Meteorol Soc Research Articles

An Asymptotic Theory for the Flow over Heterogeneous Roughness

The flow over arbitrary roughness changes is investigated, revisiting the analysis of Belcher et al. (Q J R Meteorol Soc 116:611–635, 1990) regarding surface-roughness heterogeneity. The proposed theory is restricted to steady neutral boundary layers over flat regions with changes of roughness sufficiently slow and mild to inhibit the growth of nonlinear terms. The approach is based on a triple-deck decomposition of the flow above the roughness, although only the first two layers are interactive at leading order. Two experimental datasets (one with a smooth-to-rough and the other with a rough-to-smooth transition) are used to validate the theory. The latter is further compared against two large-eddy simulations featuring chessboard patterns of alternating surface roughness with relatively short and long length scales, respectively. All the comparisons show that the proposed theory is able to reasonably assess the wind-field perturbation due to the roughness heterogeneity, supporting the use of the model to quickly assess the effect of roughness changes in the flow field.

Reliability Analysis with Proportional Hazard Model in Aeronautics

Reliability analysis is a specific field of statistics that studies failure time and its probability on a system. This branch of statistics takes an important place in many domains, especially in aeronautics. One of the most commonly used methods of reliability analysis is the hazard function which is defined for a specific instance in time, given the information up to that moment. Commonly, the most used parametric proportional hazard model is Weibull proportional hazard model. Weibull distribution makes restrictive assumptions of the baseline hazard function like monotonicity and gives average values, which can cover a large type of failures due to aircraft age and fatigue in life operation. The present work deals with an unusual phenomena, especially in the aircraft entrance in a turbulence area, see Zbrozek (Weather R Meteorol Soc 13(7):215–227, 1958). This type of situation can lead to the need of increasing–decreasing proportional hazard models, taking into account harsh conditions during the flight. The functions which could model these non-monotone phenomena are scarce in literature but remain very crucial due to their impact on flight safety. In this paper, we propose a new baseline hazard function using extreme values theory in proportional hazard model and we adjusted the flexible Weibull increasing–decreasing baseline hazard function proposed by Park* and Park (Commun Stat Theory Methods 47(4):767–778, 2018) to the proposed baseline hazard function. This function could be implemented in control system of navigation particularly the take-off and landing phase and used as a support tool for pilot or aviation controller running turbulence area. The newly suggested function is non-monotone and will be named generalized extreme values baseline hazard function and we prove that this function satisfies hazard properties and could be used as a tool in the prediction of system operations. An application on aircraft flights is done and the effects of environmental factors are studied in aircraft performance modeling with the proposed model.

A Model for the Spectrum of the Lateral Velocity Component from Mesoscale to Microscale and Its Application to Wind-Direction Variation

A model for the spectrum of the lateral velocity component $$S_v(f)$$ is developed for a frequency range from about 0.2 $$\hbox {day}^{-1}$$ to the turbulence inertial subrange, with the intent of improving the calculation of flow meandering over areas the size of offshore wind farms and clusters. These sizes can correspond to a temporal scale of several hours, much larger than the validity limit of typical boundary-layer models, such as the Kaimal model, or the Mikkelsen–Tchen model. The development of the model is based on observations from one site and verified with observations from another site up to a height of 241 m. The model describes three ranges: (1) the mesoscale from $$0.2\ \hbox {day}^{-1}$$ to about $$10^{-3}\ \hbox {Hz}$$ where a mesoscale spectral model from Larsen et al. (2013: QJR Meteorol Soc 139: 685–700) is used; (2) the spectral gap where the normalized v spectrum, $$fS_v(f)$$ , can be approximated to be a constant; (3) the high-frequency range where a boundary-layer model is used. In order to demonstrate a general applicability of the lateral velocity spectrum model to reproduce the statistics of wind-direction variability, models for both horizontal velocity components, u and v, are used through an inverse Fourier transform technique to produce time series of both components, which in theory could have been the ensemble for calculating the corresponding spectra. The ensemble is then used to calculate directional statistics, which in turn are compared with corresponding statistics from the measured time series. We demonstrate the relevance of the constructed spectral models for calculating meandering effects for large wind farms and wind-farm clusters.

