Role of the interaction between a gust front and a mesoscale air mass boundary in convection initiation: a case study

CUI Xinyan, QIN Rui, CHEN Mingxuan and HAN Lei

aCollege of Oceanic and Atmospheric Sciences,Ocean University of China,Qingdao,China;bInstitute of Urban Meteorology,China Meteorological Administration,Beijing,China; cCollege of Information Science and Engineering,Ocean University of China,Qingdao,China

ABSTRACT The local convection initiation (CI) mechanisms of a convective case that occurred on 5 August 2017 in Cangzhou, northern China, were studied using Doppler radar and automatic weather station observational analysis, along with Variational Doppler Radar Analysis System assimilation analysis.During the convective process,a gust front appeared ahead of two existing convective systems,respectively.In the warm and moist environment ahead of the gust fronts in the south, there was a mesoscale air mass boundary. With the process of a gust front moving southward, approaching the mesoscale air mass boundary, the convergence intensified in the area between the gust front and the mesoscale air mass boundary.Finally,the strong convergent updraft exceeded the level of free convection and triggered the new convection.

KEYWORDS Convection initiation; gust front; mesoscale air mass boundary; 4D variational data assimilation

1. Introduction

Understanding convective initiation (CI) mechanisms remains a great challenge due to the localized scale and sudden occurrences of strong convective weather.Several studies have shown that boundaries influence CI, and such boundaries include gust fronts, sea breeze fronts, drylines, cold fronts, horizontal convective rolls,the boundaries caused by different heating,and terraininduced convergence zones (Wilson and Roberts 2006).However, not all boundaries can trigger CI, and it is challenging to accurately predict the CI timing and location because when or where CI appears along boundaries is uncertain (Wilson and Schreiber 1986).Many studies have focused on changes along the gust front and have shown that some locations along the gust front are more likely to produce CI, such as misocyclones (Fujita 1981), ‘bulges' in the convergence lines (Hane et al. 1997), mesoscale low pressure areas(Bluestein et al. 1988), and gravity waves (Koch 1982).The generation of CI may sometimes be affected by multiple boundaries, such as the interaction of gust fronts (Wilson and Roberts 2006). However, there are few relevant studies regarding how the interaction of the gust front and mesoscale environmental boundary can lead to CI, which is the focus of this study.

Specifically, we examine the interaction process between a gust front and a mesoscale air mass boundary that occurred in Cangzhou, northern China, on 5 August 2017. High-resolution assimilation analysis based on a Variational Doppler Radar Analysis System(VDRAS) is carried out to study the CI mechanism.

This paper is organized as follows. In section 2, the data and model configurations used in this study are introduced. The CI process involved in this convective event is analyzed in section 3 through radar and automatic weather station (AWS) observations. In section 4,the assimilation analysis results are combined with the observational analysis presented in section 3 to further analyze and study the CI process. Finally, the main conclusions of this study are provided in section 5.

2. Data and simulation configurations

The CI case occurred at about 0424 UTC(UTC+8 h=LST;the same hereafter)on 5 August 2017.A VDRAS was used in this study for this convective process. VDRAS is a system based on Rapidly Refresh 4D Variational assimilation technology, using AWS and radar observations as its assimilation data and WRF model simulation results as background and boundary conditions (Sun and Crook 1997, 1998). In this study,a 730-s window was used that covered at least two radar scans from each of the seven radars assimilated(see Figure S1 in the supplementary material for the names and locations of the seven radars). These radars are run operationally, producing reflectivity and radial velocity data at nine elevation angles with a volume scan rate of 6 min.Results from a 1-km-resolution WRF model simulation were merged with the surface data to create a mesoscale background for the 4D variational(4DVar)radar data assimilation. The mesoscale background not only provided the boundary conditions, but also a first guess. The center of the VDRAS simulation was at (38.4°N, 116.5°E), and the range was 300 km×300 km,which covered most areas of Cangzhou and its vicinity(frame in Figure S1).The horizontal resolution of the simulation was 1 km and the vertical resolutionwas100 m.There were 150 verticallayers and the lowest layer was half the vertical resolution,i.e.50 m.The simulation model top was 14.95 km, the radar data were assimilated below 8.5 km,and there was a sponge boundary layer above 8.5 km.VDRAS is distinct from other 4DVar assimilation systems by its ability to provide rapid update cycles with a frequency of 15 min or less(Sun,Chen,and Wang 2010),which is important for nowcasting CI.Though VDRAS has analysis uncertainties over mountains because it cannot fully resolve the terrain, it can produce reliable analyses over plains. The CI case in this study mainly occurred over a plain,and thus VDRAS could be used with confidence. For technical details and important improvements in the VDRAS system, readers are referred to the relevant literature(Sun,Chen,and Wang 2010;Chen et al.2011),as this information will not be repeated here.

3. Observation and analysis of the CI process

Based on the radar reflectivity (left-hand column in Figure 1), there were two convection cells, A and B,and a gust front appeared ahead of them during their respective southward movements. Also, there was a boundary to the south of the gust fronts, where the wind direction changed near the ground (right-hand column in Figure 1).However, due to the lack of observations of the upper-level air,it is not known what kind of boundary it was. Two new convection cells, C1 and C2, initiated ahead of the gust front of convection cell A at 0424 UTC and 0448 UTC, respectively, between convection cell A's gust front and the boundary. The air was warm and moist and the wind field was convergent in this area, which was conducive to the initiation and development of convection. Then, C1 continued to develop into strong linear convection.The development of C2 was not as strong as that of C1. One part of C2 eventually weakened and disappeared, while the other part merged with C1 to form strong linear convection.

