Enhancing wind forecasts through assimilation of maximum wind observations in WRF-3DVar: A comparison with conventional AWS data for a severe squall line

In this paper, the Weather Research and Forecasting (WRF) Model with the three-dimensional variational data assimilation (WRF-3DVAR) system is used to investigate the impact on the near-surface wind forecast of assimilating both conventional data and Advanced Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (ATOVS) radiances compared with assimilating conventional data only. The results show that the quality of the initial field and the forecast performance of wind in the lower atmosphere are improved in both assimilation cases. Assimilation results capture the spatial distribution of the wind speed, and the observation data assimilation has a positive effect on near-surface wind forecasts. Although the impacts of assimilating ATOVS radiances on near-surface wind forecasts are limited, the fine structure of local weather systems illustrated by the WRF-3DVAR system suggests that assimilating ATOVS radiances has a positive effect on the near-surface wind forecast under conditions that ATOVS radiances in the initial condition are properly amplified. Assimilating conventional data is an effective approach for improving the forecast of the near-surface wind.

The role of the cold pool in a mountain-to-plain and plain-to-mountain squall line case in the south of the Yangtze River in China

On 13 March 1993 a powerful prefontal squall line hit western and central Cuba. At 0000 UTC, a fast-developing extratropical cyclone with a well-defined cold front was located in the northeast Gulf of Mexico. Over the eastern Gulf of Mexico there was a 300-hPa jet streak interacting with a low-level jet. A hodograph typically associated with supercells or severe squall lines was observed at Key West, Florida. The air mass over the eastern Gulf of Mexico, Florida, and Cuba was conditionally unstable. Instability was enhanced by moist air at the surface interacting with a dry intrusion in the midtroposphere from the Mexican Plateau. Lifting induced by a strong short-wave trough favored the formation a compact squall line with embedded bow-echoes and line-echo wave patterns. Damaging winds were widespread with the squall line. Thus, the event fits the definition of a serial derecho. One of the two bow echoes that affected Cuba included a small but strong mesocyclone in its forward flank, as detected by radar and surface observations. The mesocyclone caused great damage in a 20-km-wide strip over the Havana and Havana City provinces, producing a family of downbursts in its southern portion. Estimated damage up to F2 on the Fujita scale was experienced in the effected area. Another bow echo struck the central region of Cuba producing similar damage. The squall line event described in this study is the most damaging one ever recorded in Cuba.

Cloud-resolving simulation and mechanistic analysis of a squall line in East China

