# Development of a Front Identification Scheme for Compiling a Cold Front Climatology of the Mediterranean

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## Abstract

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## 1. Introduction

## 2. Description of the Identification Scheme

_{crit}during the same 6 h period.

_{crit}. Second, instead of using the change of the meridional wind component (dv), the shift of the vector wind direction is employed during a 6 h period (dφ), where φ = arctan (v/u) and a specific minimum threshold value dφ

_{crit}is also investigated. This criterion is examined to better identify zonally elongated cold fronts and at the same time, to filter out erroneously identified front segments. Third, an additional criterion of the magnitude of vector wind |U| exceeding a specific critical threshold |U|

_{crit}in each cluster of grid points is added to optimise the scheme, considering the operational experience of forecasters that the intensity of the northwesterly wind is significant behind the cold front. This criterion allows the discarding of shallow fronts.

## 3. Sensitivity Tests

_{crit}was explored. An initial threshold value dv

_{crit}= 2 m s

^{−1}was employed, as suggested by [19] in the initial version of FTS. Then, the scheme was employed for different values of dv

_{crit}increasing by 1 m s

^{−1}. Figure 1a shows the synoptic situation of 7 November 2016, 00:00 UTC. In Figure 1b, the identified fronts are depicted (red lines) for the same day and hour for dv

_{crit}= 3 m s

^{−1}. In the same Figure, the light blue regions show the areas where the wind shift criterion is satisfied.

_{crit}of 4 and 5 m s

^{−1}, the erroneous front identifications show a tendency to diminish (Figure 1c). However, when dv

_{crit}exceeds the value of 6 m s

^{−1}, the existing fronts incline to be broken into smaller fragments (Figure 1d). Therefore, moderate values of dv

_{crit}ranging between 4–6 m s

^{−1}seem to best represent Mediterranean cold fronts. Similar results were derived for the following hours. Figure 2 shows the results for 8 November 2016 at 12:00 UTC.

_{crit}starting from 20°, with steps of 10°. In Figure 3, the identified fronts are depicted for 7 November 2016, 00:00 UTC, for (a) dφ

_{crit}= 30° and (b) dφ

_{crit}= 50°. A comparison with the synoptic analysis in Figure 1a shows that the scheme indeed represents the front over Italy when dφ

_{crit}= 30°. However, it can be appreciated that using solely the dφ criterion, several erroneously identified fronts are obtained. Moreover, for dφ

_{crit}= 50°, the front over Italy is mistakenly broken into smaller fragments. Similar results were obtained for dφ

_{crit}> 40° and the best representation of fronts was obtained for dφ

_{crit}= 30° in most cases. To effectively filter out the erroneously identified frontal objects without losing the spatial continuity of the correctly identified fronts, the value dφ

_{crit}= 30° is adopted.

_{crit}. It should be noted that apart from filtering out mistaken identifications, this criterion also helped in identifying cold fronts entering from North Africa and distorted cold fronts in the Central Mediterranean that rejuvenated when entering the Aegean Sea. To find the optimum value of |U|

_{crit}, values between 0 and 10 m s

^{−1}were tested. Figure 4 depicts the results of the scheme on 7 November 2016 00:00 UTC for (a) |U|

_{crit}= 5 m s

^{−1}and (b) |U|

_{crit}= 7 m s

^{−1}. Red lines represent the identified fronts, whereas colored areas show the magnitude of the total wind |U| at the grid points where the dφ criterion is met. It is clear that a value of |U|

_{crit}= 5 m s

^{−1}effectively filters out spurious front identifications, while a larger value (e.g., |U|

_{crit}= 7 m s

^{–1}) erroneously filters out the front over Italy. From the above sensitivity tests, the combination of dφ

_{crit}= 30° and |U|

_{crit}= 5 m s

^{−1}seems to best represent cold fronts in the Mediterranean at each following synoptic time (Figure 5 and Figure 6). Similar results were derived for the other selected cases under a variety of synoptic environments. It should be noted that based on these dynamic criteria, warm frontal structures are not identified.

_{crit}= 30° and (d) the magnitude of the vector wind |U| is greater than |U|

_{crit}= 5 m s

^{−1}. It should be noted that the initial criterion of dv

_{>}dv

_{crit}used in FTS has been replaced by both the criteria of dφ > dφ

_{crit}and |U| > |U|

_{crit}.

_{crit}. The zonally elongated front over the Iberian Peninsula is also identified correctly by the scheme (Figure 7c) along with its subsequent evolvement. Figure 7d presents the results when the dv criterion is solely used. From the comparison between Figure 7c,d becomes evident that the zonal front over the Iberian is not properly identified with the dv criterion. Therefore, it is suggested that the successful identification of this front is mainly attributable to the combination of dφ

_{crit}and |U|

_{crit}criteria.

## 4. Statistical Validation

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) Synoptic surface chart over the area of interest at 00:00 UTC 7 November 2016, and identified fronts for (

**b**) dv

_{crit}= 3 m s

^{−1}, (

**c**) dv

_{crit}= 5 m s

^{−1}, and (

**d**) dv

_{crit}= 7 m s

^{−1}. The red lines show the identified fronts, whereas the areas where the wind shift criterion is satisfied are depicted with the light blue color.

**Figure 2.**(

**a**) Synoptic surface chart over the area of interest at 12:00 UTC 8 November 2016, and identified fronts for (

**b**) dv

_{crit}= 2 m s

^{−1}, (

**c**) dv

_{crit}= 4 m s

^{−1}, and (

**d**) dv

_{crit}= 6 m s

^{−1}. The red lines show the identified fronts, whereas the areas where the wind shift criterion is satisfied are depicted with the light blue color.

