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Andrew A. Taylor and Lance M. Leslie

Abstract

Error characteristics of model output statistics (MOS) temperature forecasts are calculated for over 200 locations around the continental United States. The forecasts are verified on a station-by-station basis for the year 2001. Error measures used include mean algebraic error (bias), mean absolute error (MAE), relative frequency of occurrence of bias and MAE values, and the daily forecast errors themselves. A case study examining the spatial and temporal evolution of MOS errors is also presented.

The error characteristics presented here, together with the case study, provide a more detailed evaluation of MOS performance than may be obtained from regionally averaged error statistics. Knowledge concerning locations where MOS forecasts have large errors or biases and why those errors or biases exist is of great value to operational forecasters. Not only does such knowledge help improve their forecasts, but forecaster performance is often compared to MOS predictions. Examples of biases in MOS forecast errors are illustrated by examining two stations in detail. Significant warm and cold biases are found in maximum temperature forecasts for Los Angeles, California (LAX), and minimum temperature forecasts for Las Vegas, Nevada (LAS), respectively. MAE values for MOS temperature predictions calculated in this study suggest that coastal stations tend to have lower MAE values and lower variability in their errors, while forecasts with high MAE and error variability are more frequent in the interior of the United States. Therefore, MAE values from samples of MOS forecasts are directly proportional to the variance in the observations. Additionally, it is found that daily maximum temperature forecast errors exhibit less variability during the summer months than they do over the rest of the year, and that forecasts for any one station rarely follow a consistent temporal pattern for more than two or three consecutive days. These inconsistent error patterns indicate that forecasting temperatures based on recent trends in MOS forecast errors at an individual station is usually not a good strategy. As shown in earlier studies by other authors and demonstrated again here, MOS temperature forecasts are often inaccurate in the vicinity of strong temperature gradients, for locations affected by shallow cold air masses, or for stations in regions of anomalously warm or cold temperatures.

Finally, a case study is presented examining the spatial and temporal distributions of MOS temperature forecast errors across the United States from 13 to 15 February 2001. During this period, two surges of cold arctic air moved south into the United States. In contrast to error trends at individual stations, nationwide spatial and temporal patterns of MOS forecast errors could prove to be a powerful forecasting tool. Nationwide plots of errors in MOS forecasts would be useful if made available in real time to operational forecasters.

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M. López-Puertas, M. A. López-Valverde, and F. W. Taylor

Abstract

Solar heating and thermal cooling rates by the CO2 near-infrared bands in the mesosphere and lower thermosphere are derived from measurements of the CO2 4.3 μm atmospheric emission by the Stratospheric and Mesospheric Sounder on Nimbus 7. A detailed analysis of the relaxation of the solar energy initially absorbed by the different bands, before it escapes to space or is thermalized, is included. The isotopic and hot bands of CO2 near 4.3 μm play an important role since they produce a significant heating in the mesosphere and are important emitters of the solar energy absorbed at 4.3 μm and 2.7 μm around the mesopause. The pathways followed by the fraction of the electronic energy of O(1D) that is transferred into the CO2(00°1)-N2(1) system have been studied, resulting that an important fraction of this energy is emitted by the CO2 4.3 μm fundamental band in the lower thermosphere. Sensitivity studies of the net heating rates to the atomic oxygen and carbon dioxide concentrations and to the rate of vibrational energy transfer are also presented. Global distributions of solar heating rates by CO2 for solstice and equinox situations are shown. They present a double peak structure with maxima situated around 95 and 75 km; the latter being particularly important because other components of the radiative balance have at a minimum at the same region.

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Colin M. Zarzycki, Christiane Jablonowski, and Mark A. Taylor

Abstract

A statically nested, variable-mesh option has recently been introduced into the Community Atmosphere Model’s (CAM's) Spectral Element (SE) dynamical core that has become the default in CAM version 5.3. This paper presents a series of tests of increasing complexity that highlight the use of variable-resolution grids in CAM-SE to improve tropical cyclone representation by dynamically resolving storms without requiring the computational demand of a global high-resolution grid. As a simplified initial test, a dry vortex is advected through grid transition regions in variable-resolution meshes on an irrotational planet with the CAM subgrid parameterization package turned off. Vortex structure and intensity is only affected by grid resolution and no spurious artifacts are observed. CAM-SE model simulations using an idealized tropical cyclone test case on an aquaplanet show no numerical distortion or wave reflection when the cyclone interacts with an abrupt transition region. Using the same test case, the authors demonstrate that a regionally refined mesh with significantly fewer degrees of freedom can produce the same local results as a globally uniform grid. Additionally, the authors discuss a more complex aquaplanet experiment with meridionally varying sea surface temperatures that reproduces a quasi-realistic global climate. Tropical cyclogenesis is facilitated without the need for vortex bogusing in a high-resolution patch embedded within a global grid that is otherwise too coarse to resolve realistic tropical cyclones in CAM.

