scholarly journals A Convection Parameterization for Low-CAPE Environments

2020 ◽  
Vol 148 (12) ◽  
pp. 4917-4941
Author(s):  
Ron McTaggart-Cowan ◽  
Paul A. Vaillancourt ◽  
Leo Separovic ◽  
Shawn Corvec ◽  
Ayrton Zadra
Keyword(s):  
Numerical Models ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Heavy Precipitation ◽  
Squall Line ◽  
Cumulus Convection ◽  
Moist Convection ◽  
Flux Parameterization ◽  
Convection Schemes ◽  
Convection Parameterization

AbstractNumerical models that are unable to resolve moist convection in the atmosphere employ physical parameterizations to represent the effects of the associated processes on the resolved-scale state. Most of these schemes are designed to represent the dominant class of cumulus convection that is driven by latent heat release in a conditionally unstable profile with a surplus of convective available potential energy (CAPE). However, an important subset of events occurs in low-CAPE environments in which potential and symmetric instabilities can sustain moist convective motions. Convection schemes that are dependent on the presence of CAPE are unable to depict accurately the effects of cumulus convection in these cases. A mass-flux parameterization is developed to represent such events, with triggering and closure components that are specifically designed to depict subgrid-scale convection in low-CAPE profiles. Case studies show that the scheme eliminates the “bull’s-eyes” in precipitation guidance that develop in the absence of parameterized convection, and that it can represent the initiation of elevated convection that organizes squall-line structure. The introduction of the parameterization leads to significant improvements in the quality of quantitative precipitation forecasts, including a large reduction in the frequency of spurious heavy-precipitation events predicted by the model. An evaluation of surface and upper-air guidance shows that the scheme systematically improves the model solution in the warm season, a result that suggests that the parameterization is capable of accurately representing the effects of moist convection in a range of low-CAPE environments.

scholarly journals Evaluation of the atmospheric transport in a GCM using radon measurements: sensitivity to cumulus convection parameterization

2008 ◽  
Vol 8 (10) ◽  
pp. 2811-2832 ◽  
Author(s):  
K. Zhang ◽  
H. Wan ◽  
M. Zhang ◽  
B. Wang
Keyword(s):  
Vertical Distribution ◽  
Radon Concentration ◽  
Large Scale ◽  
Atmospheric Transport ◽  
Surface Concentration ◽  
Transport Processes ◽  
Cumulus Convection ◽  
Models Comparison ◽  
Convection Schemes ◽  
Convection Parameterization

Abstract. The radioactive species radon (222Rn) has long been used as a test tracer for the numerical simulation of large scale transport processes. In this study, radon transport experiments are carried out using an atmospheric GCM with a finite-difference dynamical core, the van Leer type FFSL advection algorithm, and two state-of-the-art cumulus convection parameterization schemes. Measurements of surface concentration and vertical distribution of radon collected from the literature are used as references in model evaluation. The simulated radon concentrations using both convection schemes turn out to be consistent with earlier studies with many other models. Comparison with measurements indicates that at the locations where significant seasonal variations are observed in reality, the model can reproduce both the monthly mean surface radon concentration and the annual cycle quite well. At those sites where the seasonal variation is not large, the model is able to give a correct magnitude of the annual mean. In East Asia, where radon simulations are rarely reported in the literature, detailed analysis shows that our results compare reasonably well with the observations. The most evident changes caused by the use of a different convection scheme are found in the vertical distribution of the tracer. The scheme associated with weaker upward transport gives higher radon concentration up to about 6 km above the surface, and lower values in higher altitudes. In the lower part of the atmosphere results from this scheme does not agree as well with the measurements as the other scheme. Differences from 6 km to the model top are even larger, although we are not yet able to tell which simulation is better due to the lack of observations at such high altitudes.

