the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jan 2022
The unusually long cold spell and the snowstorm Filomena in Spain in January 2021
Abstract. In early January 2021, Spain was affected by two extreme events – an unusually long cold spell and a heavy snowfall event associated with extratropical cyclone Filomena. For example, up to 50 cm of snow fell in Madrid and the surrounding areas in 4 days. Already during 9 days prior to the snowfall event, anomalously cold temperatures at 850 hPa and night frosts prevailed over large parts of Spain. During this period, anomalously cold and dry air was transported towards Spain from central Europe and even from the Barents Sea. The storm Filomena, which was responsible for major parts of the snowfall event, developed from a precursor low-pressure system over the central North Atlantic. Filomena intensified due to interaction with an upper-level potential vorticity (PV) trough, which was the result of anticyclonic wave breaking over Europe. In turn, this wave breaking was related to an intense surface anticyclone and upper-level ridge, whose formation was strongly influenced by a warm conveyor belt outflow of a cyclone off the coast of Newfoundland. The most intense snowfall occurred on 09 January and was associated with a sharp air mass boundary with an equivalent potential temperature difference at 850 hPa across Spain exceeding 20 K. Overall, the combination of pre-existing cold surface temperatures, the optimal position of the air mass boundary, and the dynamical forcing for ascent induced by Filomena and its associated upper-level trough were all essential – and in parts physically independent – ingredients for this extreme snowfall event to occur.
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Interactive discussion
Status: closed
-
RC1: 'Comment on nhess-2021-396', Anonymous Referee #1, 07 Jan 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-RC1-supplement.pdf
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AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-AC1-supplement.pdf
-
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
-
RC2: 'Comment on nhess-2021-396', Anonymous Referee #2, 19 Jan 2022
This paper analyzes the dynamical and physical aspects on a synoptic scale that contribute to the intense snowfall and cold spell associated with the extratropical cyclone Filomena, which affected the Canary Islands and the Iberian Peninsula at the beginning of January 2021.
The authors pose three questions and the answers give substance to the article. The questions are:
- How unusual was the cold wave and what processes led to the abnormally low temperatures in Spain?
- What processes led to the formation of Filomena?
- What characteristics of Filomena facilitated the heavy snowfall?
General comments:
The authors cite a complete technical report issued by the Spanish National Weather Service (AEMET) after this episode that gives answer to the above mentioned questions:
http://www.aemet.es/es/conocermas/recursos_en_linea/publicaciones_y_estudios/estudios/detalles/informe_filomena_ola_de_frio
Nevertheless, I think that the analysis of dynamical aspects, identifying PV streamers, ridge and trough amplification, Rossby wave rupture, and WCBs is interesting and could justify the intensification of the cyclone after the tropical interaction.
The ingredients necessary for heavy snowfalls in the interior of the Iberian Peninsula are well known, cold air at low levels and humid and warm air advection. The sources of cold air mass advection over Iberian Peninsula are clearly identified too, so I think that the analysis of back trajectories does not seem to be of great interest (except for the identification of WCBs). However, the present work, based in the reanalysis and climatology of ERA-5, serves to justify the rarity of this anomalous and exceptional episode.
It is worth mentioned that the IFS and HARMONIE models successfully predicted this historic snowfall event well in advance, in the same way, the Spanish National Weather Service (AEMET) issued the corresponding forecasts and warnings well in advance.
The mesoscale characteristics, such as the complex topography of the affected area or the thermal and humidity profile in low layers were decisive in this episode, but in this work no reference is made to the snow level (around 500 m) or to the orography. Likewise, the time interval selected for the study (between 07 and 10 January) does not seem to be the most appropriate, since there were precipitations and snowfalls in the Iberian Peninsula between 06 and 10 January, and the intense snowfalls occurred on 08 and 09 January.
I think that more information should be given on the climatic characteristics of the month of January in the study area. In the months of December and January the nocturnal frosts are frequent (the period between 06 and 10 January is climatologically the lowest minimum temperature period, and at Adolfo Suarez-Barajas Madrid airport, the average number of frost days in January is 8). These frosts occur not only due to cold advection of polar or arctic air masses, but also to subsequent surface radiative processes (long nights, calm or light winds and clear skies). AEMET defines a cold spell not just an episode with minimum temperatures below 0 C in a wide area, it applies a much more demanding criteria. In this sense, according to AEMET's technical report on Filomena, only the subsequent period to Filomena is considered a significant cold spell.
Numerical model reanalysis are a useful tool, but they cannot substitute for observational data. ERA-5 has a horizontal resolution of 35 km and 37 levels in the vertical, compared to 9 km and 137 levels of the IFS-HRES model or 2.5 km and 65 vertical levels respectively of the HARMONIE limited area model. I strongly recommend the use of real observational data, taking into account the dense network of surface meteorological stations and the soundings available in the study area.