The abyssal origins of North Atlantic decadal predictability

The fundamental mechanisms that explain high subpolar North Atlantic (SPNA) decadal predictability within a particular modeling framework are described. The focus is on the Community Earth System Model (CESM), run in both a historical forced-ocean configuration as well as in a fully coupled configuration initialized from the former. The initialized prediction experiments comprise the CESM Decadal Prediction Large Ensemble (CESM-DPLE)—a 40-member set of retrospective hindcasts documented in Yeager et al. (Bull Am Meteorol Soc 99:1867–1886. https://doi.org/10.1175/bams-d-17-0098.1 , 2018). Heat budget analysis confirms the driving role of advective heat convergence in skillful prediction of SPNA upper ocean heat content out to decadal lead times. The key ocean dynamics are topographically-coupled overturning/gyre fluctuations that are geographically centered over the mid-Atlantic ridge (MAR). Long-lasting predictive skill for ocean heat transport can be related to predictable barotropic gyre and sigma-coordinate AMOC circulations, but depth-coordinate AMOC is far less predictable except in the deepest layers. The foundation of ocean memory (and circulation predictive skill) in CESM-DPLE is Labrador Sea Water thickness, which propagates predictably through interior pathways towards the MAR where large anomalies accumulate and persist. Abyssal thickness anomalies drive predictable decadal changes in the gyre circulation, including changes in sea level gradient and near surface flow, that account for the high predictability of SPNA upper ocean heat content.

Impact of volcanic aerosol hemispheric symmetry on Sahel rainfall

The semi-arid African Sahel region is highly sensitive to changes in monsoon precipitation, as much of the region’s workforce is employed in the agricultural industry (Hamro-Drotz and Programme 2011). Thus, studying the response of rainfall and aridity in this region to radiative perturbations is a matter of pressing humanitarian relevance. In addition, there is evidence to suggest that spatially asymmetric volcanic aerosols produce different hydroclimatic responses based on their hemispheric symmetry, both globally and in the Sahel. We use two different climate models, GFDL’s FLOR model (Vecchi et al. in J Clim 27(21):7994–8016, 2014) and NCAR’s CESM 1.1 model (Otto-Bliesner et al. in Bull Am Meteorol Soc 97(5):735–754, 2016), to characterize the response of rainfall in the Sahel to large volcanic eruptions based on the meridional symmetry of the volcanic eruptions. We find that in both the FLOR experiments simulating three large twentieth century eruptions and in the CESM Last Millennium Ensemble simulations of 46 historic volcanic eruptions, asymmetric Northern Hemisphere cooling causes a subsequent drying response in the Sahel, and Southern Hemisphere cooling causes a wetting, or “greening” response. Symmetric volcanic eruptions have a relatively small effect on rainfall in the Sahel. We also find that the FLOR results show a consistent response in the annual rainfall cycle in the Sahel for all three of the eruptions analyzed, with a reduction in rainfall in early summer followed by an increased rainfall in late summer. The annual cycle response of rainfall in the Sahel from the CESM experiments is different, in that the SH eruptions cause a rainfall maximum in August, NH eruptions cause a rainfall minimum in September, and symmetric eruptions show a slight increase in August and a decrease in October. Our results highlight the need for accurate meridional structures in historic volcanic forcing data used for climate models as well as the need for further study on regional effects of hemispherically asymmetric radiative forcing, especially as they might pertain to aerosol geoengineering.

Visibility Data Assimilation and Prediction Using an Observation Network in South Korea