Evaluating the conservation of energy variables in simulations of deep moist convection

2021 ◽  
Author(s):  
John M. Peters ◽  
Daniel R. Chavas
Keyword(s):  
Heat Capacity ◽  
Numerical Models ◽  
Deep Convection ◽  
Squall Line ◽  
Moist Convection ◽  
Conservation Of Energy ◽  
Deep Moist Convection ◽  
Static Energy ◽  
Hydrostatic Balance ◽  
Latent Heats

AbstractIt is often assumed in parcel theory calculations, numerical models, and cumulus parameterizations that moist static energy (MSE) is adiabatically conserved. However, the adiabatic conservation of MSE is only approximate because of the assumption of hydrostatic balance. Two alternative variables are evaluated here: MSE −IB and MSE +KE, wherein IB is the path integral of buoyancy (B) and KE is kinetic energy. Both of these variables relax the hydrostatic assumption and are more precisely conserved than MSE. This article quantifies the errors that result from assuming that the aforementioned variables are conserved in large eddy simulations (LES) of both disorganized and organized deep convection. Results show that both MSE −IB and MSE +KE better predict quantities along trajectories than MSE alone. MSE −IB is better conserved in isolated deep convection, whereas MSE −IB and MSE +KE perform comparably in squall line simulations. These results are explained by differences between the pressure perturbation behavior of squall lines and isolated convection. Errors in updraft B diagnoses are universally minimized when MSE−IB is assumed to be adiabatically conserved, but only when moisture dependencies of heat capacity and temperature dependency of latent heating are accounted for. When less accurate latent heat and heat capacity formulae were used, MSE−IB yielded poorer B predictions than MSE due to compensating errors. Our results suggest that various applications would benefit from using either MSE −IB or MSE +KE instead of MSE with properly formulated heat capacities and latent heats.

scholarly journals Evaluation of the atmospheric transport in a GCM using radon measurements: sensitivity to cumulus convection parameterization

2008 ◽  
Vol 8 (1) ◽  
pp. 2085-2127
Author(s):  
K. Zhang ◽  
H. Wan ◽  
M. Zhang ◽  
B. Wang
Keyword(s):  
Vertical Distribution ◽  
Radon Concentration ◽  
Large Scale ◽  
Atmospheric Transport ◽  
Surface Concentration ◽  
Transport Processes ◽  
Cumulus Convection ◽  
Models Comparison ◽  
Convection Schemes ◽  
Convection Parameterization

Abstract. The radioactive species radon (222Rn) has long been used as a test tracer for the numerical simulation of large scale transport processes. In this study, radon transport experiments are carried out using an atmospheric GCM with a finite-difference dynamical core, the van Leer type FFSL advection algorithm and two state-of-the-art cumulus convection parameterization schemes. Measurements of surface concentration and vertical distribution of radon collected from literature are used as references in model evaluation. The simulated radon concentrations using both convection schemes turn out to be consistent with earlier studies with many other models. Comparison with measurements indicates that at the locations where significant seasonal variations are observed in reality, the model can reproduce both the monthly mean surface radon concentration and the annual cycle quite well. At those sites where the seasonal variation is not large, the model is able to give a correct magnitude of the annual mean. In East Asia, where radon simulations are rarely reported in literature, detailed analysis shows that our results compare reasonably well with the observations. The most evident changes caused by the use of a different convection scheme are found in the vertical distribution of the tracer. The scheme associated with a weaker upward transport gives higher radon concentration up to about 6 km above the surface, and lower values in higher altitudes. In the lower part of the atmosphere results from this scheme does not agree as well with the measurements as the other scheme. Differences from 6 km to the model top are even larger, although we are not yet able to tell which simulation is better due to the lack of observations at such high altitudes.

The Use of Moisture Flux Convergence in Forecasting Convective Initiation: Historical and Operational Perspectives

2005 ◽  
Vol 20 (3) ◽  
pp. 351-366 ◽  
Author(s):  
Peter C. Banacos ◽  
David M. Schultz
Keyword(s):  
Numerical Models ◽  
Convective Available Potential Energy ◽  
Moisture Flux ◽  
Cumulus Parameterization ◽  
Moist Convection ◽  
Convective Initiation ◽  
Moisture Flux Convergence ◽  
Case Examples ◽  
Surface Data ◽  
Deep Moist Convection

