Climatological Features of Squall Line at the Borneo Coastline during Southwest Monsoon

Borneo Squall Line (BSL) is a disaster risk associated with intense rain and wind gust that affect the activities and residence near the northern coast of Borneo. Using 3-hourly rainfall from Tropical Rainfall Measuring Mission (TRMM) 3B42V7 during southwest monsoon season (May–September) from 1998–2018, a total of 629 squall days were identified. Their monthly and annual average was 6 and 30 days, respectively, with July representing the month with the highest number of squall line days. BSL is frequently initiated during midnight/predawn and terminated in the morning. Composite analyses of BSL days using the daily winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim revealed that lower tropospheric wind convergence is a crucial controlling factor for BSL formation. The position of the monsoon trough closer to the equatorial South China Sea (SCS), and strong westerly and south-westerly winds played an important role in creating this wind convergence region. Analyses of tropical cyclone (TC) data from the Regional Specialized Meteorological Centre (RSMC), Tokyo showed that nearly 72% of BSL occurred with the presence of TC. Spectral analysis exhibited prominent frequencies mainly in the 3–4- and 6-year time scale, which likely reflected the influence of interannual modulation of El-Niño Southern Oscillation (ENSO). Correlation coefficient between squall days and Sea Surface Temperature (SST) anomalies indicated that BSL increased after La-Niña events. This study is expected to have implications for real-time squall line forecasting in Malaysia and contributes toward a better understanding of BSL.

The influence of multiple groups of biological ice nucleating particles on microphysical properties of mixed-phase clouds observed during MC3E

A new empirical parameterization (EP) for multiple groups of primary biological aerosol particles (PBAPs) is implemented in the aerosol cloud model (AC) to investigate their roles as ice-nucleating particles (INPs). The EP describes the heterogeneous ice nucleation by (1) fungal spores, (2) bacteria, (3) pollen, (4) detritus of plants, animals, and viruses, and (5) algae. Each group includes fragments from the originally emitted particles. A high-resolution simulation of a midlatitude mesoscale squall line by AC is validated against airborne and ground observations. Sensitivity tests are carried out by varying the initial vertical profiles of the loadings of individual PBAP groups. The resulting changes in warm and ice microphysical parameters are investigated. Overall, PBAPs have little effect on the ice phase, especially in the convective region. In the stratiform region, increasing the initial PBAP loadings by a factor of 100 resulted in less than 60 % change in ice number concentrations. The total ice concentration is mostly controlled by various mechanisms of secondary ice production (SIP). However, when SIP is artificially prohibited in sensitivity tests, increasing the PBAP loading by a factor of 100 has no significant effect on the ice phase. Further sensitivity tests revealed that PBAPs have little effect on surface precipitation as well as on shortwave and longwave flux.

Analysis of three unusual severe weather events over Delhi during May-June, 2018 using Dual-Pol Doppler Weather Radar and GNSS data

High-impact weather events, such as thunderstorms and dust storms, are aspects of a changing climate that are likely to have an adverse effect on society. A number of such severe weather events struck Delhi and adjoining areas during the months of April, May and June of the year 2018. Three events observed during May-June have been analyzed using observations from C-Band Polarimetric Doppler Weather Radar (DWR) and ground based Global Navigational Satellite (GNSS) receiving system installed at Mausam Bhawan, New Delhi. Here, an attempt has been made to study the data regarding these unusual events from DWR observations especially of polarimetric nature and cross verify it with the data obtained from GNSS receiving system. Reflectivity of more than 60 dbZ was observed in all the events by the DWR system except on 9 June when a squall line formed with maximum reflectivity around 54 dBZ and the wind velocity increased upto 120 knots on the same date on few occasions and generally varied between 45-60 knots during the period of the storms. The height of these storms varied between 12 kms and 13.6 kms except on 9 June when the storm height was observed to be more than 15 kms by the DWR. Though the maximum reflectivity was a bit less on 9th June but the vertical extent of the clouds was greater and therefore the estimated value of IPWV from GNSS had a maximum of 67 mm as compared to the values in the range of 40 to 45 mm for other storm events. Apart from the single-pol DWR observations, the dual-pol products presented a more comprehensive ingredients of the storms in respect of the size, shape and variety of the hydrometeors and also their non-meteorological nature. The information regarding the concentration of hydrometeors has also been a positive point while analyzing through the eyes of a dual-pol radar. These multiple thunderstorms have been discussed to bring out some of their important features and a good amount of agreement has been observed between the data obtained from dual-pol DWR system and GNSS.

