Preliminary analysis of the period 5 to 10 August 2016 in Madeira
2016-08-23 (IPMA)
Following the news "exceptionally warm weather episode in Funchal" of 10th August 2016, IPMA has further assessed the weather situation which led to the occurrence of extreme temperatures and humidity values on the south slopes of the island of Madeira.
The joint action of an anticyclone located northwest of Iberia and a depression centered in Morocco influenced the weather in Madeira in the period 5-10th August. This event was caused by the advection of warm and dry air over the area (figures 1 and 2), with air temperature values around 25-26 ° C and relative humidity in the range of 10-20 %, at around 1500 meters of altitude.
In this period of 6 days, at stations Areeiro (1590 m) and the Bica da Cana (1560 m), shown in figure 3, the observed maximum temperatures at 2 m were, respectively 27.2 ° C and 28.1 ° C. In both stations, the lowest values of relative humidity at 2 m were in the range of 10 to 20%. Due to a persistent east / northeastly flow, at low level stations located on the north side of the island of Madeira, as well as on the island of Porto Santo, the maximum values of observed temperature were around 30 ° C and relative humidity was on the order of 40 %.
At the stations on the south side, the situation was more extreme, with values of relative humidity reaching 10-20% and widespread maximum temperatures of 34-36 ° C. In particular, in the Funchal area, the recorded maximum temperatures were, on August 9th, 38.2 ° C and 37.8 ° C, respectively, at Funchal / Observatory and Funchal / Lido.
More relevant is the fact that the hourly temperatures at Funchal / Observatory showed an increase of about 7 °C in one hour (between 11 pm local time on August 4 and 00 hours of August 5), with values of 30 to 32 °C throughout the night of August 5. Between the evening of August 8th and the morning of August 9th the situation was even more extreme, with observed temperatures on the range 30 to 35 °C.
The values of temperature and humidity observed in the south of the island of Madeira are not fully justified by the transport of warm, dry air induced by large scale, as during the weekend (6 and 7 of August) the maximum temperature in the south slopes of the island of Madeira was much lower, around 28-31 °C. Hence, the observations suggest that local scale phenomena must have contributed significantly to the situation of abnormally high temperatures recorded in this area.
The observations at IPMA stations in the Funchal region and in the mountains suggest that in the early hours of August 5 (Figure 4) and between 6 pm local time on August 8 and 5 am of August 10th, there was a persistent circulation with downward motion (from the mountain to the mean sea level) in the south of the Madeira island, which caused a rapid heating of the air and a sharp decline (figure 5) in the relative humidity, a phenomenon which is usually designated as foehn effect. This is suggested by IPMA's records as at 6 am of August 9th, the observed temperature at Pico Alto was around 25ºC (1118 m) and 34 °C at Funchal / Observatory, with northeasterly winds of 35 km/h and gusts of around 80 km/h. A 9 °C difference between the stations of the Pico Alto and Funchal is consistent with the warming that an air particle can suffer when dropping about 1000 meters, when the air mass is dry (process known as a dry adiabatic compression).
Given the local character of this event, it is expected that models with a higher spatial resolution will display a better forecast. IPMA will continue to study this event, in particular, the features of the atmospheric flow on the island in order to better describe it.
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