3.7. Masami Narita* and Shiro Ohmori Japan Meteorological Agency, Otemachi, Tokyo, Japan

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1 3.7 IMPROVING PRECIPITATION FORECASTS BY THE OPERATIONA NONHYDROSTATIC MESOSCAE MODE WITH THE KAIN-FRITSCH CONVECTIVE PARAMETERIZATION AND COUD MICROPHYSICS Masami Narita* and Shiro Ohmori Jaan Meteorological Agency Otemachi Tokyo Jaan 1. INTRODUCTION The Jaan Meteorological Agency (JMA has oerated a nonhydrostatic mesoscale model (MSM o 10-km horizontal grid sacing (10-km MSM since Setember 2004 (Saito et al The horizontal grid sacing has been enhanced to 5 km (5-km MSM since March 2006 and the orecast time has been extended rom 15 hours to 33 hours at the initial times o UTC with some modiications including dynamics and hysics schemes since June 2007 (Saito et al The develoment o a moist rocess scheme to imrove reciitation orecasts is continued since the major urose o MSM is to revent natural disasters such as torrential rain. Preliminary orecast tests o 10-km MSM with only cloud microhysics sometimes indicated the necessity o a convective arameterization scheme in order that the stability o time integration is assured and that weak reciitation o orecasts matches well with that o observations. To accomlish these aims the Kain-Fritsch (KF convective arameterization scheme was adoted simultaneously with a cloud microhysics scheme to MSM. As Molinari and Dudek (1992 indicated the uncertainty o treatment o convection or grid sacing rom about 3 km to km ormation o line-shaed mesoscale rainband was sometimes simulated better with only cloud microhysics without convective arameterization. According to the several orecasts however it became clear that the average objective scores or the reciitation orecasts by MSM with convective arameterization and cloud microhysics were better than those with only cloud microhysics. This act is considered to be an imortant characteristic as an oerational numerical rediction model. To imrove reciitation orecasts by 10-km MSM and 5-km MSM with the KF scheme some modiications to ormulation o reciitation and * Corresonding author address: Masami Narita Numerical Prediction Division Jaan Meteorological Agency Otemachi Chiyodaku Tokyo Jaan m_narita@nas.kishou.go.j triggering o convection were added to the original KF scheme and arameters were adjusted. As a result the orecast skill was quite imroved comared to re-oerational MSM rom the viewoint o quantitative reciitation orecasts. Full conigurations o 10-km MSM and 5-km MSM can be ound in Saito et al. (2007. In this aer the coniguration o a cloud microhysics arameterization scheme is summarized in section 2. Some modiications to the KF scheme with oerational MSM and their eects are resented in section 3. Section 4 rovides conclusions. 2. COUD MICROPHYSICS A bulk arameterization scheme o cloud microhysics based on in et al. (1983 has been adoted to the original root o MSM develoed at the Forecast Research Deertment o the Meteorological Research Institute (Ikawa and Saito In this scheme water substance is exressed by its mixing ratio and categorized into six orms: water vaor cloud water rain cloud ice snow and grauel. Although the original cloud microhysics scheme redicts the number concentrations o cloud ice snow and grauel (Murakami 1990 oerational MSM redicts only the mixing ratios. Furthermore some simliication and elimination o the original cloud microhysics scheme were alied in order to shorten the comutational time without making reciitation orecasts worse (Yamada The rognostic equations or mixing ratios o six water substances ( qv : water vaor qc : cloud water qr : rain qi : cloud ice qs : snow qg : grauel and otential temerature ( θ are as ollows: qv ADV( qv DIF( qv = Pv_ev_r Pi_de_v Ps_de_v Pg_de_v Pi_nud_v Pw_cnd_v qc ADV( qc DIF( qc = Pr_aut_w Pr_ac_r_w Pw_cnd_v Pi_rz_w Ps_ac_s_w Pg_ac_s_w Pg_ac_g_w δ Pw_mlt_i

