Abstract:
An extensive study of the climatology of frost events using an 11-year record from the Te Puke EDR, has determined that shortly after sunset in clear, calm conditions, a katabatic wind and/or land breeze from the south-southwest (200° true) was established within the lower boundary layer. If the wind speed exceeded 3 knots, this resulted in mechanical mixing with the warmer air aloft and retarded further cooling.
To maximise the katabatic flow and land breeze effects, a weak pressure gradient was required over the region, and this was commonly associated with anticyclonic conditions. As a ridge of high pressure moved onto New Zealand, there was often synoptic scale cold advection associated with the southerly flow. However, once the nocturnal surface inversion was in place, the flow aloft became insignificant due to decoupling. Clear skies permitted the greatest terrestrial heat loss during the night-time, which emphasised the importance of satellite imagery when preparing a frost forecast.
A field study at a Maketu orchard found that there could be considerable differences in temperature over short horizontal distances due to terrain effects and the location of shelterbelts. A study of the vertical temperature profile showed that there was a supply of warmer air aloft, which remained throughout the morning.