Abstract Moisture flux convergence (MFC) is a term in the conservation of water vapor equation and was first calculated in the 1950s and 1960s as a vertically integrated quantity to predict rainfall associated with synoptic-scale systems. Vertically integrated MFC was also incorporated into the Kuo cumulus parameterization scheme for the Tropics. MFC was eventually suggested for use in forecasting convective initiation in the midlatitudes in 1970, but practical MFC usage quickly evolved to include only surface data, owing to the higher spatial and temporal resolution of surface observations. Since then, surface MFC has been widely applied as a short-term (0–3 h) prognostic quantity for forecasting convective initiation, with an emphasis on determining the favorable spatial location(s) for such development. A scale analysis shows that surface MFC is directly proportional to the horizontal mass convergence field, allowing MFC to be highly effective in highlighting mesoscale boundaries between different air masses near the earth’s surface that can be resolved by surface data and appropriate grid spacing in gridded analyses and numerical models. However, the effectiveness of boundaries in generating deep moist convection is influenced by many factors, including the depth of the vertical circulation along the boundary and the presence of convective available potential energy (CAPE) and convective inhibition (CIN) near the boundary. Moreover, lower- and upper-tropospheric jets, frontogenesis, and other forcing mechanisms may produce horizontal mass convergence above the surface, providing the necessary lift to bring elevated parcels to their level of free convection without connection to the boundary layer. Case examples elucidate these points as a context for applying horizontal mass convergence for convective initiation. Because horizontal mass convergence is a more appropriate diagnostic in an ingredients-based methodology for forecasting convective initiation, its use is recommended over MFC.

scholarly journals Microphysical Characteristics of Three Convective Events with Intense Rainfall Observed by Polarimetric Radar and Disdrometer in Eastern China

2019 ◽  
Vol 11 (17) ◽  
pp. 2004 ◽  
Author(s):  
Gang Chen ◽  
Kun Zhao ◽  
Long Wen ◽  
Mengyao Wang ◽  
Hao Huang ◽  
...  
Keyword(s):  
Numerical Models ◽  
Eastern China ◽  
High Surface ◽  
Three Dimensional ◽  
Heavy Precipitation ◽  
Squall Line ◽  
Polarimetric Radar ◽  
Intense Rainfall ◽  
Rainfall Events ◽  
Freezing Level

Polarimetric radar and disdrometer observations obtained during the 2014 Observation, Prediction, and Analysis of Severe Convection of China (OPACC) field campaign are used in this study to investigate the microphysical characteristics of three primary types of organized intense rainfall events (meiyu rainband, typhoon outer rainband, and squall line) in eastern China. Drop size distributions (DSDs) of these three events on the ground are derived from measurements of a surface disdrometer, while the corresponding three-dimensional microphysical structures are obtained from the Nanjing University C-band polarimetric radar (NJU-CPOL). Although the environmental moisture and instability conditions are different, all three events possess relatively high freezing level favorable for warm-rain processes where the high medium to small raindrop concentration at low levels is consistent with the high surface rainfall rates. Convection is tallest in the squall line where abundant ice-phase processes generate large amounts of rimed particles (graupel and hail) above the freezing level and the largest surface raindrops are present among these three events. The storm tops of both the typhoon and meiyu rainbands are lower than that in the squall line, composed of less active ice processes above the freezing level. The typhoon rainrate is more intense than that of meiyu, enhanced by higher coalescence efficiency. A revised generalized intercept parameter versus mass-weighted mean diameter (Nw-Dm) space diagram is constructed to describe the DSD distributions over the three events and illustrate the relative DSD positions for heavy precipitation. DSDs of these intense rainfall convections observed in this midlatitude region of eastern Asia somewhat represent the typical DSD characteristics in low latitudes, suggesting that the parameterization of microphysical characteristics in eastern China in numerical models needs to be further investigated to improve rain fall forecasts in these heavy rainfall events.