2 qr ADV( qr DIF( qr = P_rc_r Pr_ac_r_w Pr_aut_w Pv_ev_r Pg_rz_r Pg_ac_i_r Ps_ac_s_r Pg_ac_s_r Pg_ac_g_r δ ( Pr_mlt_s Pr_mlt_g qi ADV( qi DIF( qi = Pi_nud_v Pi_rz_w Pi_de_v Pg_ac_r_i Ps_ac_s_i Pg_ac_g_i Ps_aut_i δ Pw_mlt_i qs ADV( qs DIF( qs = P_rc_s Ps_de_v Ps_aut_i Ps_ac_s_w Pg_cn_w_s Ps_ac_s_i Ps_ac_s_r Pg_ac_r_s Pg_ac_g_s δ Pr_mlt_s qg ADV( qg DIF( qg = P_rc_g Pg_de_v Pg_cn_w_s Pg_ac_s_w Pg_ac_g_r Pg_ac_g_s Pg_ac_g_w Pg_ac_g_i Pg_ac_i_r Pg_ac_r_i Pg_ac_s_r Pg_ac_r_s Pg_rz_r δ Pr_mlt_g θ ADV v = s ( θ DIF( θ ( Pv_ev_r Pw_cnd_v ( Pi_de_v Pi_nud_v Ps_de_v Pg_de_v ( Ps_ac_s_w Pg_ac_s_w Pg_ac_g_w ( Pg_ac_i_r Pi_rz_w ( Ps_ac_s_r Pg_ac_s_r Pg_ac_g_r δ ( Pw_mlt_i Pr_mlt_s Pr_mlt_g ( where t is time ADV x the advection term o x and DIF( x the diusion term o x. The symbols v s and are latent heat o vaorization sublimation and usion resectively; c the seciic heat o dry air at constant ressure π the non-dimensional ressure (Exner unction and δ = 1 when the temerature is above 0 deg C and δ = 0 otherwise. The symbol Px _ roc_ y denotes the roduction rate o water substance x (v: water vaor w: cloud water r: rain i: cloud ice s: snow g: grauel through the rocess roc (ac: accretion aut: autoconversion cn: conversion such as riming cnd: condensation de: deositional growth or evaoration ev: evaoration rz: reezing mlt: melting nud: nucleation by deosition concerning water substance y Px _ roc_ y_ z denotes the roduction rate o water substance x through the rocess roc concerning water substances y and z and P_rc_ x denotes reciitation o x. 3. MODIFICATIONS TO THE KAIN-FRITSCH CONVECTIVE PARAMETERIZATION SCHEME The Kain-Fritsch convective arameterization scheme (Kain and Fritsch 1990; Kain 2004 is adoted to reresent the eects o subgrid-scale convection. The source codes o the KF scheme have been originally develoed or the Weather Research and Forecast (WRF modeling system and imlemented to MSM with Dr. Kain s consent in Aril Minor imrovements or the original KF scheme made beore February 2003 have also been relected. Some modiications and adjustments were alied to the KF scheme in MSM. During the re-oerational orecast tests the area o reciitation redicted by MSM with only cloud microhysics was sometimes too narrow and moreover observed reciitation was sometimes missed. With the KF scheme these weaknesses were imroved. As shown in Fig. 1 distribution o accumulated reciitation orecast by 5-km MSM with only cloud microhysics is insuicient esecially over northern area o Jaan and its surrounding sea comared to that with convective arameterization and cloud microhysics. Some modiications and adjustments to the KF scheme and their eects are resented here. 3.1 Trigger unction To identiy source layers or convective clouds the KF scheme utilizes a trigger unction based on the temerature at the liting condensation level ( and the grid-scale vertical velocity (Fritsch and Chaell 1980; Kain The temerature erturbation based on the vertical velocity is given by ΔT = w Δx w z where w is a grid-scale vertical velocity at the Δx is a grid sacing and w z z w0 = 2000 w0 i z i z m > 2000m 1 where w0 = 0.02ms and z is a height o the. While the vertical velocity w o one model grid is adoted as a temerature erturbation Δ T

3 (a (b Fig. 1. Accumulated reciitation [mm/3h] rom UTC on 13 th March (a 6-hour orecast by 5-km MSM with only cloud microhysics. (b Same as (a but with the Kain-Fritsch convective arameterization scheme and cloud microhysics. (a (b Fig. 2. Accumulated reciitation [mm/3h] rom UTC on 12 th June (a 33-hour orecast by 5-km MSM with the KF scheme without relative humidity based erturbation. (b Same as (a but with relative humidity based erturbation.