Tropical cyclones form over the seas: a typical data‐sparse region for conventional observations. Therefore, satellites, especially with microwave sensors, are ideal for cyclone studies. The advanced microwave sounding unit (AMSU) , in addition to providing very valuable data over non‐precipitating cloudy regions, can provide very high horizontal resolution of the temperature and humidity soundings. Such high‐resolution microwave data can improve the poorly analysed cyclone. The objective of this study is to investigate the impact of ingesting and assimilating the AMSU data together with conventional upper air and surface meteorological observations over India on the prediction of a tropical cyclone which formed over the Arabian Sea during November 2003 using analysis nudging. The impact of assimilating the AMSU‐derived temperature and humidity vertical profiles in a mesoscale model has not been tested yet over the Indian region. Such studies are important as most weather systems over India form over the seas. The present study is unique in the sense that it addresses the impact of ingesting and assimilating microwave sounding data (together with conventional India Meteorological Department data) on the prediction of a tropical cyclone, which formed over the Arabian Sea during November 2003 using analysis nudging. Two sets of numerical experiments are designed in this study. While the first set utilizes the National Center for Environmental Prediction (NCEP) reanalysis (for the initial and lateral boundary conditions) only in the fifth‐generation mesoscale model simulation, the second set utilized the AMSU satellite and conventional meteorological upper air and surface data to provide an improved analysis through analysis nudging. The results of the two sets of model simulations are compared with one another as well as with the NCEP reanalysis and the observations. The results of the study indicated that the impact of ingesting and assimilating microwave sounding data and the conventional meteorological data through nudging resulted in an improvement in the simulation of wind asymmetries and the warm temperature anomalies. The with‐assimilation run simulated stronger wind speeds and stronger vertical velocity motion as compared with the without‐assimilation run. The time series of the minimum sea level pressure (SLP) and maximum wind speed for the simulations with the microwave sounding data and conventional meteorological data show better agreement with the observations than the simulations without the assimilation. The central minimum pressure of the simulations with the modified analysis are lower by 7 hPa as compared with the simulations without the assimilations. Even though there is not much of a difference in the maximum wind speed between the two simulations at the initial forecast time, the results indicate that the simulations with microwave sounding data and conventional meteorological data reveal a marked (9 m/s) increase in the maximum wind speed over the simulations without the assimilation. While the lowest central pressure estimated from the satellite image is 988 hPa, the simulations with microwave sounding data and conventional meteorological data show a value of 999.5 hPa for the lowest central minimum pressure. One reason for the inability of the simulation with improved analysis to achieve the observed lowest SLP is that the NCEP reanalysis had manifested an extremely weak system in the first place and, despite assimilation with microwave sounding data and conventional meteorological data, only a moderate improvement in the lowest SLP could be achieved. A proper appreciation of the impact of the microwave sounding data can be obtained by comparing with the lowest SLP obtained from the simulation without assimilation which showed a value of 1007 hPa. The initial mis‐representation in the location of the centre of the cyclone in the NCEP reanalysis with respect to the observed location has led to marked errors in the track prediction of both the model simulations. The assimilation of microwave satellite data is yet to be implemented in the current operational regional model over India and hence the results of this study may be relevant to the operational tropical cyclone forecasting community.

In the afternoon of June 17, 2020, a squall line occurred over the eastern region of Helan Mountain in Ningxia, accompanied by lightning, strong wind and short-term heavy precipitation, which affected north and middle part of Ningxia and Wuhai of Inner Mongolia. Basing on the Doppler radar data and conventional meteorological observation data, the weather process was analyzed. the results show that: (1) the cold air from the middle and high latitudes intersected with the near surface warming air in Alashan of Inner Mongolia, which promoted the development of convection and induced the formation of squall line; (2) the temperature and pressure field at high and low level were asymmetrical and the weather system presented a forward tilting structure, which was conducive to the occurrence of strong convective weathers such as squall line; (3) the squall line strengthened when moving across Helan Mountain due to the topographic uplifting and the strong vertical wind shear;(4) After moving into north and middle part of Ningxia, the convergence and divergence configuration of the squall line weakened, the circulation energy faded and the convective structure and circulation field became incomplete gradually, the inclined sinking airflow in the back of the squall line gradually weakened due to the precipitation.

The two squall lines happened in Huanghuai region of China on 17 July 2009 and 19 July 2010. The conventional and non-conventional data including radar data, sounding data, auto-weather-station, significant weather report etc. are used in this paper, to contrast and explore the genesis conditions and mechanisms for the two squall lines. The results show that the two squall lines both occurred ahead of trough at 500 hPa. There is frontal passage beyond the ground surface. The southerly jet stream exists in low level. Further dry line lies at the north of the Shandong Province in the low level. The PWAT is larger than 50 mm. Of course there are different in these two processes. Vertical wind shear is larger on 7 July 2009 than that on 19 July 2010. While Shandong Province lies under the crossing of high and low jet stream on 19 July 2010. The high wind and short duration heavy rain occurred easily under this pattern。There are different reasons for hail not happened. For the squall line occurred on 17 July 2009, the 0℃ level is high, while for the other one, the CAPE is smaller. These results can help us to forecast the squall line better in Huanghuai region of China.

