Abstract. The lack of observations near the surface is often cited as a limiting factor in the observation and prediction of deep convection. Recently, networks of personal weather stations (PWSs) measuring pressure, temperature and humidity in near-real time have been rapidly developing. Even if they suffer from quality issues, their high temporal resolution and their higher spatial density than standard weather station (SWS) networks have aroused interest in using them to observe deep convection. In this study, the PWS contribution to the observation of deep-convection features near the ground is evaluated. Four cases of deep convection in 2018 over France were considered using data from Netatmo, a PWS manufacturer. A fully automatic PWS processing algorithm, including PWS quality control, was developed. After processing, the mean number of observations available increased by a factor of 134 in mean sea level pressure (MSLP), of 11 in temperature and of 14 in relative humidity over the areas of study. Near-surface SWS analyses and analyses comprising standard and personal weather stations (SPWSs) were built. The usefulness of crowdsourced data was proven both objectively and subjectively for deep-convection observation. Objective validations of SWS and SPWS analyses by leave-one-out cross validation (LOOCV) were performed using SWSs as the validation dataset. Over the four cases, LOOCV root-mean-square errors (RMSEs) decreased for all parameters in SPWS analyses compared to SWS analyses. RMSEs decreased by 73 % to 77 % in MSLP, 12 % to 23 % in temperature and 17 % to 21 % in relative humidity. Subjectively, fine-scale structures showed up in SPWS analyses, while being partly, or not at all, visible in SWS observations only. MSLP jumps accompanying squall lines or individual cells were observed as well as wake lows at the rear of these lines. Temperature drops and humidity rises accompanying most of the storms were observed sooner and at a finer resolution in SPWS analyses than in SWS analyses. The virtual potential temperature was spatialized at an unprecedented spatial resolution. This provided the opportunity for observing cold-pool propagation and secondary convective initiation over areas with high virtual potential temperatures, i.e. favourable locations for near-surface parcel lifting.
Abstract. The case of the heavy precipitation event on 14 and 15 October 2018 which has led to severe flash flooding in the Aude watershed in south-western France is studied from a meteorological point of view using deterministic and probabilistic numerical weather prediction systems, as well as a unique combination of observations from both standard and personal weather stations. This case is typical of Mediterranean heavy precipitation events due to its classic synoptic situation and its quasi-stationary convective precipitation that regenerates continuously, but with some peculiarities such as the presence of a former hurricane and a pre-existing cold air mass close to the ground. It is shown that the positive Mediterranean sea surface temperature anomaly may have played an aggravating role in the amount of precipitation that poured into the Aude basin. On the other hand, soil moisture does not seem to have played a significant role. A study of rainfall forecasts shows that the event had limited predictability, in particular given the small size of the watersheds involved. It is shown that the stationarity of precipitation, whose estimation benefits from data from personal stations, is linked to the presence near the ground of a trough and a strong potential virtual temperature gradient, the stationarity of both of which is highlighted by a combination of observations from standard and personal stations. The forecast that comes closest to the rainfall observations contains the warmest, wettest and fastest low-level jet and also simulates near the ground a trough and a marked boundary between cold air in the west and warm air in the east, both of which are stationary.
Abstract. The case of the heavy precipitation event on 14 and 15 October 2018 which has led to severe flash flooding in the Aude watershed in south-western France is studied from a meteorological point of view using deterministic and probabilistic numerical weather prediction systems, as well as a unique combination of observations from both standard and personal weather stations. This case features typical characteristics of Mediterranean heavy precipitation events such as its classic synoptic situation and its quasi-stationary convective precipitation that regenerates continuously, as well as some peculiarities such as the presence of a former hurricane and a pre-existing cold air mass close to the ground. Mediterranean Sea surface temperature and soil moisture anomalies are briefly reviewed, as they are known to play a role in this type of hydrometeorological events. A study of rainfall forecasts shows that the event had limited predictability, in particular given the small size of the watersheds involved. It is shown that the stationarity of precipitation, whose estimation benefits from data from personal stations, is linked to the presence near the ground of a trough and a strong potential virtual temperature gradient, the stationarity of both of which is highlighted by a combination of observations from standard and personal stations. The forecast that comes closest to the rainfall observations contains the warmest, wettest, and fastest low-level jet and also simulates near the ground a trough and a marked boundary between cold air in the west and warm air in the east, both of which are stationary.
