Convection Scenarios Research Articles

Improving predictions of heat transfer in indoor environments with eddy viscosity turbulence models

Heat transfer modelling in indoor environments requires an accurate prediction of the convective heat transfer phenomenon. Because of the lower computational cost and numerical stability, eddy viscosity turbulence models are often used. These models allow modification to turbulent Prandtl number, and near wall correction which influences stagnation points, entrainment, and velocity and time scales. A modified v2–f model was made to correct the entrainment behaviour in the near wall and at the stagnation point. This new model was evaluated on six cases involving free and forced convection and room airflow scenarios and compared with the standard k–e, and k–ω–SST models. The results showed that the modification to the v2–f model provided better predictions of the buoyant heat transfer flows while the standard k–e failed to reproduce and underestimate the convective heat transfer. The k–ω–SST model was able to predict the flow field well only for a 2D square cavity room, and 3D partitioned room case, while it was poor for the other four cases.

Buoyancy-driven instabilities around miscible A+B→C reaction fronts: a general classification.

Upon contact between miscible solutions of reactants A and B along a horizontal interface in the gravity field, various buoyancy-driven instabilities can develop when an A+B→C reaction takes place and the density varies with the concentrations of the various chemicals. To classify the possible convective instability scenarios, we analyze the spatial dependence of the large time asymptotic density profiles as a function of the key parameters of the problem, which are the ratios of diffusion coefficients and of solutal expansion coefficients of species A, B, and C. We find that 62 different density profiles can develop in the reactive problem, whereas only 6 of them can be obtained in the nonreactive one.

A Pulse Compression Waveform for Improved-Sensitivity Weather Radar Observations

AbstractThe progression of phased array weather observations, research, and planning over the past decade has led to significant advances in development efforts for future weather radar technologies. However, numerous challenges still remain for large-scale deployment. The eventual goal for phased array weather radar technology includes the use of active arrays, where each element would have its own transmit/receive module. This would lead to significant advantages; however, such a design must be capable of utilizing low-power, solid-state transmitters at each element in order to keep costs down. To provide acceptable sensitivity, as well as the range resolution needed for weather observations, pulse compression strategies are required. Pulse compression has been used for decades in military applications, but it has yet to be applied on a broad scale to weather radar, partly because of concerns regarding sensitivity loss caused by pulse windowing. A robust optimization technique for pulse compression waveforms with minimalistic windowing using a genetic algorithm is presented. A continuous nonlinear frequency-modulated waveform that takes into account transmitter distortion is shown, both in theory and in practical use scenarios. Measured pulses and weather observations from the Advanced Radar Research Center’s dual-polarized PX-1000 transportable radar, which utilizes dual 100-W solid-state transmitters, are presented. Both stratiform and convective scenarios, as well as dual-polarization observations, are shown, demonstrating significant improvement in sensitivity over previous pulse compression methods.

Simulation of EarthCARE Spaceborne Doppler Radar Products Using Ground-Based and Airborne Data: Effects of Aliasing and Nonuniform Beam-Filling

This paper describes the expected performance of the Doppler cloud profiling radar being built for the Earth Cloud Aerosols Radiation Explorer (EarthCARE) mission of the Japanese Aerospace Exploration Agency and the European Space Agency. Spaceborne Doppler radar data are simulated starting from high-resolution Doppler measurements provided by ground-based and airborne Doppler radars, ranging from nonconvective to moderately convective scenarios. The method hinges upon spatial and spectral resampling to consider the specificities of the spaceborne configuration. An error analysis of the resulting Doppler product is conducted to address aliasing and nonuniform beam-filling (NUBF) problems. A perturbation analysis is applied to explore the latter problem and allow for a self-standing systematic correction of NUBF using merely the received reflectivity factor and mean Doppler velocities as measured by the instrument. The results of our simulations show that, at a horizontal integration of 1 km, after proper de-aliasing and NUBF correction, the radar will typically yield a velocity accuracy in the order of 1.3 m·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> over intertropical regions where the pulse-repetition frequency (PRF)=6.1 kHz, of 0.8 m·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> where the cloud-profiling radar (CPR) operates at PRF=7 kHz, and, of 0.7 m·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> over high latitudes where the CPR of EarthCARE will operate at PRF=7.5 kHz.