Abstract. On 1 August 1674 an active cold front moved over the Low Countries. The accompanying thunderstorms along the squall line were abnormally active, leading to large-scale damage in Europe, from northern France to the northern parts of Holland where damages were particularly severe. Using reported and pictured observations of damages and modern meteorological concepts, the reconstruction of the storm points to an exceptionally severe squall line. The orientation and the velocity of the squall line are reconstructed and shows a developed bow-echo structure. An estimate of the strength of the strongest wind gusts is ≈ 55–90 m s−1 and is based on an assessment of the damages caused by this event. A rough estimate of the return time of this event, based on observed hail size, is between 1000 and 10 000 years. This storm is compared to a more recent storm which was similar in dynamics but much less devastating. Special attention is given to the city of Utrecht which was hit hardest, and where the impact of this storm is still recognizable in the cityscape.

On 1 August 1674 an active cold front moved over the Low Countries. The accompanying thunderstorms along the squall line were abnormally active, leading to large-scale damage in Europe, from northern France to the northern parts of Holland where damages were particularly severe. The damages included destroyed bridges, like in Antwerp, to numerous churches, churchtowers and other buildings and ships lost in the harbour or at sea. The city of Utrecht was hit hardest by this event resulting in widespread damages that are still recognizable in the city landscape. Using reported and pictured observations of damages and modern meteorological concepts, the reconstruction of the storm points to an exceptionally severe squall line. The orientation and the velocity of the squall line are reconstructed and shows a developed bow-echo structure. An estimate of the strength of the strongest wind gusts is ≈ 55–90 m s−1 and is based on an assessment of the damages caused by this event. A rough estimate of the return time of this event, based on observed hail size, is between 1000 and 10,000 years.The view in this presentation disagrees with the common perception that the damages are caused by a single tornado and the arguments for this novel view will be presented. However, there is evidence of embedded vortices in the damages to the city of Utrecht. These observations are discussed in the presentation.

Comparative analysis of dry intrusion in the different position of pre-TC squall line on typhoon Lekima (1909) and Matsa (0509)

A study of three years of GOES satellite imagery has been conducted to determine whether synthesis of the imagery with surface diagnostic analyses may prove useful for predicting the precise location and time of formation of squall lines generated by a particular type of frontal circulation transverse to surface cold fronts. Existence of this circulation is inferred from the development of a thin Line of shallow Convection clouds (LC) along the front simultaneously with that of a mesoscale (<100 km wide) Clear Zone (CZ) immediately behind the front and at the leading edge of a large area of stratus clouds. The observations suggest that a thermally direct circulation transverse to the surface cold front generated the line convection and clear zone (in the upward and downward branches of the circulation, respectively) in all 15 cases which met the strict criteria for an LC/CZ. Squall lines were observed to form from the LC in 10 of the 15 cases examined, and nearly always within 90 min following the time when the CZ reached its maximum width. In addition, initial cumulonimbus development always occurred within 100 km of the diagnosed frontogenesis center at the LC. Therefore, this study suggests that both the timing and location of such squall lines should be predictable with very high accuracy. It is also shown that thermodynamic instability was insufficient for the formation of deep convection in the five non-thunderstorm cases. Our results also strongly support the hypothesis of Koch (1984) that this mesoscale circulation was generated by differential sensible heating acting in conjunction with geostrophic deformation effects. The contrast of cloudy skies behind the front (prior to CZ formation) with nearly clear skies ahead of the front is largely responsible for creation of the differential heating pattern. This suggests that forecasters should watch for such cloud patterns near cold fronts. Synoptic climatological conditions favoring the occurrence of this relatively rare phenomenon are also identified. The LC/CZ appears during the afternoon almost solely over the Great Plains states during spring and autumn. The line convection was found in all but one case to be parallel to, and either along or on the cyclonic side of, a prefrontal 850 mb jet. Although the LC/CZ is usually found on the anticyclonic side of upper-level jet streaks, it does not seem to prefer any particular jet quadrant. Diagnosis of the Sawyer-Eliassen equation for one case suggested that the mesoscale circulation was linked to a thermally direct circulation cell associated with the upper-level frontal zone. The information provided in this paper should be valuable to the operational forecaster concerned with having some guidance about specific mesoscale trigger mechanisms for squall lines. This phenomenon can be isolated with conventional surface and satellite data in real time to provide accurate and timely forecasts of the formation of squall line activity.