There have been many studies to improve visibility forecast skills using numerical models, but their performance still remains behind the forecast skills for other meteorological phenomena. This study attempted to improve visibility forecasts using a newly established automatic visibility observation network composed of 291 forward-scattering sensors in South Korea. In the analysis of recent 3-year visibility observations, clear days (visibility above 20 km) were reported for 46% of the days, and fog cases (visibility less than 1 km) accounted for 1.58% of the total observations. The Very short-range Data Assimilation and Prediction System (VDAPS) of the Korea Meteorological Administration (KMA) assimilated the visibility observations based on the Met Office Unified Model with visibility data assimilation of Clark et al. (Q J R Meteorol Soc 134:1801–1816, 2008). Prior to the data assimilation, a precipitation check eliminated visibility data with precipitation (9.4% in total, 23% for visibility less than 1 km), and a consistency check removed visibility observations that were inappropriate to relative humidity, temperature, and pressure. In a case study on two consecutive fog days, visibility forecast skills were improved by applying visibility data assimilation, mostly through modifications of aerosol concentrations. A 3-month model run in the winter of 2016 showed a positive bias in visibility predictions, especially for the low-visibility cases. Visibility data assimilation improved the prediction skills, but the positive effects were limited within 9 forecast hours and were smaller for extremely low-visibility events. Sensitivity experiments were performed using local aerosol observations with a larger number of smaller aerosol particles. Modifications in aerosol properties made better results in frequency bias for the whole forecast ranges and also improved the equitable threat score (ETS) for relatively longer forecast hours (more than 4 h).

Implementation of Tropical Cyclone Detection Scheme to CCAM model for Seasonel Tropical Cyclone Prediction over the Vietnam East Sea

Implementation of Tropical Cyclone Detection Scheme to CCAM model for Seasonel Tropical Cyclone Prediction over the Vietnam East Sea

Validation of Simplified Urban-Canopy Aerodynamic Parametrizations Using a Numerical Simulation of an Actual Downtown Area

A steady-state Reynolds-averaged Navier–Stoke computational fluid dynamics (CFD) investigation of boundary-layer flow over a major portion of downtown Abu Dhabi is conducted. The results are used to derive the shear stress and characterize the logarithmic region for eight sub-domains, where the sub-domains overlap and are overlaid in the streamwise direction. They are characterized by a high frontal area index initially, which decreases significantly beyond the fifth sub-domain. The plan area index is relatively stable throughout the domain. For each sub-domain, the estimated local roughness length and displacement height derived from CFD results are compared to prevalent empirical formulations. We further validate and tune a mixing-length model proposed by Coceal and Belcher (Q J R Meteorol Soc 130:1349–1372, 2004). Finally, the in-canopy wind-speed attenuation is analysed as a function of fetch. It is shown that, while there is some room for improvement in Macdonald’s empirical formulations (Boundary-Layer Meteorol 97:25–45, 2000), Coceal and Belcher’s mixing model in combination with the resolution method of Di Sabatino et al. (Boundary-Layer Meteorol 127:131–151, 2008) can provide a robust estimation of the average wind speed in the logarithmic region. Within the roughness sublayer, a properly parametrized Cionco exponential model is shown to be quite accurate.

Comprehensive assessment of parameterization methods for estimating clear-sky surface downward longwave radiation

Surface downward longwave radiation (SDLR) is a key variable for calculating the earth’s surface radiation budget. In this study, we evaluated seven widely used clear-sky parameterization methods using ground measurements collected from 71 globally distributed fluxnet sites. The Bayesian model averaging (BMA) method was also introduced to obtain a multi-model ensemble estimate. As a whole, the parameterization method of Carmona et al. (2014) performs the best, with an average BIAS, RMSE, and R2 of − 0.11 W/m2, 20.35 W/m2, and 0.92, respectively, followed by the parameterization methods of Idso (1981), Prata (Q J R Meteorol Soc 122:1127-1151, 1996), Brunt and Sc (Q J R Meteorol Soc 58:389-420, 1932), and Brutsaert (Water Resour Res 11:742-744, 1975). The accuracy of the BMA is close to that of the parameterization method of Carmona et al. (2014) and comparable to that of the parameterization method of Idso (1981). The advantage of the BMA is that it achieves balanced results compared to the integrated single parameterization methods. To fully assess the performance of the parameterization methods, the effects of climate type, land cover, and surface elevation were also investigated. The five parameterization methods and BMA all failed over land with the tropical climate type, with high water vapor, and had poor results over forest, wetland, and ice. These methods achieved better results over desert, bare land, cropland, and grass and had acceptable accuracies for sites at different elevations, except for the parameterization method of Carmona et al. (2014) over high elevation sites. Thus, a method that can be successfully applied everywhere does not exist.