Modeling and simulation of a solar flat plate collector as an air heater considering energy efficiency

The present research study includes thermal energy performance analysis of a flat plate solar collector, in order to cover a sensitivity analysis about effective variables contributed to performance increasing approach. To obtain that, energy and momentum governing equations on a flat plate thermal collector were developed to achieve air output temperature and velocity profiles both in model and experiment. The model theory is validated with experiments by a set of flat plate thermal collectors, those located in (35◦ 44′ 35′′ N, 50◦ 57′ 25′′ E) coordinates and then were applied to carry out experimental activities. Quantitative results depicted that the mean difference between predicted and measured output air temperature in natural and forced convection scenarii is 1.47 ◦ C (3.5%) and 0.9 ◦ C (1.5%), respectively; however earlier research works and studies mentioned in literature, include error percentages in the range of 4–10%. Another quantification about, the average error percentages of estimated amounts of output air velocity profile values in natural and forced convection scenarii is 9% and 4%, respectively. This issue realizes the developed model in forced convection scenario more accurate than natural convection scenario.

Cold-Season Thunderstorms in Finland and Their Effect on Aviation Safety

A total of 13 commercial airplanes were struck by lightning in October (10 in 1 day) and December (3 on 3 separate days) 2011 in the main Finnish Helsinki–Vantaa airport. The number of lightning-struck airplanes is extremely large, considering the time of year and the small number of flashes by the storms. This paper indicates the characteristics of these cases regarding the synoptic situation as well as their forecasting. There were remarkable differences in the operational models; the high-resolution nonhydrostatic model was superior in predicting the convective nature of the event compared to the coarser-resolution hydrostatic model. The interview of the pilots of the struck airplanes shows that the pilots did not receive detailed information to avoid the situation; also, the lightning strike affected the pilots, even causing temporary loss of sight and hearing. Luckily, no fatalities or severe damage to the airplanes occurred. The most interesting case is 19 October 2011; during this single day, a total of 10 airplanes were struck. The analysis suggests that a major cause for the large number of struck airplanes is that the planes took off directly into the convective core of the storm and the planes initialized the flashes themselves. However, the time of the year, the near position of the storm area relative to the takeoff path, and the necessity to use only a certain takeoff path because of the direction of wind makes the convective scenario difficult to predict and avoid. The pilots have expressed interest in receiving training for these cold-season thunderstorms.

NUMERICAL STUDY OF UNSTEADY AIRFLOW PHENOMENA IN A VENTILATED ROOM

Numerical simulation of airflow in an indoor environment has been carried out for forced, natural, and mixed convection modes, respectively, by using the computational fluid dynamics (CFD) approach of solving the Reynolds-averaged Navier−Stokes equations. Three empty model rooms in two-dimensional configuration were studied first; focusing on the effects of grid refinement, mesh topology, and turbulence model. It was found that structured mesh results were in better agreement with available experimental measurements for all three convection scenarios, while the renormalized group (RNG) к − e turbulence model produced better results for both forced and mixed convections and the shear stress transport (SST) turbulence model for the natural convection prediction. Further studies of air velocity and temperature distributions in a three-dimensional cubic model room with and without an obstacle have shown reasonably good agreement with available test data at the measuring points. CFD results exhibited some unsteady flow phenomena that have not yet been observed or reported in previous experimental studies for the same problem. After analyzing the time history of velocity and temperature data using fast Fourier transformation (FFT), it was found that both air velocity and temperature field oscillated at low frequencies up to 0.4 Hz and the most significant velocity oscillations occurred at a vertical height of an ankle level (0.1 m) from the floor, where temperature oscillation was insignificant. The reasons for this flow unsteadiness were possibly a higher Grashof number, estimated at 0.5 × 106 based inflow conditions, and thus strong buoyancy driven effects caused the oscillations in the flow field. The appearance of an obstacle in the room induced flow separation at its sharp edges and this would further enhance the oscillations due to the unsteady nature of detached shear-layer flow.

