Evaluation of UM microphysics using dual-polarised radar simulator

Korea Meteorological Administration (KMA) has installed two dual-polarised radars to improve weather forecasting, particularly for severe weather in 2014. One of them was installed in Youngin Testbed(YIT, 37° 12′27.13964″N, 127° 17′8.25650″E, 460m at height) for researches. In addition, more eight dual-polarised radars, as severe weather early-stage detection equipement, will be installed to replace existing single-polarised radar over South Korea by 2018. Therefore, there is a growing needs for a better understanding of polarimetric radar variables and utilisation of observation by dual-polarised radars for better weather forcasting by numerical weather predictions. In order to cope with these demand, Radar Data Analysis Division in Weather Radar Centre and Numerical Model Development Division in KMA and Centre for Analysis and Prediction of Storms (CAPS), University of Oklahoma in USA have collaboratively developed dual-polarised simulators based on numerical weather models since 2013. In this study UM-based dual-polarised radar simulator is applied to understand different type of precipitation systems through comparison of simulated and observed polarimetric variables in order to improving microphysical processes of the UM. Unified model (UM) has been developed and categorized as Global, Regional and Local Data Assimilation and Prediction Systems (GDAPS, RDAPS and LDAPS, respectively) sharing with an integrated coverage area, which is discretized by variable grids over the Korean Peninsula area, by KMA. LDAPS is, specifically, designed for extreme sever weather forecasting and implemented 1.5 km in horizontal resolution, 70 vertical levels at the top of 39 km with latent heat nudging method based on Single Column Model (SCM). However, LDAPS is inherited significant atmospheric microphysical processes based on single moment scheme, which is computed by the Large-Scale Precipitation Scheme assumed only four phases (liquid, vapour, ice aggregates and rains) in the microphysical processes (Wilkinson, J., 2012). Thus simulated outcomes are supposed that there will be limitation on classifying hydrometeors and related results, particularly calculate number of concentration of hydrometeor in each phase of precipitation forming processes, which are available by only double or triple moment scheme. As an on-going research, this study will present two case studies using the UM-based dual-polarised radar simulator and its outcomes compared to the YIT radar observations. The considered cases are 2014's summer heavy storm and 2015's Squall line accompanied lightning. Figure 1. Summer precipitation simulations on 201408202200 UTC: a. CAPPI, b.UM +4hr c. observed reflectivity, d. simulated reflectivity. The year of 2014, in which two dual-polarised radars were installed in South Korea, was very dry and there were no significantly influenced typhoons or severe Changma developed. Just some frontal summer rainfalls were occurred during summer in 2014. On 21 August, a strong precipitation echo band was developed from 201408202200UTC to 201408210200UTC due to strongly extended very saturated and warm low pressure air mass placed on the South-West Sea of Korean Peninsula (Figure 1. b). A strong convective storm cell was developed in front and stratiform rain cells were developed in rare of the precipitation band inside of the black box on Figure 1. a. These were travelled from the West Sea to Mountainous areas in the East of South Korea. The stratiform cells were getting weaken as the precipitation echo was moving toward the East, but the convective super cell was kept the strength as reflectivity (Z) at maximum 40~45 dBZ (observation) and 50~55 dBZ (simulation) (Figure 1. c &d). Differential reflectivity (ZDR) was simulated as 2.0~2.5 dB for the strong rain cells and others are simulated as 0.5 ~ 1.5 dB. ZDR is observed as 2.0 dB and 1.0~ 1.5 dB for strong and weaker rain cells, respectively. In due course the movement of these cells was well represented by simulators in terms of time, shape, location and structure. Melting layer was also quite similarly simulated about 4 km in vertical, as observed. On 2 April, 2015, there was a very remarkable Squally line developed around the Korean Peninsular. It was orginned from a heavey airmass developed from central part of mainland China and travelled south east via a few hours of stay over North Korea. When these airmass was reached North Korea, it form a multi-system composed Squall line and become linear getting toward South Korea. It was formed by a line of strong convective cells accompanied lightning, developed lower atmosphere and moved very fast to South East. Squall line system was well duplicated in time and space, although the location was placed slightly north of where the line was observed by YIT radar. Reflectivity is well matched along the Sqaull line as the maximum reflectivity was observed over 50 dBZ and simulated as 50~55dBZ at 201504021400UTC. Weaker echo around the Squall line was also simulated with similar range of difference in reflectivity. Although echo area was over estimated toward North Korea, it seems it was influenced by microphysics gained from LDAPS forecast. Differential reflectivity was simulated as 4 ~ 5 dB (supposed to be about between 4 and 6 mm in rain drop size, along the Squall line) and less or equal to 2dB for the rest of eco areas as mideum size of rain drops. A UM-based dual-polarised radar simulator was applied for two different type of precipitations to estimate evolution, structure, polarimetric variables. Simulators overall well performed to duplication location, time and structures of precipitations with slight differences on polarimetric variables, reflectivity, differential reflectivity and correlation coefficient that simulator computes as purely rain with value of 0.99 even for edges. Differences and limiations are supposed that they are influenced from the microphysics used. Therefore they will be more case studies carried out to get more information to improve microphysics for UM through more simulations with various type of precipitations. This research is supported by “Development and application of Cross governmental dual-pol radar harmonization (WRC-2013-A-1)” project of the Weather Radar Center, Korea Meteorological Administration.