Spectral cumulus parameterization based on cloud-resolving model

We have developed a spectral cumulus parameterization using a cloud-resolving model. This includes a new parameterization of the entrainment rate which was derived from analysis of the cloud properties obtained from the cloud-resolving model simulation and was valid for both shallow and deep convection. The new scheme was examined in a single-column model experiment and compared with the existing parameterization of Gregory (2001, Q J R Meteorol Soc 127:53–72) (GR scheme). The results showed that the GR scheme simulated more shallow and diluted convection than the new scheme. To further validate the physical performance of the parameterizations, Atmospheric Model Intercomparison Project (AMIP) experiments were performed, and the results were compared with reanalysis data. The new scheme performed better than the GR scheme in terms of mean state and variability of atmospheric circulation, i.e., the new scheme improved positive bias of precipitation in western Pacific region, and improved positive bias of outgoing shortwave radiation over the ocean. The new scheme also simulated better features of convectively coupled equatorial waves and Madden–Julian oscillation. These improvements were found to be derived from the modification of parameterization for the entrainment rate, i.e., the proposed parameterization suppressed excessive increase of entrainment, thus suppressing excessive increase of low-level clouds.

Estimation of Surface-Layer Scaling Parameters in the Unstable Boundary Layer: Implications for Orographic Flow Speed-Up

A method is proposed for estimating the surface-layer depth $$(z_s)$$ and the friction velocity $$(u_*)$$ as a function of stability (here quantified by the Obukhov length, L) over the complete range of unstable flow regimes. This method extends that developed previously for stable conditions by Argain et al. (Boundary-Layer Meteorol 130:15–28, 2009), but uses a qualitatively different approach. The method is specifically used to calculate the fractional speed-up $$(\varDelta S)$$ in flow over a ridge, although it is suitable for more general boundary-layer applications. The behaviour of $$z_s \left( L\right) $$ and $$u_*\left( L\right) $$ as a function of L is indirectly assessed via calculation of $$\varDelta S\left( L\right) $$ using the linear model of Hunt et al. (Q J R Meteorol Soc 29:16–26, 1988) and its comparison with the field measurements reported in Coppin et al. (Boundary-Layer Meteorol 69:173–199, 1994) and with numerical simulations carried out using a non-linear numerical model, FLEX. The behaviour of $$\varDelta S$$ estimated from the linear model is clearly improved when $$u_*$$ is calculated using the method proposed here, confirming the importance of accounting for the dependences of $$z_s\left( L \right) $$ and $$u_*\left( L \right) $$ on L to better represent processes in the unstable boundary layer.

Drones in California research and extension

Drones in California research and extension

Sensitivity test of parameterizations of subgrid-scale orographic form drag in the NCAR CESM1

Turbulent drag caused by subgrid orographic form drag has significant effects on the atmosphere. It is represented through parameterization in large-scale numerical prediction models. An indirect parameterization scheme, the Turbulent Mountain Stress scheme (TMS), is currently used in the National Center for Atmospheric Research Community Earth System Model v1.0.4. In this study we test a direct scheme referred to as BBW04 (Beljaars et al. in Q J R Meteorol Soc 130:1327–1347, 2004. doi:10.1256/qj.03.73), which has been used in several short-term weather forecast models and earth system models. Results indicate that both the indirect and direct schemes increase surface wind stress and improve the model’s performance in simulating low-level wind speed over complex orography compared to the simulation without subgrid orographic effect. It is shown that the TMS scheme produces a more intense wind speed adjustment, leading to lower wind speed near the surface. The low-level wind speed by the BBW04 scheme agrees better with the ERA-Interim reanalysis and is more sensitive to complex orography as a direct method. Further, the TMS scheme increases the 2-m temperature and planetary boundary layer height over large areas of tropical and subtropical Northern Hemisphere land.

