Intense precipitation in Lisbon - September 22, 2014
2014-09-23 (IPMA)
Following the intense precipitation event occurred in Lisbon, the afternoon of 22nd September 2014, IPMA reports that the most significant amounts of rainfall in the district of Lisbon between 13:15 local time were as follows:
Lisbon / Geofísico - 13.2 mm
Lisbon / Gago Coutinho - 18.7 mm
Lisbon / Amoreiras - 8.0 mm
Amadora - 7.0 mm
The highest amount of rainfall in 10 minutes was recorded in Lisbon / Geofísico station with 6 mm between 14:00 and 14:10 local time and the highest amount of rainfall in 1 hour was recorded at the Lisbon / Gago Coutinho station with 17.3 mm between 13:00 and 14:00 local time.
Radar suggests that precipitation values in Lisbon may have been higher, exceeding 20 mm per hour between 13 and 15 LT. A preliminary analysis indicates that, in some places in the city of Lisbon, precipitation may have exceeded 40 mm in two hours (Figures 1 and 2).
Such large precipitation values were due to building up of strong convective cells in very localized areas and with temporal persistence, resulting in irregular distribution of precipitation, particularly in the Greater Lisbon area; similar phenomena happened elsewhere in the portuguese territory.
When compared with the intense rainfall episode occurred in Lisbon on 18th February 2008, the following can be noted (Figure 1):
- i) Precipitation amounts recorded in IPMA meteorological stations network on 22nd September 2014 are lower than the values recorded on 18th February 2008 (eg, 12.2 mm in 10 minutes in Lisbon / Benfica in 2008 and 6.0 mm in 10 minutes in Lisbon / Geofísico in 2014);
- ii) Radar indicate that the most intense rainfall in 2014 did not happen on the locations of IPMA observations network, while in 2008 the values recorded at the IPMA stations encompassed the highest values of precipitation;
- iii) Both in 2008 as in 2014, radar images suggest the occurrence of two 1-hour periods when rainfall could have exceeded 20 mm in one hour: between 02 and 03h and between 04 and 05h, local time, on 18th February 2008 and between 13 and 14h and between 14 and 15h local time on 22nd September 2014;
- iv) When aggregated for 6 and 24 hours intervals (which included the most critical period) on 18th February 2008, precipitation at IPMA meteorological stations network, was about 3 times greater than that of 22nd September 2014. For example, in Lisbon / Gago Coutinho station, in six-hourly periods, maximum values of 86.0 mm occurred in 2008 and 23.0 mm in 2014, and in 24-hour periods, 137.8 mm occurred in 2008 and 43.6 mm in 2014.
A preliminary data analysis suggests that the most intense two-hourly rainfall occurred on 22nd September 2014 in some areas of Lisbon, might have matched the two-hourly rainfall occurred on 18th February 2008; nevertheless 6 and 24 hours precipitation in 2014 was much smaller than in 2008.
As an additional comparison, we note that the maximum precipitation occurred in two episodes of intense rainfall in Lisbon in 2008 and in 2014 was lower than that occurring in the episode of intense rainfall in Madeira on February 20, 2010: Funchal: 11.0 mm in 10 minutes, 51.3 mm in one hour, 111.5 mm in 6 hours and 144.3 in 24 hours; Areeiro: 15.4 mm in 10 minutes, 78.5 mm in 1 hour, 272.1 mm in 6 hours and 387.1 mm in 17 hours.
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