Direct verification of forecasts from a very high resolution numerical weather prediction (NWP) model

Summary The University of New South Wales (UNSW) High Resolution numerical weather prediction model (HIRES) is run routinely, on a daily basis, at a horizontal resolution of 25 km. The output is made available to the New South Wales (NSW) regional office of the Australian Bureau of Meteorology in Sydney. HIRES has been used to forecast mean wind direction and speed at a height of 12 metres for a number of events in the past. The opportunity was used in December 1997 to run the model for the annual Sydney to Hobart yacht race area. For the 1997 Sydney to Hobart yacht race the model was run at 25 km horizontal resolution and the output was made available to all competitors on the morning of the race, namely December 26th. It was also decided by the authors to expand the verification to include all observations available both from land and sea within the model domain an well as those available from a moving single point at sea, namely a yacht. After the event, the model was run once at the increased resolution of 10 km, out to 5 days ahead. Both model runs were subjected to detailed verification by one of the authors (KLB) who participated in the race aboard the maxi-yacht Nicorette and who carried out a pre-arranged observational program during the race. Surface synoptic weather maps prepared in the NSW office of the Bureau of Meteorology were also consulted in order to extend the verification scheme. The model predicted winds were verified on a six-hourly basis utilising instrumentation on the yacht as well as surface observations plotted in standard World Meterological Organisation (WMO) format on surface synoptic weather maps. The yacht carried wind sensors situated on top of the mast at a height of 30 metres above the water. The authors were most interested in the accuracy of the wind velocity forecast by the model. It is important to note that forecasting for points over the ocean at widely separated time intervals represents a very difficult challenge. The verifications reveal that the model gave overall forecast guidance of very good to excellent quality and was particularly accurate early in the race, when a Southerly Buster event occurred during the evening of the first day. However, a caveat should be added that this forecast, despite its remarkable accuracy, is not a claim that accurate very high resolution regional prediction is a solved problem. Rather, it is a demonstration that in particular instances current models are now capable of achieving high levels of skill a number of days ahead. Received September 28, 1999 Revised November 25, 1999     

Initialization of cloud water in a numerical weather prediction model

Summary In recent years many studies have shown the importance of treating condensation processes in a consistent manner in numerical weather prediction models. Among emerging improvements is the explicit treatment of cloud water, and in some cases precipitating water. An unresolved problem then is how to initialize the cloud water, especially since this quantity is not treated in the most commonly used analysis schemes.In this study, a method for initializing the cloud water in a numerical weather prediction (NWP) model will be presented and tested. The implications for the model's spin-up are investigated. Information from an earlier run (first guess fields) is used, together with satellite data. If necessary, humidity enhancement is performed where clouds are indicated by those sources. The results indicate that initialization of the cloud water field by itself does not have a large effect on the spin-up of precipitation and clouds. A much larger effect is obtained when the humidity field is enhanced. The spin-up time for precipitation is then reduced from 12 to 6 hours, while for cloud cover it is reduced to only 1–2 hours. The method is computationally very efficient, and is particularly useful over data-sparse areas, such as the oceans.An investigation of the different terms in the cloud water tendency equation is done and the results interpreted in terms of spin-up of cloud parameters. These tests confirm that the cloud water field only accounts for a small part of the spin-up effect. These also show that the production of cloud water per time step increases throughout the simulation.With 9 Figures     

高分辨率区域模式降水预报在云南的检验

为了解高分辨率区域数值模式降水预报在云南的预报效果和误差特点,针对华南中尺度模式、华东区域数值预报业务模式和中央气象台GRAPES-Meso模式对2017年9月—2018年12月云南降水预报进行检验分析。结果表明:华东模式降水预报效果整体最好,其降水的振幅接近实况,晴雨准确率也是最高,而华南和GRAPES模式空报率和漏报率普遍偏高。三种模式对滇东北、滇中西部、滇西北北部≥0.1 mm降水预报评分普遍较低,对滇南、滇西南、滇西边缘地区的评分普遍较高。在滇东北北部、滇中西部、及滇西北北部地区三种模式对≥10 mm降水TS评分普遍较低。对于≥25 mm降水,华东模式和华南模式在滇中、滇西地区的TS评分高于GRAPES模式。对于≥50 mm降水,华东模式和华南模式在滇东南、滇西南、滇西边缘及金沙江河谷沿线TS评分高于GRAPES模式。对于云南强降水天气过程,≥0.1 mm降水华南模式预报效果较好,但10 mm和25 mm以上量级降水华东模式的预报效果较好,≥50.0 mm则是GRAPES模式更具参考价值。     