Forecasting challenges during the severe weather outbreak in Central Europe on 25 June 2008

On 25 June 2008, severe thunderstorms caused widespread damage and two fatalities in the Czech Republic. Significant features of the storms included numerous downbursts on a squall line that exhibited a bow echo reflectivity pattern, with sustained wind gusts over 32 m/s at several reporting stations. Moreover, a tornado and several downbursts of F2 intensity occurred within the convective system, collocated with the development of mesovortices within the larger scale bow echo. The extent of the event was sufficient to call it a derecho, as the windstorm had affected Eastern Germany, Southern Poland, Slovakia, Austria and Northern Hungary as well. Ahead of the squall line, several well-organized isolated cells occurred, exhibiting supercellular characteristics, both from a radar and visual perspective. These storms produced large hail and also isolated severe wind gusts. This paper deals mostly with the forecasting challenges that were experienced by the meteorologist on duty during the evolution of this convective scenario. The main challenge of the day was to identify the region that would be most affected by severe convection, especially as the numerical weather prediction failed to anticipate the extent and the progress of the derecho-producing mesoscale convective systems (MCSs). Convective storms developed in an environment conducive to severe thunderstorms, with strong wind shear confined mostly to the lower half of the troposphere. These developments also were strongly influenced by mesoscale factors, especially a mesolow centered over Austria and its trough stretching to Eastern Bohemia. The paper demonstrates how careful mesoscale analysis could prove useful in dealing with such convective situations. Remote-sensing methods are also shown to be useful in such situations, especially when they can offer sufficient lead time to issue a warning, which is not always the case.

Evaluation of an Integrated Traffic Flow Management Decision Making Approach

An integrated traffic flow management decision-making approach that sequentially schedules and reroutes flights subject to actual and forecast weather induced airspace capacity constraints is presented, and used to conduct 80 fast-time simulation experiments. These experiments considered variations in the severity and location of en route convective weather, and the traffic flow management strategies implemented to mitigate these weather impacts. Strategic flight scheduling, in which pre-departure and airborne delays were assigned to individual flights, was found to be effective at alleviating capacity constraints in sectors and at airports. However, tactical rerouting was found to be more effective than flight scheduling at avoiding en route weather hazards when the scheduling algorithms used weather impacted sector capacities as a proxy for weather location. In general, the distribution of the delays amongst the users was found to be most equitable when scheduling flights using a heuristic scheduling algorithm, such as ration-by-schedule. On the other hand, equity decreased by 77% when using a scheduling algorithm that took into account the number of seats aboard each flight. Interestingly, traffic flow management solutions that included scheduling and rerouting increased the equity of the solutions by 9% to 18% over comparable solutions that involved flight scheduling alone. Finally, the modeled results were compared with actual levels of delay, sector congestion and the number of weather incursions under three historical convective weather scenarios. The agreement between the modeled and actual results was found to be strongly dependent on the weather scenario and the number of constraints that were binding in the scheduling algorithm.

Mars geodesy, rotation and gravity

This review provides explanations of how geodesy, rotation and gravity can be addressed using radioscience data of an orbiter around a planet or of the lander on its surface. The planet Mars is the center of the discussion. The information one can get from orbitography and radioscience in general concerns the global static gravitational field, the time variation of the gravitational field induced by mass exchange between the atmosphere and the ice caps, the time variation of the gravitational field induced by the tides, the secular changes in the spacecraft's orbit induced by the little moons of Mars named Phobos and Deimos, the gravity induced by particular targets, the Martian ephemerides, and Mars' rotation and orientation. The paper addresses as well the determination of the geophysical parameters of Mars and, in particular, the state of Mars' core and its size, which is important for understanding the planet's evolution. Indeed, the state and dimension of the core determined from the moment of inertia and nutation depend in turn on the percentage of light elements in the core as well as on the core temperature, which is related to heat transport in the mantle. For example, the radius of the core has implications for possible mantle convection scenarios and, in particular, for the presence of a perovskite phase transition at the bottom of the mantle. This is also important for our understanding of the large volcanic province Tharsis on the surface of Mars.