Asymmetric Structures of a Squall-Line MCS over Taiwan with Significant Hydraulic Jumps

On 19 April 2019, a mature squall-line mesoscale convective system (MCS) with the characteristics of a leading convective line and trailing stratiform landed on Taiwan, resulting in strong gust wind and heavy rainfall. This squall-line MCS became asymmetric after landfall on Taiwan. Two sets of idealized numerical simulations (mountain heights and low-level vertical wind shear) using the Weather Research and Forecasting (WRF) model were conducted to examine the impacts of realistic Taiwan topography on a squall-line MCS. Results showed numerous similarities between the idealized simulations and real-case observations. The low-level Froude number which considered the terrain height (Fmt) was calculated to examine the blocking effect of the Taiwan terrain, and the cold pool (determined by − 1.5 K isotherm) was found to be completely blocked by the 500-m height contour. The northeast-southwest orientation of the Snow Mountain Range (SMR), and the north–south orientation of the Central Mountain Range (CMR) led to the upwind side asymmetry. On the other hand, the lee-side asymmetry was associated with different intensities and occurrence locations of the hydraulic jump between the SMR and southern CMR, and the cold-pool Froude number (Fcp) indicated the flow-regime transition from subcritical to supercritical.

Assessment of the data assimilation framework for the Rapid Refresh Forecast System v0.1 and impacts on forecasts of convective storms

The Rapid Refresh Forecast System (RRFS) is currently under development and aims to replace the National Centers for Environmental Prediction (NCEP) operational suite of regional and convective scale modeling systems in the next upgrade. In order to achieve skillful forecasts comparable to the current operational suite, each component of the RRFS needs to be configured through exhaustive testing and evaluation. The current data assimilation component uses the Gridpoint Statistical Interpolation (GSI) system. In this study, various data assimilation algorithms and configurations in GSI are assessed for their impacts on RRFS analyses and forecasts of a squall line over Oklahoma on 4 May 2020. Results show that a baseline RRFS run without data assimilation is able to represent the observed convection, but with stronger cells and large location errors. With data assimilation, these errors are reduced, especially in the 4 and 6 h forecasts using 75 % of the ensemble background error covariance (BEC) and with the supersaturation removal function activated in GSI. Decreasing the vertical ensemble localization radius in the first 10 layers of the hybrid analysis results in overall less skillful forecasts. Convection and precipitation are overforecast in most forecast hours when using planetary boundary layer pseudo-observations, but the root mean square error and bias of the 2 h forecast of 2 m dew point temperature are reduced by 1.6 K during the afternoon hours. Lighter hourly accumulated precipitation is predicted better when using 100 % ensemble BEC in the first 4 h forecast, but heavier hourly accumulated precipitation is better predicted with 75 % ensemble BEC. Our results provide insight into current capabilities of the RRFS data assimilation system and identify configurations that should be considered as candidates for the first version of RRFS.

Simulation of the Dynamic and Thermodynamic Structure and Microphysical Evolution of a Squall Line in South China

A squall line that occurred in south China on 31 March 2014 was simulated with the Weather Research and Forecasting model. The microphysical processes had an important influence on the dynamic and thermodynamic structure of the squall line. The process of water vapor condensation (PCC+) provided heat for the ascending movement inside the squall line. The forward movement of the heating area of PCC+ was an important reason for the squall line’s tilting. The convergence of the outflow of the cold pool and the warm and wet air constantly triggered new convection cells in the front of the cold pool, which made the squall line propagate forwards. The cooling process of graupel melting into rain corresponded closely with the rear inflow jet. During the mature period of the squall line, the effect of cooling strengthened the rear inflow jet. This promoted low-layer inflow and a convective ascending motion, thus further promoting the development of the squall line system. During the decay period, the strong backflow center of the stratospheric region cut off the forward inflow of the middle and low layer towards the high layer, and cooperated with the cold pool to cut off the warm and wet air transport of the low layer, making the system decline gradually.

Convective Cloud Clusters and Squall Lines along coastal Amazon

Mesoscale convective cloud clusters develop and organize in the form of squall lines along the coastal Amazon in the afternoon hours and propagate inland during the evening hours. The frequency, location, organization into lines and movement of the convective systems are determined by analyzing the “precipitation features” obtained from the TRMM satellite for the period 1998-2014. The convective clusters and their alignments into Amazon coastal squall lines are more frequent from December through July and they mostly stay within 170 km from the coast line. Their development and movement in the afternoon and evening hours of about 14 m s-1 are helped by the sea breeze. Negative phase of Atlantic Dipole and La Niña combined increase the frequency of convective clusters over coastal Amazon. Composite environmental conditions of 13 large Amazon coastal squall line cases in April show that conditional instability increases from 09 LT to 12 LT and the wind profiles show a jet like structure in low levels. The differences in the vertical profiles of temperature and humidity between the large squall line composites and no-squall line composites are weak. However, appreciable increase in the mean value of CAPE from 09 LT to 15 LT is found in large squall line composite. The mean mixing ratio of mixed layer at 09 LT in La Niña situations is significantly larger in the large squall line composite. Thus, CAPE and mixed layer mixing ratio are considered promising indicators of the convective activity over the coastal belt of the Amazon Basin.