Assimilation of radar‐based nowcast into a HIRLAM NWP model

The present study introduces a nowcast scheme that assimilates radar extrapolation data (RED) into a nowcasting version of the high resolution limited area model (HIRLAM) numerical weather prediction (NWP) model covering the area of Denmark. The RED are based on the Co-TREC (tracking radar echoes by correlation) methodology and are generated from cleaned radar mosaics from the Danish weather radar network. The assimilation technique is a newly developed method that increases model precipitation by increasing low-level convergence and decreasing convergence aloft in order to increase the vertical velocity. A selection criterion based on spatial coverage and temporal duration is applied to identify rain events for evaluation. Eight events from August 2010 comply with the criteria and additionally, an extreme event on 2 July 2011 is also evaluated. The RED assimilated NWP model is verified against a non-assimilated NWP reference run using the fractional skill score. In all cases, the predictability of the RED assimilated NWP model has been increased compared with the non-RED assimilated NWP model. After ending the RED assimilation, the positive effects continue until the skill converges towards the reference run. The level of improved predictability relies on the RED quality, which again relies on the type of event.

[1]  Assimilation of the radar-derived water vapour mixing ratio into the LM COSMO model with a high horizontal resolution , 2009 .

[2]  I. Zawadzki,et al.  Scale-Dependence of the Predictability of Precipitation from Continental Radar Images. Part I: Description of the Methodology , 2002 .

[3]  F. Mesinger,et al.  Four-dimensional variational assimilation of precipitation data , 1995 .

[4]  Juanzhen Sun,et al.  Impacts of Initial Estimate and Observation Availability on Convective-Scale Data Assimilation with an Ensemble Kalman Filter , 2004 .

[5]  J. Mahfouf,et al.  The ISBA land surface parameterisation scheme , 1996 .

[6]  A study on WRF radar data assimilation for hydrological rainfall prediction , 2013 .

[7]  L. Li,et al.  Nowcasting of Motion and Growth of Precipitation with Radar over a Complex Orography , 1995 .

[8]  T. N. Krishnamurti,et al.  Physical initialization for numerical weather prediction over the tropics , 1991 .

[9]  A. Seed,et al.  STEPS: A probabilistic precipitation forecasting scheme which merges an extrapolation nowcast with downscaled NWP , 2006 .

[10]  M. R. Rasmussen,et al.  A Numerical Method to Generate High Temporal Resolution Precipitation Time Series by Combining Weather Radar Measurements with a Nowcast Model , 2014 .

[11]  S. Planton,et al.  A Simple Parameterization of Land Surface Processes for Meteorological Models , 1989 .

[12]  Jialin Lin,et al.  Doppler Radar Observations of Mesoscale Wind Divergence in Regions of Tropical Convection , 2005 .

[13]  Juanzhen Sun,et al.  Dynamical and Microphysical Retrieval from Doppler Radar Observations Using a Cloud Model and Its Adjoint. Part I: Model Development and Simulated Data Experiments. , 1997 .

[14]  B. Macpherson.  Operational experience with assimilation of rainfall datain the Met Office Mesoscale model , 2001 .

[15]  I. Zawadzki,et al.  Scale Dependence of the Predictability of Precipitation from Continental Radar Images. Part II: Probability Forecasts , 2004 .

[16]  N. Roberts,et al.  Scale-Selective Verification of Rainfall Accumulations from High-Resolution Forecasts of Convective Events , 2008 .

[17]  V. M. Karyampudi,et al.  The Impact of Assimilating Satellite-Derived Precipitation Rates on Numerical Simulations of the ERICA IOP 4 Cyclone , 1994 .

[18]  B. Macpherson,et al.  A latent heat nudging scheme for the assimilation of precipitation data into an operational mesoscale model , 1997 .

[19]  M. Xue,et al.  The Diurnal Cycle of Precipitation from Continental Radar Mosaics and Numerical Weather Prediction Models. Part II: Intercomparison among Numerical Models and with Nowcasting , 2012 .

[20]  J. Marshall,et al.  THE DISTRIBUTION OF RAINDROPS WITH SIZE , 1948 .

[21]  Juanzhen Sun,et al.  Convective‐scale assimilation of radar data: progress and challenges , 2005 .

[22]  Juanzhen Sun,et al.  Dynamical and Microphysical Retrieval from Doppler Radar Observations Using a Cloud Model and Its Adjoint. Part II: Retrieval Experiments of an Observed Florida Convective Storm , 1998 .

[23]  Z. Sokol,et al.  Nowcasting of precipitation by an NWP model using assimilation of extrapolated radar reflectivity , 2012 .