Dealing with uncertainty: turbulent parameterizations and grid-spacing effects in numerical modelling of deep moist convective processes

Abstract. Computer power has grown to the point that very-fine-mesh mesoscale modelling is now possible. Going down through scales is clumsily supposed to reduce uncertainty and to improve the predictive ability of the models. This work provides a contribution to understand how the uncertainty in the numerical weather prediction (NWP) of severe weather events is affected by increasing the model grid resolution and by choosing a parameterization which is able to represent turbulent processes at such finer scales. A deep moist convective scenario, a supercell, in a simplified atmospheric setting is studied by mean of high resolution numerical simulations with COSMO-Model. Different turbulent closures are used and their impacts on the space-time properties of convective fields are discussed. The convective-resolving solutions adopting Large Eddy Simulation (LES) turbulent closure converge with respect to the overall flow field structure when grid spacing is properly reduced. By comparing the rainfall fields produced by the model on larger scales with those at the convergence scales it's possible to size up the uncertainty introduced by the modelling itself on the predicted ground effects in such simplified scenario.

Polar wander of the Earth associated with the Quaternary glacial cycle on a convecting mantle

SUMMARY Observed true polar wander (TPW) at the present-day, with the speed of ∼1° Myr−1 towards Hudson Bay, has been generally examined based on the glacial isostatic adjustment (GIA) for the Quaternary glacial cycle. The observation is, however, affected by the present-day melting events of polar ice sheets and mountain glaciers and convective processes in the mantle. In this study, I examine TPW due to GIA and convective processes by developing the Liouville equation including these two processes, referred to as generalized Liouville equation here. The generalized Liouville equation makes it possible to evaluate the effects of convective related non-forcing inertia elements (I11, I22, I33 and I12) and forcing elements (I13 and I23) on the polar wander for the Quaternary ice age phase, and clearly indicates that the excess flattening of the Earth, inferred from the non-hydrostatic geoid, stabilizes the polar motion as discussed by Mitrovica et al. (2005). Also, numerical experiments for examining the effects of time-dependent convective processes on TPW indicate that reasonable convection scenarios with a time dependence in the inertia tensor of ∼1031 kg m2 Myr−1 (Ricard et al. 1993) insignificantly contribute to the observed present-day TPW; moreover, the predicted TPW with magnitude of convective related forcing rates, dI13/dt and dI23/dt, larger than ∼5 × 1031 kg m2 Myr−1 is significantly different from the observation. These results may provide a framework for separating the contributions of GIA and convection to the observed present-day polar wander.

An observing system simulation experiment for the study on attenuation of C‐band radar measurements

AbstractA new radar simulation model based on a three‐dimensional polarimetric radar simulator model (RSM‐POL) coupled with a very high resolution cloud resolving model is presented. RSM‐POL is able to model reflectivity measurements by using some numerically‐generated meteorological fields as input: temperature, pressure, water vapour content, cloud water content, cloud ice content and rain, snow and graupel sedimentation fluxes and the number density intercept parameter No for each precipitating species. In this study, the radar simulation model is applied to assess the influence of attenuation on radar measurements at C‐band and its effect (1) in the computation of rainfall rate, (2) vertical profile of reflectivity and (3) in the retrieval of microphysical parameters. At the present time, the importance of radar data assimilation in numerical weather prediction models is continuously growing, the latter two points appear to be a fundamental research issue and a rigorous approach able to separate the contribution of different causes is needed as a starting point for developing correction algorithms. Results obtained on an idealized deep convective meteorological scenario, run at very high horizontal resolution (500 m), are presented. Copyright © 2008 Royal Meteorological Society