**Figure 3.**Identified fronts at 00:00 UTC 07 November 2016, for (

**a**) dφ

_{crit}= 30° and (

**b**) dφ

_{crit}= 50°. The red lines show the identified fronts, whereas the areas where the wind shift criterion is satisfied are depicted with the light blue color.

**Figure 4.**Identified fronts at 00:00 UTC 7 November 2016, for dφ

_{crit}= 30°, (

**a**) |U|

_{crit}= 5 m s

^{−1}and (

**b**) |U|

_{crit}= 7 m s

^{–1}. Red lines represent the identified fronts, whereas colored areas show the magnitude of the total wind |U| at the grid points where the dφ criterion is met.

**Figure 5.**(

**a**) Synoptic surface chart at 12:00 UTC 08 November 2016, and (

**b**) identified fronts for dφ

_{crit}= 30° and |U|

_{crit}= 5 m s

^{–1}. Red lines represent the identified fronts, whereas colored areas show the magnitude of the total wind |U| at the grid points where the dφ criterion is met.

**Figure 6.**(

**a**) Synoptic surface chart over the area of interest at 00:00 UTC 9 November 2016, and (

**b**) identified fronts for dφ

_{crit}= 30° and |U|

_{crit}= 5 m s

^{–1}. Red lines represent the identified fronts, whereas colored areas show the magnitude of the total wind |U| at the grid points where the dφ criterion is met.

**Figure 7.**(

**a**) Satellite image (IR 12μm) of the Mediterranean sea at 19 March 2018, 12:00UTC, (

**b**) synoptic surface chart over the area of interest at the same time, (

**c**) identified fronts for dφ

_{crit}= 30° and |U|

_{crit}= 5 m s

^{–1}. Red lines represent the identified fronts, whereas colored areas show the magnitude of the total wind |U| at the grid points where the dφ criterion is met. (

**d**) Respective results for the case of solely the dv criterion for dv

_{crit}= 6 m s

^{−1}.

**Figure 8.**Mean annual cycle of the number of cold fronts identified by the MedFTS (model) and the synoptic charts (analysis) over Greece for the period 2007–2016.

**Table 1.**Definition of statistic indices used for the comparison of the fronts identified by the algorithm and the fronts appearing in the synoptic charts.

Symbol | Statistic Index | Explanation |
---|---|---|

a | hits | Front exists in synoptic charts and identified |

b | false alarms | Front identified but not appearing in charts |

c | misses | Front appearing in charts not identified |

d | correct rejection | No front identified and no front in charts |

n | a+b+c+d | Sample size |

**Table 2.**Definition of the statistical metrics used of the validation of the algorithm’s capability.

Metric | Definition | Range | Perfect Score |
---|---|---|---|

Frequency Bias Index (FBI) | $\frac{a+b}{a+c}$ | 0 ÷ ∞ | 1 |

Probability of Detection (POD) | $\frac{a}{a+c}$ | 0 ÷ 1 | 1 |

False Alarm Ratio (FAR) | $\frac{b}{a+b}$ | 0 ÷ 1 | 0 |

Critical Success Index (CSI) | $\frac{a}{a+b+c}$ | 0 ÷ 1 | 1 |

True Skill Statistics (TSS) | $\frac{ad-bc}{\left(a+c\right)\left(b+d\right)}$ | −1 ÷ 1 | 1 |

Heidke Skill Score (HSS) | $\frac{2\left(ad-bc\right)}{\left(a+c\right)\left(c+d\right)+\left(a+b\right)\left(b+d\right)}$ | −∞ ÷ 1 | 1 |

Equitable Threat Score (ETS) | $\frac{a-{a}_{r}}{a+b+c-{a}_{r}}$ where ${a}_{r}=\frac{\left(a+b\right)\left(a+c\right)}{n}$ | −1/3 ÷ 1 | 1 |

**Table 3.**Values of the indices of Table 1, as they are counted for the decade 2007–2016.

Number of Fronts | Fronts Appearing in Synoptic Charts | Fronts Not Appearing in Synoptic Charts |
---|---|---|

Fronts appearing in the scheme | a = 436 | b = 111 |

Fronts not appearing in the scheme | c = 75 | d = 3031 |

**Table 4.**Values of the metrics that are defined in Table 2 for the decade 2007–2016.

Metric | Value |
---|---|

FBI | 1.070 |

POD | 0.853 |

FAR | 0.203 |

CSI | 0.701 |

TSS | 0.818 |

HSS | 0.794 |

ETS | 0.659 |

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**MDPI and ACS Style**

Bitsa, E.; Flocas, H.; Kouroutzoglou, J.; Hatzaki, M.; Rudeva, I.; Simmonds, I. Development of a Front Identification Scheme for Compiling a Cold Front Climatology of the Mediterranean. *Climate* **2019**, *7*, 130.
https://doi.org/10.3390/cli7110130

**AMA Style**

Bitsa E, Flocas H, Kouroutzoglou J, Hatzaki M, Rudeva I, Simmonds I. Development of a Front Identification Scheme for Compiling a Cold Front Climatology of the Mediterranean. *Climate*. 2019; 7(11):130.
https://doi.org/10.3390/cli7110130

**Chicago/Turabian Style**

Bitsa, E., H. Flocas, J. Kouroutzoglou, M. Hatzaki, I. Rudeva, and I. Simmonds. 2019. "Development of a Front Identification Scheme for Compiling a Cold Front Climatology of the Mediterranean" *Climate* 7, no. 11: 130.
https://doi.org/10.3390/cli7110130