Tuesday, 29 August 2023
Boundary Waters (Hyatt Regency Minneapolis)
Handout (2.5 MB)
Abstract
Integrated radar-instrument analyses were conducted for a number of mesoscale convective storms (MCS) that affected Hong Kong in 2021-2022 using different frequency bands of weather radars, wind profiler as well as anemometer, lightning and weather camera networks operated by the Hong Kong Observatory (HKO). The radars included two S-band (3 GHz) weather radars at Tai Mo Shan (TMS) and Tate’s Cairn (TC) respectively, a C-band (5 GHz) Terminal Doppler Weather Radar (TDWR) at Brothers Point (BP) and an X-band (10 GHz) Phased Array Weather Radar (PAWR) at Sha Lo Wan (SLW). The S-band radar at TC and the PAWR were dual-polarisation weather radars.
Three different MCS cases causing high impact weather in Hong Kong including hail, waterspout, microburst and windshear that occurred chronologically on 16 September 2021, 8 June 2022 and 18 September 2022 respectively have been examined. For the 1st case, the PAWR enabled detailed observations of the structures of convective cells with hails detected at an altitude range of 3 to 7 km and the 0C freezing height was estimated to be around 4 km. Hail stones fell on the ground with sizes of around 1 cm or less were reported by members of the public. The presence of mixed ice crystals and graupels promoted cloud electrification, resulting in active cloud-to-cloud and cloud-to-ground lightnings. Hails could also be identified from the hydro-classification products from the S-band weather radar at TC but it was difficult to analyse the detailed structures of the convective cells.
For the 2nd case, the waterspout whose lifetime was less than half an hour was well captured by HKO’s weather camera and the PAWR. The horizontal extent and vertical height of the associated velocity couplet observed by the PAWR were over 1 km and 5 km respectively while the maximum velocity was estimated to be above 16 m/s. In this case, signals from the two S-band radars showing the potential formation of a vortex tube associated with the waterspout were not as strong as the PAWR.
The 3rd case was related to thundery showers triggered by intense insolation and it caused the development of microburst and significant windshear as detected by the BP TDWR. During the impact of microburst, the BP TDWR once outputted shear magnitude of over 36 m/s although the sign of an upper-level mesocyclone was not apparent. The differential reflectivity (ZDR) from the PAWR was a good indicator for diagnosing vertical motion with positive and negative ZDR indicating updraughts and downdraughts respectively. In connection to the microburst-producing downdraught, the PAWR showed negative ZDR column in the height range of 0.5 to 4 km. Based on the cross-sections of PAWR images, a region of high winds was observed to descend downwards from about 2 km to the surface in less than 20 minutes and the touch down caused strengthening of surface winds to strong force level as well as around 5C drop of air temperatures as recorded by one of the HKO’s automatic weather stations. Coincidentally, the wind profiler showed a vertical downward velocity of about 8 m/s at a height range of 0.9–1.4 km. Regarding the maximum radar reflectivity of the intense echo triggering the microburst, by comparing the respective low-level PPI images, the two S-band radars showed similar level of around 55 dBZ. It was 1-2 dBZ lower for the C-band BP TDWR which was reasonable as there was no correction of attenuation caused by precipitation. For the X-band PAWR, it was 4-5 dBZ higher which suggested that the associated attenuation correction was overdone.
In summary, a comparison of the different frequency bands of weather radars suggested that the PAWR was more able to capture the mesoscale features of convective cells such as waterspout due to high spatial resolution (30 m) of the PAWR imagery. Also, the PAWR could depict more clearly different hydrometeors, for examples, hails, ice crystals and graupels embedded within convective cells. While the BP TDWR detected the occurrence of microburst and determined the severity of windshear to support HKO’s aviation weather services for the operation of the Hong Kong International Airport (HKIA), the complementary use of dual-polarisation PAWR enabled a better appreciation of the positions and strength of updrafts and downdraughts through the use of ZDR columns and using S-band radars to better assess the intensity of radar echoes. Coupled with other instruments’ observations, a more thorough understanding on the evolution of MCS was achieved. More in-depth meteorological analysis and performance evaluation of the weather radars in the above three cases would be covered in this paper.