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Cornelia Klein, Francis Nkrumah, Christopher M. Taylor, and Elijah A. Adefisan

Abstract

Mesoscale convective systems (MCSs) are the major source of extreme rainfall over land in the tropics and are expected to intensify with global warming. In the Sahel, changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. Here we extend that analysis to southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. We identify clear trends in intense MCSs since 1983 and their associated atmospheric drivers. We also find a marked annual cycle in the drivers, linked to changes in the convective regime during the progression of the West African monsoon. Before the peak of the first rainy season, we identify a shear regime where increased temperature gradients play a crucial role for MCS intensity trends. From June onward, SWA moves into a less unstable, moist regime during which MCS trends are mainly linked to frequency increase and may be more influenced by total column water vapor. However, during both seasons we find that MCSs with the most intense convection occur in an environment with stronger wind shear, increased low-level humidity, and drier midlevels. Comparing the sensitivity of MCS intensity and peak rainfall to low-level moisture and wind shear conditions preceding events, we find a dominant role for wind shear. We conclude that MCS trends are directly linked to a strengthening of two distinct convective regimes that cause the seasonal change of SWA MCS characteristics. However, the convective environment that ultimately produces the most intense MCSs remains the same.

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John R. Taylor, Katherine M. Smith, and Catherine A. Vreugdenhil

Abstract

We use idealized large-eddy simulations (LES) and a simple analytical theory to study the influence of submesoscales on the concentration and export of sinking particles from the mixed layer. We find that restratification of the mixed layer following the development of submesoscales reduces the rate of vertical mixing which, in turn, enhances the export rate associated with gravitational settling. For a neutral tracer initially confined to the mixed layer, subinertial (submesoscale) motions enhance the downward tracer flux, consistent with previous studies. However, the sign of the advective flux associated with the concentration of sinking particles reverses, indicating reentrainment into the mixed layer. A new theory is developed to model the gravitational settling flux when the particle concentration is nonuniform. The theory broadly agrees with the LES results and allows us to extend the analysis to a wider range of parameters.

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S. K. Mukkavilli, A. A. Prasad, R. A. Taylor, A. Troccoli, and M. J. Kay

Abstract

Direct normal irradiance (DNI) is the main input for concentrating solar power (CSP) technologies—an important component in future energy scenarios. DNI forecast accuracy is sensitive to radiative transfer schemes (RTSs) and microphysics in numerical weather prediction (NWP) models. Additionally, NWP models have large regional aerosol uncertainties. Dust aerosols can significantly attenuate DNI in extreme cases, with marked consequences for applications such as CSP. To date, studies have not compared the skill of different physical parameterization schemes for predicting hourly DNI under varying aerosol conditions over Australia. The authors address this gap by aiming to provide the first Weather and Forecasting (WRF) Model DNI benchmarks for Australia as baselines for assessing future aerosol-assimilated models. Annual and day-ahead simulations against ground measurements at selected sites focusing on an extreme dust event are run. Model biases are assessed for five shortwave RTSs at 30- and 10-km grid resolutions, along with the Thompson aerosol-aware scheme in three different microphysics configurations: no aerosols, fixed optical properties, and monthly climatologies. From the annual simulation, the best schemes were the Rapid Radiative Transfer Model for global climate models (RRTMG), followed by the new Goddard and Dudhia schemes, despite the relative simplicity of the latter. These top three RTSs all had 1.4–70.8 W m−2 lower mean absolute error than persistence. RRTMG with monthly aerosol climatologies was the best combination. The extreme dust event had large DNI mean bias overpredictions (up to 4.6 times), compared to background aerosol results. Dust storm–aware DNI forecasts could benefit from RRTMG with high-resolution aerosol inputs.