Regarding the adverse impact of Filomena, there were barely mentioned some effects that were very significant in the Canary Islands (intense rainfall and strong gusts of wind). The heavy snowfalls occurred on 08 and 09 January, but there was notable rainfall the previous days (06 and 07 January) in the southeast of the Iberian Peninsula, affected by a warm front associated to Filomena. The exceptionality of the snowfall, according to the climatology of the model, is reflected in the Extreme Forecast Index (EFI) developed by the ECMWF. In this sense, the snowfall was exceptional, fundamentally in the center of the Iberian Peninsula.
Specific commentaries.
Please also note the supplement to this comment:
-
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-AC1-supplement.pdf
-
RC3: 'Comment on nhess-2021-396', Anonymous Referee #3, 31 Jan 2022
In this paper an attempt is made to investigate the large scale dynamics of a cold spell in Spain along with a following snowstorm and to examine their characteristics on a climatological basis. The paper deals with an interesting topic and the authors have investigated it to some extent. However, I have the following queries:
- The study provided an analysis of basic large mechanisms that are well known for the occurrence of snowfall in the Mediterranean region. From this point, the study does not contribute any new knowledge. The only interesting feature I found is the analysis of the role of WCB. I think that the study should include mesoscale processes that facilitated the snowfall. Alternatively, the authors should perform a full analysis of other similar studies and comparison among them to exhibit similarities and/or differences. For instance, the authors refer briefly two cases in section 6 in comparison to Filomena, with obvious orographic forcing without further discussion. Furthermore, a comparison of the snowfall with forecast and investigation of possible forecast failure would be an interesting topic.
- The authors employed short term ECMWF forecasts for precipitation and snowfall during the examined period. Furthermore, they employed ERA5 reanalysis data with resolution 0.5°5° for the climatological part of their study during the period 1979-2021. I suspect that they employed the ERA 5 reanalysis data for plotting θe and PV (Figures 6, 7 and 8). However, they performed comparison of the specific snowfall event and the cold spell with other events employing different datasets, probably providing biases in their results (e.g section 2.3 or 4). I am wondering why they did not use ERA5 reanalysis data throughout the whole study.
- The analysis of trajectories was useful only for the part of WCBs. The source of the cold air mass can be easily seen from the synoptic analysis. More details should be provided about the use of the LAGRANTO model on a climatological basis
- There are some points where there is no justification. For instance: the anticyclonic wave breaking (Fig 6e), the content of the clouds (Figure 8)
- Observational data (station data, radionsonde data or radar data) should be incorporated to justify findings from the reanalysis plots.
More specific comments
- Figures 6-7: These figures contain many details (coloured parameters, contours, labels) and it is difficult for the reader to follow
- Figure 6: the letters L1, L2, F should be annotated what they stand for
Citation: https://doi.org/10.5194/nhess-2021-396-RC3 -
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-AC1-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on nhess-2021-396', Anonymous Referee #1, 07 Jan 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-RC1-supplement.pdf
-
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-AC1-supplement.pdf
-
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
-
RC2: 'Comment on nhess-2021-396', Anonymous Referee #2, 19 Jan 2022
This paper analyzes the dynamical and physical aspects on a synoptic scale that contribute to the intense snowfall and cold spell associated with the extratropical cyclone Filomena, which affected the Canary Islands and the Iberian Peninsula at the beginning of January 2021.
The authors pose three questions and the answers give substance to the article. The questions are:
- How unusual was the cold wave and what processes led to the abnormally low temperatures in Spain?
- What processes led to the formation of Filomena?
- What characteristics of Filomena facilitated the heavy snowfall?
General comments:
The authors cite a complete technical report issued by the Spanish National Weather Service (AEMET) after this episode that gives answer to the above mentioned questions:
http://www.aemet.es/es/conocermas/recursos_en_linea/publicaciones_y_estudios/estudios/detalles/informe_filomena_ola_de_frio
Nevertheless, I think that the analysis of dynamical aspects, identifying PV streamers, ridge and trough amplification, Rossby wave rupture, and WCBs is interesting and could justify the intensification of the cyclone after the tropical interaction.
The ingredients necessary for heavy snowfalls in the interior of the Iberian Peninsula are well known, cold air at low levels and humid and warm air advection. The sources of cold air mass advection over Iberian Peninsula are clearly identified too, so I think that the analysis of back trajectories does not seem to be of great interest (except for the identification of WCBs). However, the present work, based in the reanalysis and climatology of ERA-5, serves to justify the rarity of this anomalous and exceptional episode.