The lack of observations near the surface is often cited as a limiting factor in the observation and prediction of deep convection. Recently, networks of personal weather stations (PWSs) measuring pressure, temperature and humidity in nearreal time have been rapidly developing. Even if they suffer from quality issues, their high temporal resolution and their higher spatial density than standard weather station (SWS) networks have aroused interest in using them to observe deep convection.In this study, the PWSs contribution to the observation of deep-convection features near the ground is evaluated. Four cases 5 of deep convection in 2018 over France were considered using data from Netatmo, a PWS manufacturer. A fully automatic PWS processing algorithm, including PWS quality control, was developed. After processing, the mean number of observations available increased by a factor of 134 in mean sea level pressure (MSLP), of 11 in temperature and of 14 in relative humidity over the areas of study. Near-surface SWS analyses, and analyses comprising standard and personal weather stations (SPWS) were built. The usefulness of crowdsourced data was proven both objectively and subjectively for deep convection observation. 10Objective validations of SWS and SPWS analyses by leave-one-out cross-validation (LOOCV) were performed using SWSs as the validation dataset. Over the four cases, LOOCV root mean square errors (RMSEs) decreased for all parameters in SPWS analyses compared to SWS analyses. RMSEs decreased by 73 to 77 % in MSLP, 12 to 23 % in temperature and 17 to 21 % in relative humidity. Subjectively, fine-scale structures showed up in SPWS analyses, while partly or not at all visible in SWS observations only. MSLP jumps accompanying squall lines or individual cells were observed, as well as wake lows at the rear 15 of these lines. Temperature drops and humidity rises accompanying most of the storms were observed sooner and at a finer resolution in SPWS analyses than in SWS analyses. The virtual potential temperature was spatialized at an unprecedented spatial resolution. It gave the opportunity to observe cold pool propagation and secondary convective initiation over areas with high virtual potential temperatures, i.e. favorable locations for near surface parcel lifting.
Abstract. In south-eastern France, the Mediterranean coast is regularly affected by heavy-precipitation events. On 14–15 October 2018, in the Aude department, a back-building quasi-stationary mesoscale convective system produced up to about 300 mm of rain in 11 h. At synoptic scale, the former Hurricane Leslie was involved in the formation of a Mediterranean surface low that channelled conditionally unstable air towards the coast. At mesoscale, convective cells focused west of a decaying cold front that became quasi-stationary and downwind of the terrain. To investigate the roles of the moisture provided by Leslie, orography and evaporative cooling among the physical processes that led to the location and intensity of the observed rainfall, numerical simulations are run at 1 km and 500 m horizontal grid spacing and evaluated with independent near-surface analyses including novel crowd-sourced observations of personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updraughts mainly originated from areas east of the Balearic Islands, over the Mediterranean Sea, whereas in a second part, an increasing number originated from Leslie's remnants. Air masses from areas east of the Balearic Islands appeared as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moistening mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department did not play any substantial role in the stationarity of the quasi-stationary front. Regarding lifting mechanisms, the advection of conditionally unstable air by a low-level jet towards the quasi-stationary front, confined to altitudes below 2 km, reactivated convection along and downwind of the front. Most of the air parcels found inside strong updraughts near the location of the maximum rainfall were lifted above the quasi-stationary front. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting were maintained by low-level leeward convergence, mountain lee waves and a favourable directional wind shear; when terrain is flattened, rainfall is substantially reduced. The location of the exceptional precipitation was primarily driven by the location of the quasi-stationary front and secondarily by the location of convective bands downwind of orography.
Abstract. In southeastern France, the Mediterranean coast is regularly affected by heavy precipitation events. On 14–15 October 2018, in the Aude department, a back-building quasi-stationary mesoscale convective system produced up to about 300 mm of rain in 11 h. The synoptic situation was perturbed by the former hurricane Leslie, involved in the formation of a Mediterranean surface low that focused the convective activity. At mesoscale, convective cells focused west of a quasi-stationary cold front and downwind of the terrain. To investigate the roles of Leslie, orography and evaporative cooling in the processes that led to the observed rainfall, numerical simulations are run and evaluated with near-surface analyses comprising standard and personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updrafts mainly originate from the Mediterranean Sea, east of 4.5° E, whereas in a second part, an increasing number originates from Leslie's remnants. Air masses from east of 4.5° E appear as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moisten mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department does not play any substantial role in the stationarity of the cold front. Regarding lifting mechanisms, most of the air parcels found inside strong updrafts near the location of the maximum rainfall are lifted above the cold front, attesting its key role in focusing convection. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting seem to be maintained by low-level leeward convergence, mountain lee waves and a favourable directional wind shear; when terrain is flattened, rainfall is substantially reduced. The location of the exceptional precipitation appears to be driven primarily by the location of the quasi-stationary cold front and secondarily by the location of convective bands downwind of the orography.
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