The Effect of Upstream Convection on Downstream Precipitation

Abstract Operational forecasters in the southeast and mid-Atlantic regions of the United States have noted a positive quantitative precipitation forecast (QPF) bias in numerical weather prediction (NWP) model forecasts downstream of some organized, cold-season convective systems. Examination of cold-season cases in which model QPF guidance exhibited large errors allowed identification of two representative cases for detailed analysis. The goals of the case study analyses are to (i) identify physical mechanisms through which the upstream convection (UC) alters downstream precipitation amounts, (ii) determine why operational models are challenged to provide accurate guidance in these situations, and (iii) suggest future research directions that would improve model forecasts in these situations and allow forecasters to better anticipate such events. Two primary scenarios are identified during which downstream precipitation is altered in the presence of UC for the study region: (i) a fast-moving convective (FC) scenario in which organized convective systems oriented parallel to the lower-tropospheric flow are progressive relative to the parent synoptic system, and appear to disrupt poleward moisture transport, and (ii) a situation characterized by slower-moving convection (SC) relative to the parent system. Analysis of a representative FC case indicated that moisture consumption, stabilization via convective overturning, and modification of the low-level flow to a more westerly direction in the postconvective environment all appear to contribute to the reduction of downstream precipitation. In the FC case, operational Eta Model forecasts moved the organized UC too slowly, resulting in an overestimate of downstream moisture transport. A 4-km explicit convection model forecast from the Weather Research and Forecasting model produced a faster-moving upstream convective system and improved downstream QPF. In contrast to the FC event, latent heat release in the primary rainband is shown to enhance the low-level jet ahead of the convection in the SC case, thereby increasing moisture transport into the downstream region. A negative model QPF bias was observed in Eta Model forecasts for the SC event. Suggestions are made for precipitation forecasting in UC situations, and implications for NWP model configuration are discussed.

End‐of‐storm oscillation in tropical air mass thunderstorms

Characteristics of the end‐of‐storm oscillation (EOSO) in the atmospheric surface electric field observed beneath isolated air mass thunderstorms at a tropical station, Pune, are investigated. Typically, the value of the electric field at the ground during the EOSO periods has been observed to range between −3 and +4 kV m−1. Durations of the EOSO at Pune are 24% of those for the large quasi‐stationary thunderstorms occurring at Florida and 55% of the convective air mass thunderstorms at New Mexico. During the EOSO periods, frequency of lightning decreases very much but some flashes exhibit unique features of charge transportation associated with them. Observation of a new phenomenon, the inverted EOSO, is reported in a storm with inverted polarity of electrical dipole. Our observations are found to be consistent with the convective mechanism scenario of the EOSO in which the surface field meters are exposed to the upper positive charge of the cloud by the downdrafts.

Linear and nonlinear convection in solidifying ternary alloys

In this paper we consider buoyancy-driven flow and directional solidification of a ternary alloy in two dimensions. A steady flow can be established by forcing liquid downward at an average rate V through a temperature gradient that is fixed in the laboratory frame of reference and spans both the eutectic and liquidus temperature of the material being solidified. Our results include both a linear stability analysis and numerical solution of the governing equations for finite-amplitude steady states. The ternary system is characterized by two distinct mushy zones – a primary layer with dendrites composed of a single species and, beneath the primary layer, a secondary layer with a dendritic region composed of two species. The two layers have independent effective Rayleigh numbers, which allows for a variety of convection scenarios.

Impact of Cloud Cover on Solar Radiative Biases in Deep Convective Regimes

Abstract Conflicting claims have been made concerning the magnitude of the bias in solar radiative transfer calculations when horizontal photon transport is neglected for deep convective scenarios. The difficulty of obtaining a realistic set of cloud scenes for situations of complex cloud geometry, while certain characteristics such as total cloud cover are systematically controlled, has hindered the attempt to reach a consensus. Here, a simple alternative approach is adopted. An ensemble of cloud scenes generated by a cloud resolving model are modified by an idealized function that progressively alters the cirrus anvil coverage without affecting the realism of the scene produced. Comparing three-dimensional radiative calculations with the independent column approximation for all cloud scenes, it is found that the bias in scene albedo can reach as much as 22% when the sun is overhead and 46% at low sun angles. The bias is an asymmetrical function of cloud cover with a maximum attained at cirrus anvil cloud cover of approximately 30%–40%. With a cloud cover of 15%, the bias is half its maximum value, while it is limited for coverage exceeding 80%. The position of the peak occurs at the cloud cover coinciding with the maximum number of independent clouds present in the scene. Increasing the cloud cover past this point produces a decrease in the number of isolated clouds because of cloud merging, with a consequential bias reduction. With this systematic documentation of the biases as a function of total cloud cover, it is possible to identify two contributions to the total error: the geometrical consequences of the effective cloud cover increase at low sun angles and the true 3D scattering effect of photons deviating from the original path direction. An attempt to account for the former geometrical contribution to the 1D bias is made by performing a simple correction technique, whereby the field is sheared by the tangent of the solar zenith angle. It is found that this greatly reduces the 1D biases at low sun angles. Because of the small aspect ratio of the cirrus cloud deck, the remaining bias contribution is small in magnitude and almost independent of solar zenith angle.