4 or the KF scheme in the WRF model the horizontally-averaged vertical velocity o the grid and surrounding eight grids is adoted as w in oerational MSM to eliminate an inluence o the grid-scale intensiied vertical velocity. Even though this temerature erturbation is ormulated as a unction o horizontal grid sacing a value calculated with original ormulation may be too large or MSM with 5-km or 10-km horizontal grid sacing and causes sometimes undesirable reciitation in regions where no reciitation was observed. Thus the temerature erturbation deending on the vertical velocity is reduced by a certain amount o the values determined in the original ormulation. The original KF scheme alied to the humid climate area o Jaan and its surrounding sea sometimes ails to initiate arameterized convection when the lowest atmoshere is wet and dynamical orcing is weak. To eliminate this weakness a temerature erturbation based on the relative humidity has been added to the trigger unction. The temerature erturbation based on the relative humidity has been imlemented or the High Resolution imited Area Model (HIRAM Unden et al and is given by Δ T v RH 0 i Rh < ( Rh 0.75 Qmix QS T = i 0.75 Rh 0.95 ( 1 Rh 1 Qmix i Rh > 0.95 QS T where Rh is the relative humidity at the T is the temerature Q S is the saturation mixing ratio at the is the mixing ratio o udrat Q mix source layer. The temerature erturbation is reduced a certain amount o the value determined by the ormulation o Unden et al. (2002 in 5-km MSM. Figure 2 shows accumulated reciitation orecasts by 5-km MSM. According to Fig. 2 (a by 5-km MSM with the original KF scheme too narrow and too intensiied reciitation was calculated at the western sea o Taiwan (286 mm/3h southern sea o Okinawa island (111 mm/3h and so on. On the other hand too intensiied reciitation caused by grid-oint storm was eliminated by 5-km MSM with the modiied KF scheme as shown in Fig. 2 (b. Inclusion o the temerature erturbation deending on the relative humidity also imroved the orecast o diurnal convective rain. Figure 3 shows an examle o reciitation orecasts by 5-km MSM with the original and modiied KF schemes. Even though the amount o reciitation was not enough comared to observation accumulated reciitation orecasted by the modiied KF scheme was better than that by the original KF scheme. 3.2 Preciitation ormation In the original KF scheme reciitation is continuously roduced in convective udrat regardless o the content o condensate by a method roosed by Ogura and Cho (1973. As shown in Fig. 4 (b the ormation o reciitation by 10-km MSM with the original KF scheme sometimes roduces unnatural distribution such as elongated regions whose orientations are erendicular to major rainbands observed by radar and rain gauge as shown in Fig. 4 (a. To eliminate this weakness the method or calculation o reciitation is relaced by the Kessler-tye autoconversion scheme in the modiied KF scheme. In this scheme reciitation ormation is adoted such that condensate in convective udrat is converted into reciitation when its amount exceeds (a (b (c Fig. 3. Accumulated reciitation [mm/3h] rom UTC on 13 th July (a Observation (derived rom radar data corrected by rain gauge data. (b 33-hour orecast by 5-km MSM with the KF scheme without relative humidity based erturbation. (c Same as (a but with relative humidity based erturbation.

5 (a (b (c Fig. 4. Accumulated reciitation [mm/3h] rom UTC on 28 th August (a Observation (derived rom radar data corrected by rain gauge data. (b 15-hour orecast by 10-km MSM with the original KF scheme. (c Same as (b but with the Kessler-tye autoconversion. (a (b (c Fig. 5. Accumulated reciitation [mm/3h] rom UTC on 20 th January (a Observation (derived rom radar data corrected by rain gauge data. (b 6-hour orecast by 5-km MSM with the original KF scheme time scale or shallow convection = 2400 seconds. (c Same as (b but time scale or shallow convection = 600 seconds. a threshold value. As shown in Fig. 4 (c the unnatural distribution o accumulated reciitation is eliminated by the modiied KF scheme. 4 1 The threshold value kg kg in 10-km 3 1 MSM is increased to kg kg in 5-km MSM to eliminate reciitation calculated by the KF scheme and to imrove the reresentation o weak reciitation in summer season. 3.3 Time scale or convection The KF scheme assumes that the convection consumes the convective available otential energy (CAPE in a certain time scale. This time scale within 1800 to 3600 seconds is based on the advective time scale and is used to determine the heating and moistening ratios. The original ormulation o the KF scheme assigns this time scale o 1800 seconds or a ine mesh model with horizontal grid sacing less than 10 km. A shorter time scale o 900 seconds imroves the reciitation orecast by rain rate requency. The imortance o the time scale or shallow convection is aeared in the reciitation orecasts during the cold air outbreak o winter monsoon. Under this condition shallow convective clouds develoed over the sea bring about reciitation. With a value o 2400 seconds assigned or the original ormulation the band-shaed weak rain is sometimes excessively roduced. With a shorter time scale o 600 seconds the excess weak reciitation is ameliorated well as shown in igure CONCUSIONS An oerational nonhydrostatic mesoscale model o 10-km and 5-km horizontal grid sacing with cloud microhysics and the Kain-Fritsch convective arameterization scheme has been develoed at the Jaan Meteorological Agency. The result o re-oerational orecast tests with only cloud microhysics showed some weaknesses. On the other hand the orecast scores or reciitation were imroved by the Kain-Fritsch scheme with some

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