Climatology of cloud and radiation fields in a numerical weather prediction model

Summary Satellite-derived datasets are used to verify the cloud cover and radiation field generated by a T62 (horizontal resolution) version of the operational global model at the National Meteorological Centre (NMC). An ensemble of five day forecasts for July 1985 is used, as well as 30 day climatological forecasts for July 1985, October 1985, January 1986, and April 1986.Monthly averages of radiation fields are compared with Earth Radiation Budget Experiment (ERBE) data. For the four months examined, clear-sky outgoing longwave radiation (clear-sky OLR) and absorbed shortwave radiation (clear-sky SW) tend to agree roughly with ERBE. Model global mean OLR, however, exceeds that of ERBE by 10 W m^–2.Comparison of effective cloud cover to corresponding fields cataloged by the International Satellite Cloud Climatology Project (ISCCP C1) reveals deficiencies in the amount of supersaturation cloudiness and the vertical distribution of convective clouds. Large inaccuracies in model radiation fields are closely related to deficiencies in the cloud parameterization. An inventory of model cloudiness, in comparison to satellite data, is conducted.With 18 Figures     

陡峭地形有限区域数值预报模式设计

本文设计了一个考虑陡峭地形的E-网格有限区域数值预报模式。对如何减小由地形坡度带来的计算误差,我们从地形表示方式和差分格式构造两个方面给出了有效的处理方法。并用青藏高原的两个背风气旋实例对模式作了预报检验,同时比较了有、无地形的模拟结果。     

The adjustment of the wind field to small scale topography in a numerical weather prediction model

Numerical experiments are performed to test one reasonably economical method of producing regional forecasts. Starting with initial conditions interpolated from a 20 hour coarse grid Northern Hemisphere forecast, a fine mesh model is integrated for a further period of 4 hours over a limited area. The fine mesh is located over the north‐eastern part of North America and its resolution is sufficient to re‐produce topographic features such as the St. Lawrence and Richelieu Valleys. The resulting forecast at hour 24 is then compared with the coarse mesh prediction for the same time. The comparison reveals how the horizontal and vertical components of the wind are affected by the small scale topography. In particular, the channelling effect of the main valleys is demonstrated.     

Towards urbanisation of the non-hydrostatic numerical weather prediction model Lokalmodell (LM)

Numerical weather prediction models are increasingly employed for providing meteorological data for urban air quality applications. Model resolution, physiographic parameters and surface-layer parameterisations need to be adapted to the requirements of the urban boundary layer. The Lokalmodell of the German Weather Service was triple-nested down to a horizontal grid resolution of 1.1 km, urbanised physiographic parameters were implemented, and an additional anthropogenic heat source was introduced. Results of a sensitivity study for a spring dust episode in Helsinki show a clear urbanisation effect of these measures on temperature, humidity and the partitioning of surface fluxes, leading to an increased Bowen ratio and heat storage and an urban heat island effect.     

A study of planetary wave errors in a spectral numerical weather prediction model

Abstract

A series of fifteen 96‐h forecasts made with a spectral numerical weather prediction model is studied with reference to errors in the planetary wavelengths. The major contributor to the short (less than 48‐h) range forecast error is identified as an external mode. The medium range forecast error (96 h) is internal in character and reflects a deficiency in the simulation of the quasi‐stationary components.     