COOLING EFFECTS ON PROCESSED CHEESE FUNCTIONALITY

ABSTRACT Textural and functional properties of processed cheese are affected by a final production step – cooling. Rheological data demonstrate a firmer cheese at slower cooling rates. To simulate industrial production, five‐pound cheese loaves were cooled in an environment at 5C under free and forced convection. Slice‐ability was estimated by cutting loaves at different locations using a wire‐cutting device, and melt‐ability was determined by the Schreiber method. Cooling rates, estimated from a heat transfer model, did not show a large difference within the five‐pound loaf, and no obvious trends in slice‐ability and melt‐ability were observed. Comparing forced with free convection, a smaller force was required to slice the cheese, and a higher melt score was experienced for the forced convection scenario. Cheese manufacturers can benefit from this research by manipulating cooling schedules to achieve desired textural attributes of processed cheese.

The Nowcasting of Precipitation during Sydney 2000: An Appraisal of the QPF Algorithms

Statistical and case study–oriented comparisons of the quantitative precipitation nowcasting (QPN) schemes demonstrated during the first World Weather Research Programme (WWRP) Forecast Demonstration Project (FDP), held in Sydney, Australia, during 2000, served to confirm many of the earlier reported findings regarding QPN algorithm design and performance. With a few notable exceptions, nowcasting algorithms based upon the linear extrapolation of observed precipitation motion (Lagrangian persistence) were generally superior to more sophisticated, nonlinear nowcasting methods. Centroid trackers [Thunderstorm Identification, Tracking, Analysis and Nowcasting System (TITAN)] and pattern matching extrapolators using multiple vectors (Auto-nowcaster and Nimrod) were most reliable in convective scenarios. During widespread, stratiform rain events, the pattern matching extrapolators were superior to centroid trackers and wind advection techniques (Gandolf, Nimrod). There is some limited case study and statistical evidence from the FDP to support the use of more sophisticated, nonlinear QPN algorithms. In a companion paper in this issue, Wilson et al. demonstrate the advantages of combining linear extrapolation with algorithms designed to predict convective initiation, growth, and decay in the Auto-nowcaster. Ebert et al. show that the application of a nonlinear scheme [Spectral Prognosis (S-PROG)] designed to smooth precipitation features at a rate consistent with their observed temporal persistence tends to produce a nowcast that is superior to Lagrangian persistence in terms of rms error. However, the value of this approach in severe weather forecasting is called into question due to the rapid smoothing of high-intensity precipitation features.

Ionospheric signatures of split reconnection X‐lines during conditions of IMF Bz &lt; 0 and |By| ∼ |Bz|: Evidence for the antiparallel merging hypothesis

We present measurements of the convection electric field in the dayside ionosphere during extended intervals when the interplanetary magnetic field (IMF) has a negative Bz component and a significant By component (|By| ∼ |Bz|). Under these conditions, distinct differences in the ionospheric projection of the magnetopause reconnection X‐line are predicted by the antiparallel and subsolar merging hypotheses. Close to solstice, in the winter hemisphere the antiparallel merging hypothesis predicts two ionospheric merging lines which are significantly separated either side of noon. This prediction is not expected from any subsolar merging hypothesis (e.g., component merging). Using the method of global convection mapping with line‐of‐sight velocity data from the SuperDARN HF radar network, we present evidence of two distinct merging regions on the dayside magnetopause under these IMF and seasonal conditions. The two merging regions manifest themselves in the ionosphere as two regions of convection flow with a strong poleward component separated by a region of convection flow with either a weak poleward or equatorward component corresponding to an adiaroic portion of the polar cap boundary. The spatiotemporal nature of the actual convection scenario can be complicated, but the predicted pattern represents a recurring feature of the convection electric field under these IMF and seasonal conditions. These observations lend support to the hypothesis that antiparallel merging is the dominant form of magnetopause reconnection during these steady state IMF intervals. This suggests that the magnetic shear between magnetosheath and magnetospheric magnetic field lines plays the major role in determining the location of reconnection on the dayside magnetopause.