A Synoptic Scale Meteorological Investigation of Springtime Frost Incidence in Cromwell

The onset of a frost can cause millions of dollars of lost profit due to damage to the grapevine buds utilized for the production of world class wines. The aim of this thesis is to assess the synoptic controls on springtime frost incidence in the complex terrain of Cromwell, Central Otago, New Zealand/Aotearoa (45.03°S and 169.20°E). The main research objectives of the research are to (1) identify a statistically significant synoptic signal that leads to springtime frost incidence using synoptic weather types and (2) to investigate the meteorological variables associated with specific synoptic weather types to separate a frost event from a non-frost event. An understanding of the relationships between synoptic weather types and the incidence of frost is desirable because of the practical, operational, and economic implications that frost incidence poses in this region. The implications are particularly important given that there is an acknowledged increase in the synoptic types (anticyclones) over New Zealand that are associated with the incidence of frost. Kidson types were temporally aligned with a historical weather dataset from NIWA’s CliFLo network (1950 - 2017) in such a manner that each frost or non-frost in the Austral spring (September, October, and November) were preceded by preceding noon Kidson type (PNKT) and midnight Kidson type (MKT). The model utilized was classified as being an excellent predictor given the metric of the receiver operator curve (ROC- score of 80.4%). The top ten most probable Kidson transitions were characterized by southwesterly winds (HW, SW, TSW Kidson types) at noon. The most probable midnight Kidson types to render a frost are characterized by anticyclonic activity (HW, HSE, R, NE). On the contrary, the ten least likely Kidson transitions to render a frost are those that are characterized by either noontime westerlies (H, W) or noontime northerly quarter winds (TNW, HE). Following this, the most probable midnight Kidson types to render a non-frost are characterized by being part of the trough (T, TSW, TNW) or zonal regime (W). The ERA-Interim reanalysis dataset was used to investigate the meteorological variables (mean sea level pressure (MSLP), MSLP anomalies, wind speed and direction, total column water (TCW) anomalies, and relative humidity (RH)) that separate a frost event from a non frost event, thereby achieving objective 2. Frost events are consistently characterized by higher pressures than that of non-frost events with regards to both MSLP and MSLP anomalies. Frosts are consistently characterized by less atmospheric moisture in both absolute and relative metrics (negative TCW anomalies, and lower RH values) when compared to non-frost events. With regards to wind, windspeeds are consistently lower within the interior of the South Island, and the incident synoptic winds are more meridional/southerly during fros events than non-frosts. The statistical link between the incidence of frost and synoptic types in this study was achieved in complex terrain, suggesting that it successfully captured the local response (particularly katabatic drainage winds) to synoptic processes with regards to the incidence of frost, and may have potential for utilization elsewhere in New Zealand. The climate reanalysis component of this study proved to be effective, as it was able to discern the meteorological variables that are influential on the incidence of frost. Thus, it is concluded that this thesis makes a significant contribution towards developing an understanding that leads to springtime frost incidence in Cromwell, and provides a means to differentiate the controlling factors on such events