Numerical Weather Predication (NWP) is the main way to forecast the surface-layer wind of wind power farm. In order to improve the accuracy of the forecast with the existing observations, the WRF-3DVAR is used in two-way nested simulations with the data from National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analyses as initial and boundary conditions. Conventional data (sounding and surface observation) are assimilated through 3h assimilation cycles. The impacts of assimilation are examined through direct verification of the forecasts against the observations from conventional stations and the wind tower. It is found that the data assimilation can improve the accuracy of the forecast of surface-Layer wind up to 36h, and the improvement at conventional stations is more prominent than that at the wind tower.

To investigate the process of convective merger (CM) and its effect on thunderstorms evolution and corresponding electrical activity, a severe squall line that took place on 27 July 2015 over the Beijing Metropolitan Region (BMR) is simulated and studied using the Weather Research and Forecasting (WRF) model coupled with an explicit electrification lightning scheme (E‐WRF). The model‐simulated radar reflectivity reasonably captured the whole squall line evolution, including the merging of individual cells and storms. The variation of normalized flash frequency simulated by E‐WRF is highly consistent with the occurrence of a prominent flash rate surge during the CM both in the observation and the simulation. Cloud bridge is one of the regions where lightning events increase most. The upstream anvil and the sinking outflow led to the rapid connection and formation of new convective cells between the two older main storms. Subsequently, the mass of graupel and snow increased substantially at the middle level, and the mass of upper‐level ice was horizontally advected from upstream. During the merger, the in‐cloud charge distribution evolved from a staggered charge pocket structure into a vertically stratified five‐layer structure. This study supports the hypothesized role of CM in enhancing lightning activity, and further expands our understanding of CM effects on microphysics and charge‐redistribution.

By using the observation data, satellite data, lightening data and radar data etc., the severe squall line which occurred on Jun 3 in 2009 in Henan, Anhui and Jiangsu is analyzed. Results demonstrate that under the favorable cold vortex in North east of china, the outflow verge of thunderstorm formed by thermal convection in the afternoon and convergence line at the surface cause the mesoscale severe squall line in south of Shanxi and Henan province. The squall line followed by gale, hail etc. cause the casualty of people and loss of belongings.

In preparation for the Aeolus Doppler Wind Lidar satellite mission, single‐component wind information from conventional observations was assimilated into the ECMWF data assimilation system. Various Observing System Experiments have been designed and performed in order to estimate the impact of such information in numerical weather prediction. The evaluation used various adjoint diagnostic tools and traditional statistical verification methods. The importance of assimilating wind observations as either single component or full vector wind is evaluated. Comparisons between the assimilation of wind and mass observations were also made. Wind observations can lead to significant improvement in the upper troposphere, lower stratosphere and in the Tropics. Mass data are more valuable in the midlatitudes, particularly in the lower part of the atmosphere. The comparison of additional mass and wind observations in the Global Observing System is useful to understand the importance of future wind observations. The investigation also highlighted that the impact of zonal wind observations is larger than the meridional wind (Aeolus will mostly measure wind components near the zonal direction). Root mean‐squared errors of temperature and wind forecasts when the single wind component (zonal) is assimilated show around 35% degradation up to day 2 forecasts and around 20% after day 2 as compared to assimilating full vector wind. Therefore the single (zonal) wind components can provide a large fraction of the vector wind impact particularly for the medium‐range forecast, which is promising for Aeolus.