Improving heavy precipitation forecasting over the western Mediterranean: Benefits of stochastic techniques for model error sampling

2021 ◽  
Author(s):  
Alejandro Hermoso ◽  
Victor Homar ◽  
Robert Plant
Keyword(s):  
Flash Flood ◽  
Numerical Models ◽  
Cloud Condensation Nuclei ◽  
Heavy Precipitation ◽  
Western Mediterranean ◽  
Cloud Formation ◽  
Stochastic Methods ◽  
Moist Convection ◽  
State Variables ◽  
Stochastic Perturbations

<p>The western Mediterranean region is frequently disrupted by heavy precipitation and flash flood episodes. Designing convection-permitting ensembles capable of accurately forecasting socially relevant aspects of these natural hazards such as timing, location, and intensity at basin scales of the order of a few hundred of squared kilometers is an extremely challenging effort. The usual forecast underdispersion prevailing at these scales motivates the research of sampling methodologies which are able to provide an adequate representation of the uncertainties in the initial atmospheric state and its time-integration by means of numerical models. This work investigates the skill of multiple techniques to sample model uncertainty in the context of heavy precipitation in the Mediterranean. The performance of multiple stochastic schemes is analyzed for a singular event occurred on 12 and 13 September 2019 in València, Murcia, and Almería (eastern Spain). This remarkable and enlightening episode caused seven casualties, the flooding of hundreds of homes and economic exceeding 425 million EUR.</p><p>Stochastic methods are compared to the popular multiphysics strategy in terms of both diversity and skill. The considered techniques include stochastic parameterization perturbation tendencies of state variables and perturbations to specific and influential parameters within the microphysics scheme (cloud condensation nuclei, fall speed factors, saturation percentage for cloud formation). The introduction of stochastic perturbations to the microphysics parameters results in an increased ensemble spread throughout the entire simulation. A conclusion of special relevance for the western Mediterranean, where local topography and deep moist convection play an essential role, is that stochastic methods significantly outperform the multiphysics-based ensemble, indicating a clear potential of stochastic parameterizations for the short-range forecast of high-impact events in the region.</p>

scholarly journals Evaluation of the WRF Double-Moment 6-Class Microphysics Scheme for Precipitating Convection

2010 ◽  
Vol 2010 ◽  
pp. 1-10 ◽  
Author(s):  
Song-You Hong ◽  
Kyo-Sun Sunny Lim ◽  
Yong-Hee Lee ◽  
Jong-Chul Ha ◽  
Hyung-Woo Kim ◽  
...  
Keyword(s):  
Great Plains ◽  
Leading Edge ◽  
Heavy Precipitation ◽  
Summer Monsoon Rainfall ◽  
Squall Line ◽  
Convective System ◽  
Moist Convection ◽  
Microphysics Scheme ◽  
Evolutionary Features ◽  
Double Moment

This study demonstrates the characteristics of the Weather Research and Forecasting (WRF) Double-Moment 6-Class (WDM6) Microphysics scheme for representing precipitating moist convection in 3D platforms, relative to the WSM6 scheme that has been widely used in the WRF community. For a case study of convective system over the Great Plains, the WDM6 scheme improves the evolutionary features such as the bow-type echo in the leading edge of the squall line. We also found that the WRF with WDM6 scheme removes spurious oceanic rainfall that is a systematic defect resulting from the use of the WSM6 scheme alone. The simulated summer monsoon rainfall in East Asia is improved by weakening (strengthening) light (heavy) precipitation activity. These changes can be explained by the fact that the WDM6 scheme has a wider range in cloud and rain number concentrations than does the WSM6 scheme.

scholarly journals Evaluating the Performance of Cumulus Convection Parameterization Schemes in Regional Climate Modeling System

2021 ◽  
Author(s):  
Sridhara Nayak ◽  
Suman Maity
Keyword(s):  
Climate Modeling ◽  
Regional Climate ◽  
Regional Climate Modeling ◽  
Horizontal Resolution ◽  
Cumulus Convection ◽  
Convection Scheme ◽  
Parameterization Schemes ◽  
Global Precipitation Climatology Centre ◽  
Convection Schemes ◽  
Convection Parameterization

