Preliminary study on terrain uncertainty and its perturbing scheme

1.Introduction

A key issue in developing the ensemble prediction technique is the recognition of uncertain factors in numerical forecasting and how to use appropriate perturbation techniques to reflect these uncertain processes and improve ensemble prediction levels.Plenty of corresponding perturbation techniques have been developed. Such as Initial uncertainty and model uncertainty,In addition to the influence of IC and model uncertainty ,precipitation is closely related to terrain.The influence of terrain on the heavy rain includes the following three aspects:(1) The terrain has significant effect on the climatic distribution of precipitation.(2)The windward slope and leeward slope and other dynamic effects generated by terrain impact the triggering and intensity of precipitation.(3)The thermal effect is triggered by the heating of land surface of terrain at different height and latent heat release when airflow rises ,and this thermal action makes mountain precipitation closely related to terrain distribution .What are the terrain uncertainties in the model?(1)Different vertical coordinate systems lead to significant differences in terrain treatment(2)The conversion from real terrain to model terrain is closely associated with the resolution of the model and different terrain interpolation schemes, and it affects the simulation results of precipitation .(3)Measuring error of real terrain, etc.In this report, A terrain perturbation scheme (ter) has been firstly incorporated into an ensemble prediction system (EPS) and preliminarily tested in the simulation of the extremely heavy rain event occurred on 21 July, 2012 in Beijing, along with other three perturbation schemes.

2.Case,data and schemes

(1)Case: Based on the extremely heavy rain case in Beijing on July 21,2012, maximum precipitation center more than 400mm.(2)Data: GEPS of NCEP were used as initial background fields and lateral boundary condition , surface and upper-level observation of GTS,Rain gauge etc.(3)Model: WRFv4.3, 9km horizontal resolution ,511*511 grid point, 51 vertical layers,KF Eta,WSM6,etc(4)Experiments schemes: Four different perturbation schemes were used in the experiments and six members in each experiment. Sch_1(IC) considered the IC uncertainty ,the parameterization schemes were same but IC/LBC came from different GEPS members. Sch_2(phy) considered the Phy uncertainty ,the IC/LBC were same but PHY schemes were comprised of different parameterization schemes. Sch_3-4(ter and icter) considered the terrain uncertainty ,the second aspect of terrain uncertainty was considered in this study. Two different model terrain smoothing schemes and 3 terrain interpolation schemes were used to reflect the forecast error caused by terrain height. Icter is the mixed scheme of ter and ic.

3.Preliminary test and results

(1)Precipitation is closely related to terrain, terrain uncertainties have significant effect on the intensity and falling area of precipitation.(2) Only a simple terrain perturbation can produce a significant forecast spread , and its ensemble mean forecast is also improved compared with control forecast. for this case, it has a slightly positive contribution to the spread and probability forecast of precipitation on the basis of not impacting the quality of ensemble mean forecast.(3) In this case, the magnitude of spread generated by the terrain perturbation scheme is significantly smaller than that generated by the initial perturbation and physics process perturbation schemes.