Therefore,the initiation of the new convection cell,C1,may have been influenced by convection cell A's gust front and the boundary in the south. However, the role of the gust front and the boundary in the CI process can be further revealed by VDRAS analysis.VDRAS assimilates radar and AWS data while blending with the background fields from the WRF simulation,producing rapid updated 3D meteorological analyses.In the following section,fine microphysical and thermodynamic fields of the VDRAS assimilation analysis are used to study the primary initiation mechanism of convection cell C1.

4. VDRAS analysis results

From the VDRAS analysis fields(Figure 2),at 0430 UTC,a gust front appeared in front of convection cells A and B, and there was a boundary in the south,which was consistent with observations. There was clear wind shear near the ground, but the wind shear was not as obvious as the humidity and temperature gradient above 0.5 km.Moreover,the temperature and humidity could not be observed above the ground, so it was difficult to determine whether there was a humidity and temperature gradient above the ground. However, the trend of the southern part of the wind shear in the AWS observations and the southern part of the humidity and temperature gradient above 0.5 km in the VDRAS analysis results were consistent(Figure 2(e,f)). Therefore,it was a mesoscale air mass boundary (Markowski and Richardson 2010)there. In the process of the gust front moving southward,the convergence between the gust front and the mesoscale air mass boundary intensified, some new discrete convection cells appeared at 0424 UTC, and then continued to develop into linear convection(Figure 2(c,d)). Based on the cross sections of the analysis fields of VDRAS, the convergence of the boundary reached up to 2 km, and the gust front of convection cell A moved southward, approaching the boundary in the south(left-hand column in Figure S2).Then, because the convergence of the wind fields strengthened on both sides close to the gust front and mesoscale air mass boundary, respectively, the updraft intensified between the gust front and the boundary at the positions of the new convection cells, C1 and C2 (Figure S3). The enhanced updraft exceeded the level of free convection (LFC) and eventually triggered CI (right-hand column in Figure S2).Then, the new convection cells, C1 and C2, merged into strong linear convection.

Figure 1. Cangzhou radar observations. Left: radar reflectivity (color shaded; units: dBZ) of the lowest elevation angle (0.5°). Right:water vapor mixing ratio (colored dots; units: g kg-1) and surface wind (barbs; units: m s-1 (full barb = 5 m s-1; half barb = 2.5 m s-1)) of the AWS observations. Black solid bold lines and black dashed bold lines indicate the positions of the gust fronts and the mesoscale air mass boundary, respectively. ‘+' indicates the location of Cangzhou radar location. (a) 0342 UTC; (b) 0340 UTC; (c)0424 UTC; (d) 0425 UTC; (e) 0448 UTC; (f) 0450 UTC.

Figure 2.VDRAS analysis results at 0430 UTC:(a)maximum reflectivity(color shaded;units:dBZ);(b)water vapor mixing ratio at the lowest level of 50 m (color shaded; units: g kg-1); (c) divergence at the lowest level of 50 m (color shaded; units: m s-1 km-1); (d)vertical velocity at the level of 3.05 km (color shaded; units: m s-1); (e) equivalent potential temperature at the level of 1.95 km(color shaded;units:K);(f)water vapor mixing ratio at the level of 1.95 km(color shaded;units:g kg-1).The wind is overlaid in all panels (arrowed black vectors; length proportional to wind speed). Black solid bold lines and black dashed bold lines indicate the positions of the gust fronts and the mesoscale air mass boundary, respectively. The red solid line ‘L1' in (a) is the position of the cross sections in Figures S2 and S3 in the supplementary material.

Figure 3.Schematic diagram of CI generated by the interaction between the gust front and the mesoscale air mass boundary.In the warm and moist environment ahead of the gust front in the south, there is a mesoscale air mass boundary. There is convergence along the gust front and in the banded zone of the mesoscale air mass boundary,but this convergence alone is not enough to trigger CI. With the process of the gust front moving southward, approaching the mesoscale air mass boundary, the convergence intensifies in the area between the gust front and the mesoscale air mass boundary,and finally the strong updraft exceeds the LFC and triggers the new convection.

Therefore, based on observational analysis and the fine thermodynamic fields of VDRAS assimilation analysis, a schematic summary of the CI mechanism can be provided (Figure 3). In this convective process, a gust front appeared ahead of two existing convective systems, separately. In the warm and moist environment ahead of the gust front in the south, there was a mesoscale air mass boundary. The convergence along the gust front and in the banded zone of the mesoscale air mass boundary alone was not enough to trigger CI. With the process of the gust front moving southward,approaching the mesoscale air mass boundary, the convergence intensified in the area between the gust front and the mesoscale air mass boundary.Finally, the strong updraft exceeded the LFC and triggered the new convection.

5. Conclusion

The mechanisms of CI were studied using Doppler radar and AWS observational analysis, along with VDRAS assimilation analysis, based on a case that occurred in Cangzhou, northern China. The CI process involved the interaction between the gust front and the mesoscale air mass boundary.

During the convective process, a gust front appeared ahead of two existing convective systems,separately. In the warm and moist environment ahead of the gust front in the south, there was a mesoscale air mass boundary. With the process of the gust front moving southward, approaching the mesoscale air mass boundary, the convergence intensified in the area between the gust front and the mesoscale air mass boundary. Finally, the strong convergent updraft exceeded the LFC and triggered the new convection. Also, the VDRAS assimilation analysis showed the development of the two existing convective systems and their gust fronts along with the mesoscale air mass boundary correctly,indicating the initiation process of the new convection. Therefore, VDRAS can be used as an effective tool for analyzing the role of the interaction of the gust front and the mesoscale air mass boundary in CI.

Funding

This work was supported by the Beijing Municipal Science and Technology Project [grant number Z171100004417008] and the National Natural Science Foundation of China[grant numbers 41575050, 41875049, and 41805034].