Updates of convection scheme in the 5km resolution operational system Kengo Matsubayashi Numerical Prediction Division, Japan Meteorological Agency with Tabito Hara, Kohei Aranami and Kohei Kawano November 30, 2016 4th International Workshop on Nonhydrostatic Models Hakone, Japan

Current operational NWP systems at JMA NWP systems at JMA (deterministic) Newly developed non-hydrostatic numerical model called ASUCA has been launched into operation as the 2km LFM since 2015. JMA even plans to introduce ASUCA into the 5km grid spacing operational model (MSM) this winter. Parameterization schemes including convection are also updated, and many improvements are made. Global Meso Local Objectives Short- and Medium-range forecast Disaster reduction Short-range forecast Aviation forecast Disaster prevention NWP model Global Spectral Model(GSM) Meso-Scale Model(MSM) Local Forecast Model (LFM) Horizontal resolution TL959 (0.1875 deg) 5 km (817x611) 2 km (1581x1301) Vertical levels / Top 100 0.01 hPa 48 21.8 km 58 20.2 km Forecast Hours (Initial time) 264 hours 39 hours (every 3 hours) 9 hours (every hour) Forecast model GSAM JMA-NHM ASUCA Domain Global Meso Local 2 2

Improvement in a precipitation forecast Observation Radar obs. and MSLP(analysis) New MSM Current MSM

Improvement in the precipitation forecast Threat score Bias score overestimated better Updates of physical processes improved accuracy of precipitation forecasts measured by the threat score remarkably. In particular the convection scheme strongly affects prediction accuracy.

Convection parameterization 4 Parameterized processes convection initiation updraft entrainment detrainment microphysics condensation, evaporation freezing, melting sedimentation downdraft compensational subsidence 5 7 2 3 6 1

Convection scheme in MSM Based on Kain and Fritsch(1990; KF) Overhauling the KF scheme in detail Just entirely changing the current scheme with another one seldom enhances the accuracy because other compensational errors often arise. It is necessary to thoroughly analyze the current scheme. The current scheme was carefully inspected using not only 3D model but also SCM, which exposed a lot of problems.

Overhauling Too high convective cloud top Large difference of heating rate between deep and shallow convection. Too much precipitation over windward side of mountains Less sedimentation produced in convective updraft around freezing level. Parcel produced by entrainment below cloud base is not identical to updraft parcel at cloud base. Double count between resolved and parameterized vertical transport. In the tropics, heating rate is relatively weak compared to midlatitudes. Shallow convection is triggered even in zero CAPE condition. KF keeps heating rate constant for convectional lifetime(~30mins). Ice and water categories KF produces

Too high convectional cloud top Cloud top height(CTH) of the parameterized convection in the MSM is often too higher than that estimated by satellite observations. [m] [m] [m] CTH estimated by satellite (Himawari 8) observation parameterized convection CTH parameterized - observed

The relation between CTH and entrainment Entrainment dilutes buoyancy and vertical velocity of updraft. Weaker entrainment rate leads to larger Available Buoyant Energy(ABE) and higher CTH. Too high CTH implies entrainment is too weak. In the KF scheme, the strength of convection highly depends on ABE (CAPE closure). 𝐾 𝑧 𝜃 𝑒𝑠 : enrivonment 𝜃 𝑒 weak entrainment large ABE strong entrainment small ABE updraft parcel

Too high convectional cloud top One of the causes of excessively high parameterized CTH is related to too small entrainment rate. Entrainment rate 𝜀 𝑢 in the KF scheme 𝑅 is a constant and the only parameter to change, but not observable. 𝜀 𝑢 ∝ (−0.03𝛿𝑝) 𝑅 𝑅:updraft radius

Too high convectional cloud top Histogram of the parameterized CTH – observed CTH changed 𝑅 such that the parameterized CTH becomes consistent with observation. Since the original 𝑅 (weak entrainment) caused too strong convection, the modified 𝑅 weaken the strength of convection. As a result dry bias over lower troposphere is also reduced. original 𝑅(1000m) [m] modified 𝑅(750m) [m]

Convection Parameterization(CP) updates in MSM The updated CP scheme significantly improves quantitative precipitation forecasts and reduces dry bias over the lower troposphere. Refinement of entrainment rate, triggering, treatment of sedimentation were critical for this improvement. Threat score Bias score Qv ME Qv RMSE [kg/kg] [kg/kg]

The importance of CP in 5km resolution However relatively heavy precipitation is still overestimated. With aggressively exerted CP (weaken entrainment rate over lower layer) predicting precipitation accuracy can be further enhanced. however dry bias becomes more serious. Threat score Bias score [kg/kg]

The importance of CP in 5km resolution reference w/ aggressively exerted CP Observation The precipitation predicted by reference is too strong and it occurs in few grid (grid point storm). On the other hand, aggressive CP can stabilize before grid point storm arise. It is estimated that grid scale vertical updraft lacks processes such as entrainment which weaken vertical updraft.

The importance of CP in 5km resolution Entrainment weakens convection activity by incorporating dry air outside cumuli and dilutes moist air inside, however it is too small to resolve. Detrainment and convective initiation are also unresolved. If stabilization by CP is weak, poorly resolved entrainment/detrainment and convective initiation lead to frequent excessively strong updrafts and too intense precipitation.

Convection parameterization in high resolution In high resolution model, convectional vertical transport is partially resolved. Convection permitting Scales of processes such as entrainment/detrainment, convective initiation are too small to resolve. These processes should be still parameterized. Grey zone

Summary Overhauled convection scheme in MSM. rather than entirely changing the current scheme with another one SCM reveals a lot of problems in the KF scheme. Exposed problems are fixed and forecast accuracy is improved. CP is inevitable at 5km resolution NWP model. Even in high resolution models which can resolve vertical transport, smaller unresolved phenomena around convection should be still parameterized.