It is worth mentioned that the IFS and HARMONIE models successfully predicted this historic snowfall event well in advance, in the same way, the Spanish National Weather Service (AEMET) issued the corresponding forecasts and warnings well in advance.
The mesoscale characteristics, such as the complex topography of the affected area or the thermal and humidity profile in low layers were decisive in this episode, but in this work no reference is made to the snow level (around 500 m) or to the orography. Likewise, the time interval selected for the study (between 07 and 10 January) does not seem to be the most appropriate, since there were precipitations and snowfalls in the Iberian Peninsula between 06 and 10 January, and the intense snowfalls occurred on 08 and 09 January.
I think that more information should be given on the climatic characteristics of the month of January in the study area. In the months of December and January the nocturnal frosts are frequent (the period between 06 and 10 January is climatologically the lowest minimum temperature period, and at Adolfo Suarez-Barajas Madrid airport, the average number of frost days in January is 8). These frosts occur not only due to cold advection of polar or arctic air masses, but also to subsequent surface radiative processes (long nights, calm or light winds and clear skies). AEMET defines a cold spell not just an episode with minimum temperatures below 0 C in a wide area, it applies a much more demanding criteria. In this sense, according to AEMET's technical report on Filomena, only the subsequent period to Filomena is considered a significant cold spell.
Numerical model reanalysis are a useful tool, but they cannot substitute for observational data. ERA-5 has a horizontal resolution of 35 km and 37 levels in the vertical, compared to 9 km and 137 levels of the IFS-HRES model or 2.5 km and 65 vertical levels respectively of the HARMONIE limited area model. I strongly recommend the use of real observational data, taking into account the dense network of surface meteorological stations and the soundings available in the study area.
Regarding the adverse impact of Filomena, there were barely mentioned some effects that were very significant in the Canary Islands (intense rainfall and strong gusts of wind). The heavy snowfalls occurred on 08 and 09 January, but there was notable rainfall the previous days (06 and 07 January) in the southeast of the Iberian Peninsula, affected by a warm front associated to Filomena. The exceptionality of the snowfall, according to the climatology of the model, is reflected in the Extreme Forecast Index (EFI) developed by the ECMWF. In this sense, the snowfall was exceptional, fundamentally in the center of the Iberian Peninsula.
Specific commentaries.
Please also note the supplement to this comment:
-
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-AC1-supplement.pdf
-
RC3: 'Comment on nhess-2021-396', Anonymous Referee #3, 31 Jan 2022
In this paper an attempt is made to investigate the large scale dynamics of a cold spell in Spain along with a following snowstorm and to examine their characteristics on a climatological basis. The paper deals with an interesting topic and the authors have investigated it to some extent. However, I have the following queries:
- The study provided an analysis of basic large mechanisms that are well known for the occurrence of snowfall in the Mediterranean region. From this point, the study does not contribute any new knowledge. The only interesting feature I found is the analysis of the role of WCB. I think that the study should include mesoscale processes that facilitated the snowfall. Alternatively, the authors should perform a full analysis of other similar studies and comparison among them to exhibit similarities and/or differences. For instance, the authors refer briefly two cases in section 6 in comparison to Filomena, with obvious orographic forcing without further discussion. Furthermore, a comparison of the snowfall with forecast and investigation of possible forecast failure would be an interesting topic.
- The authors employed short term ECMWF forecasts for precipitation and snowfall during the examined period. Furthermore, they employed ERA5 reanalysis data with resolution 0.5°5° for the climatological part of their study during the period 1979-2021. I suspect that they employed the ERA 5 reanalysis data for plotting θe and PV (Figures 6, 7 and 8). However, they performed comparison of the specific snowfall event and the cold spell with other events employing different datasets, probably providing biases in their results (e.g section 2.3 or 4). I am wondering why they did not use ERA5 reanalysis data throughout the whole study.
- The analysis of trajectories was useful only for the part of WCBs. The source of the cold air mass can be easily seen from the synoptic analysis. More details should be provided about the use of the LAGRANTO model on a climatological basis
- There are some points where there is no justification. For instance: the anticyclonic wave breaking (Fig 6e), the content of the clouds (Figure 8)
- Observational data (station data, radionsonde data or radar data) should be incorporated to justify findings from the reanalysis plots.
More specific comments
- Figures 6-7: These figures contain many details (coloured parameters, contours, labels) and it is difficult for the reader to follow
- Figure 6: the letters L1, L2, F should be annotated what they stand for
Citation: https://doi.org/10.5194/nhess-2021-396-RC3 -
AC1: 'Response to all reviewers', Philipp Zschenderlein, 22 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-396/nhess-2021-396-AC1-supplement.pdf
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Cited
Philipp Zschenderlein
Heini Wernli
This preprint has been withdrawn.
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