTC 22 Oct, with positions of buoy (white cross) and Hokitika (black cross) marked; (e) NOAA-12 infrared imagery for 0700 UTC 22 Oct. The vertical lines in (a) (c) mark the times of the images in (d) and (e). May were characterized by high local spatial variability near the buoy, with standard deviations in direction of 20.7 and 15.6, respectively. The selected ambiguities, however, did not match the wind direction at the buoy, indicating a more northerly orientation to the wind near the buoy. These winds for 1100 UTC on 21 May are shown in Fig. 11d. A reassessment of the chosen wind ambiguities was made as per the previous case, selecting the wind directions to be consistent with those at the buoy. The data now indicate a shear line extending out from the coast just north of the buoy as shown in Fig. 11e. e. 31 May 1997 In this case, a trough crossed South Island late on 30 May This was immediately followed by airflow with heavy convection moving up the west coast. The pressure at the buoy over this period (Fig. 12a) shows considerable fluctuation and does not rise until the first clearance occurred about 0600 UTC on 31 May. The pressure at Hokitika was not affected in this way. A second cold surge up the west coast was also identified by convective cloud in GMS imagery. This surge reached Haast at 0600 UTC on 31 May and Hokitika at 0900 UTC. The pressure dip under consideration developed at about 1200 UTC on 31 May and a maximum Hokitika buoy pressure difference of 6.9 hpa was reached at 1800 UTC. The 1800 UTC 800-hPa wind at Hokitika was 4.1 m s 1 from 245 true. GMS cloud imagery shows a changing cloud system over the buoy position. For the 3 4 h during the maximum pressure dip a convective cell appeared to be

12 380 WEATHER AND FORECASTING VOLUME 14 FIG. 10. Data for case c, 31 Oct 1996: (a) (c) as for Fig. 8; (d) GMS infrared imagery for 1200 UTC 31 Oct with NSCAT winds and ECMWF surface pressure field as for Fig. 8e, also at 1200 UTC. The NSCAT winds ambiguities shown are those selected manually; (e) NSCAT wind data for 1200 UTC 31 Oct as for Fig. 8e, using the winds selected by the ambiguity removal algorithm. The vertical line in (a) (c) marks the time of the image and NSCAT data in (d) and (e). positioned over the buoy (Fig. 12d). The sea air temperature difference also reached a maximum during this period (Fig. 12c). The clearance at the buoy coincided with the sharp rise in pressure, within2hofthepressure low. A loop of hourly GMS imagery suggests that during the 4-h period from 1400 to 1800 UTC there is a small rotating system moving along the coastline toward the northeast. After the cool change passed Haast, a light southeast wind became established there at 1100 UTC suggesting the onset of local offshore flow. At 1800 UTC, just south of Hokitika a clearance expands out from the coast, over the buoy (Fig. 12e). NSCAT winds during this clearance are shown in Fig. 5. They are light and variable near the buoy but in other areas show a moderate southwesterly flow. 6. Discussion This paper presents data associated with observed pressure dips at a moored buoy close to the west coast of South Island, New Zealand. The pressure dips are frequent and, as can be readily seen in the case studies, often occur in groups. The magnitudes of these pressure events are often large enough to have a major impact on the positioning of isobars on weather charts for the area. Consequently, if the new information is included in analyses, predicted wind fields are likely to be much affected. For example, after a frontal passage it may often be incorrect to draw isobars to the Hokitika pressure and to space them uniformly out from the coast. Both buoy pressures and scatterometer winds indicate that in these circumstances a fairly uniform field of southwesterlies often exists over the open sea, west of the buoy, but the field may be quite complex nearer the coast. Ways in which analyses can be adapted to include these large but relatively transitory disturbances will need to be developed. Four of the case studies presented in this paper can be described under one scenario, which is probably the

13 381 FIG. 11. Data for case d, 21 May 1997: (a) (c) as for Fig. 8; (d) NSCAT winds for and ECMWF surface pressure field as for Fig. 10d, at 1100 UTC 21 May; (e) NSCAT winds as for Fig. 10e, at 1100 UTC 21 May. The vertical line in (a) (c) marks the time of the NSCAT data in (d) and (e). most important for the development of the disturbances. The scenario follows the passage of a major front, with a southwesterly flow parallel with the coast. Outbreaks of cold air within the southwesterlies often precede the development of pressure dips at the buoy. After the cold surges move up the coast, suppression of the convection is apparent in satellite pictures, possibly because of developing flow off the land. At this stage, new cloud features often seem to develop over the sea in the central West Coast Bight, where the buoy was moored. These cloud features are associated with dips in the surface pressure and, in three of the four cases, coincide with sea minus air temperature maxima (the air temperature falls rather than sea temperature rises). In one case there is clear evidence of a cyclonic vortex in the cloud features. However, the imagery alone can be inconclusive, as the height of the dominating cloud systems may mask low-level circulations. In the other cases, it is the combination of buoy and satellite winds and cloud features in satellite imagery that provide evidence of small cyclonic vortices. The fifth case study (case d) is for a northeasterly flow. The ambiguity in the satellite winds that is most consistent with the buoy winds suggests a shear line just north of the buoy. It might be inferred that the mechanisms involved in the development of the pressure dip are different in this case compared with the southwesterly cases. However, we note again that the nighttime sea air temperature differences were positive, and like the other cases there may well be a small-scale vortex present over the sea near the buoy. Visual evidence of vortices has been obtained from satellite imagery (e.g., Gjevik 1980). The formation of such cloud vortices has been particularly associated with small islands. Dynamically, Baines (1995, p. 421) considers that island vortex wakes in the atmosphere are essentially the same as vortex shedding in fluid flow past an obstacle. A precondition for the formation of