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R. L. H. Essery, M. J. Best, R. A. Betts, P. M. Cox, and C. M. Taylor

Abstract

A land surface scheme that may be run with or without a tiled representation of subgrid heterogeneity and includes an implicit atmospheric coupling scheme is described. Simulated average surface air temperatures and diurnal temperature ranges in a GCM using this surface model are compared with climatology. Surface tiling is not found to give a clear improvement in the simulated climate but offers more flexibility in the representation of heterogeneous land surface processes. Using the same meteorological forcing in offline simulations using versions of the surface model with and without tiling, the tiled model gives slightly lower winter temperatures at high latitudes and higher summer temperatures at midlatitudes. When the surface model is coupled to a GCM, reduced evaporation in the tiled version leads to changes in cloud cover and radiation at the surface that enhance these differences.

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Lisa S. Alexander, David M. L. Sills, and Peter A. Taylor

Abstract

The relationship between low-level mesoscale boundaries and convective storm initiation was investigated in southwestern Ontario, Canada. The influence of lake-breeze fronts, a type of boundary that frequently affects this region of the Great Lakes watershed in summer, presented a particular interest.

Radar data were processed using thunderstorm cell identification and tracking algorithms. The distances between the locations of storm cells reaching an intensity of 40 dBZ and the closest low-level mesoscale boundary were measured. Considering only days not influenced by a warm front, more than 75% of cells developed within 30 km of a low-level mesoscale boundary. Further examination by boundary type showed that cell initiations associated with moving boundaries and storm gust fronts occurred most often 0–5 km behind the boundaries. However, cell initiations associated with lake-breeze fronts most often occurred 0–5 km ahead of the boundaries. The analysis also suggested that lake-breeze fronts would often initiate the first storms of the day, which in turn generated gust fronts that could initiate subsequent storms.

Overall, the results were similar to a previous study investigating storm initiation in the vicinity of low-level mesoscale boundaries in eastern Colorado and include some new findings in relation to lake-breeze fronts. The findings can be used by forecasters as well as automated nowcasting algorithms in order to improve predictions of storm initiation.

Open access
H. Bonekamp, G. J. Komen, A. Sterl, P. A. E. M. Janssen, P. K. Taylor, and M. J. Yelland

Abstract

The surface-drag coefficients of two versions of the ECMWF's atmosphere–wave model are compared with those of uncoupled model versions and with those of inertial-dissipation measurements in the open ocean made by the RRS Discovery. It is found that the mean drag resulting from the latest coupled version is on average equal to that of the uncoupled version. However, both have a positive bias when compared with the RRS Discovery observations. This bias is discussed, also in the light of other observational open ocean data. In the second part of the paper, bulk parameterizations with and without parameters of collocated sea-state data are validated against the Discovery observations. Using published estimates of the error in friction velocity and the neutral 10-m winds, all bulk parameterizations score low on goodness-of-fit tests. The lowest scores are obtained for the constant Charnock parameter case, whereas the highest scores are obtained for a wave-age-dependent parameterization. On–off experiments are made for the corrections to the inertial-dissipation data that have been proposed in previous studies. These corrections concern the measurement height and the direct wave-induced turbulence in the lower atmosphere. The first correction results in a slightly better agreement, but the second reduces the goodness-of-fit of the bulk parameterizations.

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Colin M. Zarzycki, Christiane Jablonowski, Diana R. Thatcher, and Mark A. Taylor

Abstract

Using the spectral element (SE) dynamical core within the National Center for Atmospheric Research–Department of Energy Community Atmosphere Model (CAM), a regionally refined nest at 0.25° (~28 km) horizontal resolution located over the North Atlantic is embedded within a global 1° (~111 km) grid. A 23-yr simulation using Atmospheric Model Intercomparison Project (AMIP) protocols and default CAM, version 5, physics is compared to an identically forced run using the global 1° (~111 km) grid without refinement. The addition of a refined patch over the Atlantic basin does not noticeably affect the global circulation. In the area where the refinement is located, large-scale precipitation increases with the higher resolution. This increase is partly offset by a decrease in precipitation resulting from convective parameterizations, although total precipitation is also slightly higher at finer resolutions. Equatorial waves are not significantly impacted when traversing multiple grid spacings. Despite the grid transition region bisecting northern Africa, local zonal jets and African easterly wave activity are highly similar in both simulations. The frequency of extreme precipitation events increases with resolution, although this increase is restricted to the refined patch. Topography is better resolved in the nest as a result of finer grid spacing. The spatial patterns of variables with strong orographic forcing (such as precipitation, cloud, and precipitable water) are improved with local refinement. Additionally, dynamical features, such as wind patterns, associated with steep terrain are improved in the variable-resolution simulation when compared to the uniform coarser run.

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