Integrated radar-instrument analyses were conducted for a number of mesoscale convective storms (MCS) that affected Hong Kong in 2021-2022 using different frequency bands of weather radars, wind profiler as well as anemometer, lightning and weather camera networks operated by the Hong Kong Observatory (HKO). The radars included two S-band (3 GHz) weather radars at Tai Mo Shan (TMS) and Tate’s Cairn (TC) respectively, a C-band (5 GHz) Terminal Doppler Weather Radar (TDWR) at Brothers Point (BP) and an X-band (10 GHz) Phased Array Weather Radar (PAWR) at Sha Lo Wan (SLW). The S-band radar at TC and the PAWR were dual-polarisation weather radars.
Three different MCS cases causing high impact weather in Hong Kong including hail, waterspout, microburst and windshear that occurred chronologically on 16 September 2021, 8 June 2022 and 18 September 2022 respectively have been examined. For the 1st case, the PAWR enabled detailed observations of the structures of convective cells with hails detected at an altitude range of 3 to 7 km and the 0C freezing height was estimated to be around 4 km. Hail stones fell on the ground with sizes of around 1 cm or less were reported by members of the public. The presence of mixed ice crystals and graupels promoted cloud electrification, resulting in active cloud-to-cloud and cloud-to-ground lightnings. Hails could also be identified from the hydro-classification products from the S-band weather radar at TC but it was difficult to analyse the detailed structures of the convective cells.
For the 2nd case, the waterspout whose lifetime was less than half an hour was well captured by HKO’s weather camera and the PAWR. The horizontal extent and vertical height of the associated velocity couplet observed by the PAWR were over 1 km and 5 km respectively while the maximum velocity was estimated to be above 16 m/s. In this case, signals from the two S-band radars showing the potential formation of a vortex tube associated with the waterspout were not as strong as the PAWR.
The 3rd case was related to thundery showers triggered by intense insolation and it caused the development of microburst and significant windshear as detected by the BP TDWR. During the impact of microburst, the BP TDWR once outputted shear magnitude of over 36 m/s although the sign of an upper-level mesocyclone was not apparent. The differential reflectivity (ZDR) from the PAWR was a good indicator for diagnosing vertical motion with positive and negative ZDR indicating updraughts and downdraughts respectively. In connection to the microburst-producing downdraught, the PAWR showed negative ZDR column in the height range of 0.5 to 4 km. Based on the cross-sections of PAWR images, a region of high winds was observed to descend downwards from about 2 km to the surface in less than 20 minutes and the touch down caused strengthening of surface winds to strong force level as well as around 5C drop of air temperatures as recorded by one of the HKO’s automatic weather stations. Coincidentally, the wind profiler showed a vertical downward velocity of about 8 m/s at a height range of 0.9–1.4 km. Regarding the maximum radar reflectivity of the intense echo triggering the microburst, by comparing the respective low-level PPI images, the two S-band radars showed similar level of around 55 dBZ. It was 1-2 dBZ lower for the C-band BP TDWR which was reasonable as there was no correction of attenuation caused by precipitation. For the X-band PAWR, it was 4-5 dBZ higher which suggested that the associated attenuation correction was overdone.
In summary, a comparison of the different frequency bands of weather radars suggested that the PAWR was more able to capture the mesoscale features of convective cells such as waterspout due to high spatial resolution (30 m) of the PAWR imagery. Also, the PAWR could depict more clearly different hydrometeors, for examples, hails, ice crystals and graupels embedded within convective cells. While the BP TDWR detected the occurrence of microburst and determined the severity of windshear to support HKO’s aviation weather services for the operation of the Hong Kong International Airport (HKIA), the complementary use of dual-polarisation PAWR enabled a better appreciation of the positions and strength of updrafts and downdraughts through the use of ZDR columns and using S-band radars to better assess the intensity of radar echoes. Coupled with other instruments’ observations, a more thorough understanding on the evolution of MCS was achieved. More in-depth meteorological analysis and performance evaluation of the weather radars in the above three cases would be covered in this paper.
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