14 382 WEATHER AND FORECASTING VOLUME 14 FIG. 12. Data for case e, 31 May 1 June 1997: (a) (c) as for Fig. 8; (d) GMS infrared imagery for 1630 UTC 31 May with crosses marking positions of buoy and Hokitika; (e) NOAA-12 infrared imagery for 1745 UTC 31 May. The vertical lines in (a) (c) mark the times of the images in (d) and (e). such vortices is stable stratification (Chopra 1973), particularly below the level of the mountains on the islands. Revell (1983) has identified vortex streets over New Zealand coastal waters in satellite imagery and noted a significant eddy off the west coast in the wake of South Island. Further, areas of high vorticity at the margins of the wake have been observed in scatterometer data by Laing and Brenstrum (1996). In all these cases stability and blocking criteria were noted as being important. In the present study soundings from Invercargill indicate that in four of the five specific cases shown in this study there was an inversion below 800 hpa, although this fluctuated in height and was quite transitory in some cases. The characteristic clearing of open cell convection from the coastal waters near the buoy before the pressure dips suggests that a stable layer in the lower atmosphere is also a locally important feature. Blocking of the airflow by the Southern Alps may generate vortices associated with the end-barrier effects, but there may also be a contribution from gap flows. In particular, a gap near Haast appears to have some role, and surface winds from the southeast at Haast are one of the observed characteristics. Hokitika, on the other hand, is in the lee of the highest part of the Southern Alps, where southeasterlies are obstructed up to about the 700-hPa level. South to southeast flows across the lower mountains or through the gap near Haast appear to veer to become southwesterly near Hokitika. This is exemplified in the satellite imagery [see Fig. 9d and McGregor et al. (1992)]. The outflows of such low-level

15 383 jets are associated with high shear vorticity and will be conducive to the initiation and enhancement of cyclonic vortices in the region. The evidence suggests that such vortices are associated with the observed pressure dips at the buoy, that is, mesoscale lows. It is likely that heat fluxes have some role in the evolution of these lows. In most of the cases the lows coincided with substantial sea air temperature differences. In high latitudes, pressure signatures from small polar lows are very similar to the present phenomenon (Rasmussen et al. 1992). However, in the genesis of polar lows by air sea interaction instability air sea temperatures of 25 C and fluxes of over 1000 W m 2 are encountered (Businger and Reed 1989). In the present cases the latent heat flux peaks at about 250 W m 2 and sensible heat fluxes are less than 100 W m 2. Nevertheless, the development of cyclonic features by air sea interaction instability (Emanuel 1986) cannot be completely discounted. This mechanism can theoretically produce pressure dips of up to 5 hpa in the type of environment encountered in this study, namely, sea air temperature differences of 2 4 C, with 60% 80% relative humidity. In the present environment heat fluxes probably contribute in a lesser role, one of delaying the decay of these features by a few hours. The typical spindown time [e-folding decay time; see Holton (1972)] for cyclonic features extending over the lower kilometer of the atmosphere is about 12 h. This is sufficient to permit small orographically generated features to survive and drift across the buoy site. 7. Conclusions In a data period covering little more than 5 months, about 30 cases have been found in which the pressure at a meteorological buoy 50 km from the coast dips more than 5 hpa below the pressure at the nearest coastal station (70 km from the buoy). There are numerous other cases of lesser pressure dips. In five case studies, certain common features have been established: the pressure dips tend to follow cold outbreaks, which themselves follow the passages of major troughs or depressions; flow around the adjacent mountain ranges seems to provide the initial impetus for the pressure dips; and high sea minus air temperature differences are a common feature. In one case study, where there was no southwesterly cold outbreak, the sea air temperature difference was nevertheless high. Cyclonic rotating wind systems are the most likely explanation for the pressure dips, and in one of the case studies satellite cloud imagery shows such a system unambiguously. In other cases, satellite wind and cloud data provide supporting evidence for mesoscale lows. Acknowledgments. This study was funded from the New Zealand Public Good Science Fund under Contract CO1620. 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