Observation impact analysis methods for storm surge forecasting systems

This paper presents a simple method for estimating the impact of assimilating individual or group of observations on forecast accuracy improvement. This method is derived from the nsemble-based observation impact analysis method of Liu and Kalnay (Q J R Meteorol Soc 134:1327–1335, 2008). The method described here is different in two ways from their method. Firstly, it uses a quadratic function of model-minus-observation residuals as a measure of forecast accuracy, instead of model-minus-analysis. Secondly, it simply makes use of time series of observations and the corresponding model output generated without data assimilation. These time series are usually available in an operational database. Hence, it is simple to implement. It can be used before any data assimilation is implemented. Therefore, it is useful as a design tool of a data assimilation system, namely for selecting which observations to assimilate. The method can also be used as a diagnostic tool, for example, to assess if all observation contributes positively to the accuracy improvement. The method is applicable for systems with stationary error process and fixed observing network. Using twin experiments with a simple one-dimensional advection model, the method is shown to work perfectly in an idealized situation. The method is used to evaluate the observation impact in the operational storm surge forecasting system based on the Dutch Continental Shelf Model version 5 (DCSMv5).

Stratified Flow Past a Hill: Dividing Streamline Concept Revisited

The Sheppard formula (Q J R Meteorol Soc 82:528–529, 1956) for the dividing streamline height $$H_\mathrm{s}$$ assumes a uniform velocity $$U_\infty $$ and a constant buoyancy frequency N for the approach flow towards a mountain of height h, and takes the form $$H_\mathrm{s}/h=\left( {1-F} \right) $$ , where $$F=U_{\infty }/Nh$$ . We extend this solution to a logarithmic approach-velocity profile with constant N. An analytical solution is obtained for $$H_\mathrm{s}/h$$ in terms of Lambert-W functions, which also suggests alternative scaling for $$H_\mathrm{s}/h$$ . A ‘modified’ logarithmic velocity profile is proposed for stably stratified atmospheric boundary-layer flows. A field experiment designed to observe $$H_\mathrm{s}$$ is described, which utilized instrumentation from the spring field campaign of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. Multiple releases of smoke at $$F\approx 0.3$$ –0.4 support the new formulation, notwithstanding the limited success of experiments due to logistical constraints. No dividing streamline is discerned for $$F\approx 10$$ , since, if present, it is too close to the foothill. Flow separation and vortex shedding is observed in this case. The proposed modified logarithmic profile is in reasonable agreement with experimental observations.

Selected Spectral Characteristics of Turbulence over an Urbanized Area in the Centre of Łódź, Poland

We present the turbulence spectra and cospectra derived from more than five years of eddy-covariance measurements at two urban sites in Łódź, central Poland. The fast response wind velocity components were obtained using sonic anemometers placed on narrow masts at 37 and 42 m above ground level. The analysis follows Kaimal et al. (Q J R Meteorol Soc 98:563–589, 1972) who established the spectral and cospectral properties of turbulent flow in atmospheric surface layer on the basis of the Kansas experiment. Our results illustrate many features similar to those of Kaimal et al., but some differences are also observed. The velocity (co)spectra from Łódź show a clear inertial subrange with \(-2/3\) slope for spectra and \(-4/3\) slope for cospectra. We found that an appropriate stability function for the non-dimensional dissipation of turbulent kinetic energy calculated from spectra in the inertial subrange differs from that of Kaimal et al., and it can be satisfactorily estimated with the assumption of local equilibrium using standard functions for the non-dimensional shear production. A similar function for the cospectrum corresponds well to Kaimal et al. for unstable and weakly stable conditions. The (co)spectra normalized by their spectral values in the inertial subrange are in general similar to those of Kaimal et al., but they peak at lower frequencies in strongly stable conditions. Moreover, our results do not confirm the existence of a clear “excluded region” at low frequencies for the transition from stable to unstable conditions, for longitudinal and lateral wind components. The empirical models of Kaimal et al. with adjusted parameters fit well to the vertical velocity spectrum and the vertical momentum flux cospectrum. The same type of function should be used for longitudinal and lateral wind spectra because of their sharper peak than occurs for the Kansas data. Finally, it should be stressed that the above relationships are well-defined for averaged values. The results for individual 1-h periods are very scattered and can be significantly different from the generalized functions.

Assessment of mixed layer models embedded in an ocean general circulation model

The atmospheric mixed layer obtained using the Mellor–Yamada model grows slower and becomes shallower than observed, which motivated Nakanishi and Niino (J Meteorol Soc Jpn 87:895–912, 2009) to present a modified version of the Mellor–Yamada model. In this study, incorporating each of the Mellor–Yamada and the Nakanishi–Niino models into an ocean general circulation model, we evaluate its performance in the ocean. Comparing the numerical results with the observed ones in the western North Pacific, the Nakanishi–Niino model is shown to exhibit a better performance than the Mellor–Yamada model under strong wind forcing and sea surface cooling during winter and after passage of typhoons during summer.