地形跟随坐标下的中尺度模式气压梯度力计算误差分析及其改进方案

GRAPES_Meso模式预报存在南风偏大、虚假降水偏多等问题,且在大地形下游地区异常明显。平缓-混合坐标可以有效减小气压梯度力计算误差以及平流输送误差,而这两种误差与风场和水汽场预报密切相关。基于GRAPES_Meso模式选择四种平缓-混合坐标对一次典型的高原东部准静止锋降水过程进行模拟分析。模拟结果表明,较弱的天气形势演变下,平缓-混合坐标的改进效果比较明显,可以有效缓解风场预报偏差、虚假降水、虚假天气系统等问题,个例模拟的结果与实况更接近。

     

Generalized inversion of a global numerical weather prediction model, II: Analysis and implementation

Summary This is a sequel to Bennett, Chua and Leslie (1996), concerning weak-constraint, four-dimensional variational assimilation of reprocessed cloud-track wind observations (Velden, 1992) into a global, primitive-equation numerical weather prediction model. The assimilation is performed by solving the Euler-Lagrange equations associated with the variational principle. Bennett et al. (1996) assimilate 2436 scalar wind components into their model over a 24-hour interval, yielding a substantially improved estimate of the state of the atmosphere at the end of the interval. This improvement is still in evidence in forecasts for the next 48 hours.The model and variational equations are nonlinear, but are solved as sequence of linear equations. It is shown here that each linear solution is precisely equivalent to optimal or statistical interpolation using a background error covariance derived from the linearized dynamics, from the forcing error covariance, and from the initial error covariance. Bennett et al. (1996) control small-scale flow divergence using divergence dissipation (Talagrand, 1972). It is shown here that this approach is virtually equivalent to including a penalty, for the gradient of divergence, in the variational principle. The linearized variational equations are solved in terms of the representer functions for the wind observations. Diagonalizing the representer matrix yields rotation vectors. The rotated representers are the array modes of the entire system of the model, prior covariances and observations. The modes are the observable degrees of freedom of the atmosphere. Several leading array modes are presented here. Finally, appendices discuss a number of technical implementation issues: time convolutions, convergence in the presence of planetary shear instability, and preconditioning the essential inverse problem.With 9 Figures     

GPS掩星观测的发展及其在气象业务中的应用现状

GPS无线电掩星探测技术已经发展成为一种强有力的、相对经济的观测全球大气的方法.多年对GPS/MET概念卫星、CHAMP和SAC-C等GPS掩星观测的研究表明,GPS掩星观测资料与其他卫星资料相比具有很高的观测精度和垂直分辨率等优点,并且GPS掩星观测不受天气状况的影响,因此对其在天气预报及气候的应用有独特的优势.GP...     

Disaggregation of screen-level variables in a numerical weather prediction model with an explicit simulation of subgrid-scale land-surface heterogeneity

The earth’s surface is characterized by small-scale heterogeneity attributable to variability in land cover, soil characteristics and orography. In atmospheric models, this small-scale variability can be partially accounted for by the so-called mosaic approach, i.e., by computing the land-surface processes on a grid with an explicit higher horizontal resolution than the atmosphere. The mosaic approach does, however, not account for the subgrid-scale variability in the screen-level atmospheric parameters, part of which might be related to land-surface heterogeneity itself. In this study, simulations with the numerical weather prediction model COSMO are shown, employing the mosaic approach together with a spatial disaggregation of the atmospheric forcing by the screen-level variables to the subgrid-scale. The atmospheric model is run with a 2.8?km horizontal grid resolution while the land surface processes are computed on a 400-m horizontal grid. The disaggregation of the driving atmospheric variables at screen-level is achieved by a three-step statistical downscaling with rules learnt from high-resolution fully coupled COSMO simulations, where both, atmosphere and surface, were simulated on a 400-m grid. The steps encompass spline interpolation of the grid scale variables, conditional regression based on the high-resolution runs, and an optional stochastic noise generator which restores the variability of the downscaled variables. Simulations for a number of case studies have been carried out, with or without mosaic surface representation and with or without atmospheric disaggregation, and evaluated with respect to the surface state variables and the turbulent surface exchange fluxes of sensible and latent heat. The results are compared with the high-resolution fully coupled COSMO simulations. The results clearly demonstrate the high importance of accounting for subgrid-scale surface heterogeneity. It is shown that the atmospheric disaggregation leads to clear additional improvements in the structures of the two-dimensional surface state variable fields, but to only marginally impacts on the simulation of the turbulent surface exchange fluxes. A detailed analysis of these results identifies strongly correlated errors in atmospheric and surface variables in the mosaic approach as the main reason for the latter. The effects of these errors largely cancel out in the flux parameterization, and thus explain the comparably good results for the fluxes in the mosaic approach without atmospheric disaggregation despite inferior performance for the surface state variables themselves. Inserting noise in the disaggregation scheme leads to a deterioration of the results.     