Physically based evaluation of climate models over the Iberian Peninsula

A novel approach is proposed for evaluating regional climate models based on the comparison of empirical relationships among model outcome variables. The approach is actually a quantitative adaptation of the method for evaluating global climate models proposed by Betts (Bull Am Meteorol Soc 85:1673–1688, 2004). Three selected relationships among different magnitudes involved in water and energy land surface budgets are firstly established using daily re-analysis data. The selected relationships are obtained for an area encompassing two river basins in the southern Iberian Peninsula corresponding to 2 months, representative of dry and wet seasons. The same corresponding relations are also computed for each of the thirteen regional simulations of the ENSEMBLES project over the same area. The usage of a metric based on the Hellinger coefficient allows a quantitative estimation of how well models are performing in simulating the relations among surface magnitudes. Finally, a series of six rankings of the thirteen regional climate models participating in the ENSEMBLES project is obtained based on their ability to simulate such surface processes.

Flow Over a Narrow Ridge Covered with a Plant Canopy: A Comparison Between Wind-Tunnel Observations and Linear Theory

We revisit the analytical model for atmospheric boundary-layer flow over a hill covered with a canopy of Finnigan and Belcher (Q J R Meteorol Soc 130:1–29, 2004). Remaining within the overall scope of that analysis we extend in two ways. First we include the impacts of the advection terms within the upper canopy in a simple, but approximate, manner. Second we establish a modification for the associated pressure perturbation. Both extensions allow us to extend the parameter range wherein the analytical framework can be expected to reasonably hold. The within-canopy advection terms act to provide a downstream shift, and decreased magnitude, to the flow perturbations within the canopy as compared to the predictions from the original analysis. Through continuity, similar, but smaller, impacts are seen above the canopy. Together these act to reduce the differences in the streamwise positions of the topographic speed-up seen above and within the canopy. The modified pressure perturbation also acts to decrease the magnitude of the flow perturbations. The predicted topographic influence on the flow is reduced from that given in the original analysis but, importantly, the positions where the topographic influences most strongly affect the flow, and by extension the scalar concentration fields, are also changed. Predictions from the revised analysis are shown to be in good agreement with wind-tunnel data for flow over an isolated narrow ridge covered by a partially dense canopy.

Limited area NWP and regional climate modeling: a test of the relaxation vs Eta lateral boundary conditions

With very few exceptions, just about all limited area models (LAMs) used in operational NWP and regional climate modeling use the Davies (Q J R Meteorol Soc 102:405–418, 1976) relaxation lateral boundary conditions (LBCs), even though they make no effort to respect the basic mathematics of the problem. While in the early stages of the primitive equation LAM development in the seventies numerous schemes have been proposed and tested, LAM communities have eventually for the most part settled on the relaxation LBCs with few questions asked. An exception is the Eta model used extensively at NCEP and several other centers, in which the Mesinger (Contrib Atmos Phys 50:200–210, 1977) LBCs are used, designed and based on knowledge available before the introduction of the relaxation scheme. They prescribe variables along the outermost row of grid points only; all of them at the inflow points and one less at the outflow points where the tangential velocity components are extrapolated from inside of the model domain. Additional schemes are in place to suppress separation of gravity-wave solutions on C-subgrids of the model’s E-grid. A recent paper of Veljovic et al. (Meteor Zeitschrift 19:237–246, 2010) included three 32-day forecasts done with both the Eta and the relaxation LBCs and the comparison of some of their verification results. Here we extend this experiment by three additional forecasts to arrive at an ensemble of six members run with both schemes, and present a more complete discussion of results. We in addition show results of one of these forecasts in which the linear change of relaxation coefficients was replaced by the change following the recommendation of Lehmann (Meteorol Atmos Phys 52:1–14, 1993). We feel that the results of our two verification schemes strongly suggest the advantage of the Eta over the conventional relaxation scheme, thereby raising doubts as to the justification for its use.