Parametrization and influence of subgridscale orography in general circulation and numerical weather prediction models

Summary The problem of representing the drag due to subgridscale orography is examined. Results from model simulations are used to illustrate clear deficiencies in the global angular momentum budgets and possible ways of correcting for these deficiencies are considered. It is argued that a formulation for the stress due to subgridscale gravity waves is required, as was first recognized by Boer et al. (1984a, b), and the impact of a scheme based on Palmer et al. (1986) is presented. The scheme is improved by using directionally-dependent subgridscale orographic variances. Results from 90-day integrations using the ECMWF and UKMO models with similar resolutions are very much in accord both with and without a wave drag parametrization scheme; showing much improved wintertime circulations.The relationship between the wave drag and the model orography is examined with the use of idealized stress profiles whereby the wave drag is limited to either near the surface or in the stratosphere. A combination of parametrized wave drag with an envelope orography performs best at this stage of development.Results are presented from a substantial series of ten-day forecast experiments with the ECMWF operational model using mean and (1 ) envelope orographies, these show significant improvements in forecast skill.With 26 Figures     

On the assimilation of satellite sounder data in cloudy skies in numerical weather prediction models

Satellite measurements are an important source of global observations in support of numerical weather prediction (NWP). The assimilation of satellite radiances under clear skies has greatly improved NWP forecast scores. However, the application of radiances in cloudy skies remains a significant challenge. In order to better assimilate radiances in cloudy skies, it is very important to detect any clear field-of-view (FOV) accurately and assimilate cloudy radiances appropriately. Research progress on both clear FOV detection methodologies and cloudy radiance assimilation techniques are reviewed in this paper. Overview on approaches being implemented in the operational centers and studied by the satellite data assimilation research community is presented. Challenges and future directions for satellite sounder radiance assimilation in cloudy skies in NWP models are also discussed.     

Spatial prediction model and its application to chemistry of atmospheric precipitation in Jordan

This study is concerned with the spatial variability of some wet atmospheric precipitation parameters such as; pH, conductivity (EC). The study also depicts the spatial variability of some ions (cations and anions) of atmospheric precipitation in Jordan such as, Ca²⁺, Mg²⁺, Na⁺ and K⁺, HCO₃⁻, Cl⁻, NO₃⁻ and SO₄²⁻. The basis of the work is to establish a relationship through the cumulative semivariogram technique between the distance ratios and the spatial dependence structure of the chemical composition of atmospheric precipitation. All semivariogram models are constructed in this study in order to understand the behavior of the spatial distribution. The spatial distributions of rainwater parameters show differences from station to station which is expressed in terms of angle, where the larger the angle the weaker the correlation. The semivariogram (SV) models are constructed to show the variation of the rainfall chemistry in Jordan. The SV models show weak correlation between mountain and leeside mountain stations, i.e. mountain and desert stations. On the other hand, good correlations are observed when transferring from south to north of the country. The larger is the found angle, the weaker is the correlation. For most of the SV model the correlation is found to be very weak between desert and mountainous locality. The Standard Regional Dependence Factor (SRDF) is used for prediction of the distribution of rain fall parameters. It shows the relative error between observed and predicted values of rainwater parameters. The overall regional relative error between the observed and estimated concentrations remains less than 15%.     

成都区域气象中心数值天气预报工作20年综述

本文对成都区域中心开展数值预报工作历程作了一个简要回顾.四川开展数值天气预报工作起步早,且从未间断.数值预报工作从无到有,经历了4个阶段,最终迎来了新的发展阶段.本文最后指出,在西南地区复杂地形条件下开展区域数值天气预报是一个科学难题,有待数值预报工作者为之而努力.     

成都区域气象中心数值天气预报工作20年综述

本文对成都区域中心开展数值预报工作历程作了一个简要回顾。四川开展数值天气预报工作起步早，且从未间断。数值预报工作从无到有，经历了4个阶段，最终迎来了新的发展阶段。本文最后指出，在西南地区复杂地形条件下开展区域数值天气预报是一个科学难题，有待数值预报工作者为之而努力。