In this study, we explored the performance of the cumulus convection parameterization schemes of Regional Climate Modeling System (RegCM) towards the Indian summer monsoon (ISM) of a catastrophic year through various numerical experiments conducted with different convection schemes (Kuo, Grell amd MIT) in RegCM. The model is integrated at 60KM horizontal resolution over Indian region and forced with NCEP/NCAR reanalysis. The simulated temperature at 2m and the wind at 10m are validated against the forced data and the total precipitation is compared with the Global Precipitation Climatology Centre (GPCC) observations. We find that the simulation with MIT convection scheme is close to the GPCC data and NCEP/NCAR reanalysis. Our results with three convection schemes suggest that the RegCM with MIT convection scheme successfully simulated some characteristics of ISM of a catastrophic year and may be further examined with more number of convection schemes to customize which convection scheme is much better.

scholarly journals Mechanisms Governing the Persistence and Diurnal Cycle of a Heavy Rainfall Corridor

2014 ◽  
Vol 71 (11) ◽  
pp. 4102-4126 ◽  
Author(s):  
Stanley B. Trier ◽  
Christopher A. Davis ◽  
Richard E. Carbone
Keyword(s):  
Mississippi River ◽  
Large Scale ◽  
Convective Available Potential Energy ◽  
Sensible Heat Flux ◽  
Spatial Coherence ◽  
Warm Season ◽  
Heavy Precipitation ◽  
Surface Front ◽  
Vertical Displacements ◽  
Upper Level

Abstract Observations and convection-permitting simulations are used to study a 12-day warm-season heavy precipitation corridor over the central U.S. plains and Mississippi River valley regions. Such precipitation corridors, defined by narrow latitudinal widths (~3°–4°) and only modest north–south drifts of their centroids (<2° day−1), often yield extreme total precipitation (100–250 mm), resulting in both short-term and seasonal impacts on the regional hydrologic cycle. The corridor precipitation is predominately nocturnal and located several hundred kilometers north of a quasi-stationary surface front. There, hot, dry air from the daytime boundary layer located underneath a persistent upper-level anticyclone requires large vertical displacements along the axis of the southerly low-level jet (LLJ) above the front to eliminate convection inhibition (CIN). Composites reveal ~500 J kg−1 of average convective available potential energy (CAPE) when this air reaches the southern edge of the precipitation corridor. Despite the relatively modest CAPE, convection is favored by the large reductions in CIN along the vertical displacements and by high ambient midtropospheric relative humidity located above, which is influenced by persistent nightly convection in the region. Though internal feedbacks resulting from the large nightly spatial coherence of convection (including enhanced midtropospheric relative humidities and frontogenetic daytime sensible heat flux gradients owing to residual cloudiness) are favorable for maintaining the corridor, its persistence is most sensitive to large-scale external factors. Here, changes to the intensity and position of the large-synoptic upper-tropospheric anticyclone are associated with changes in the frequency of strong LLJs and the surface frontal position, dramatically affecting the intensity and stationarity of the precipitation corridor.

NAM Model Forecasts of Warm-Season Quasi-Stationary Frontal Environments in the Central United States

2010 ◽  
Vol 25 (4) ◽  
pp. 1281-1292 ◽  
Author(s):  
Shih-Yu Wang ◽  
Adam J. Clark
Keyword(s):  
United States ◽  
Mesoscale Model ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Correlation Coefficients ◽  
Precipitable Water ◽  
Wind Fields ◽  
Vapor Flux ◽  
Convective Systems ◽  
Central United States

Abstract Using a composite procedure, North American Mesoscale Model (NAM) forecast and observed environments associated with zonally oriented, quasi-stationary surface fronts for 64 cases during July–August 2006–08 were examined for a large region encompassing the central United States. NAM adequately simulated the general synoptic features associated with the frontal environments (e.g., patterns in the low-level wind fields) as well as the positions of the fronts. However, kinematic fields important to frontogenesis such as horizontal deformation and convergence were overpredicted. Surface-based convective available potential energy (CAPE) and precipitable water were also overpredicted, which was likely related to the overprediction of the kinematic fields through convergence of water vapor flux. In addition, a spurious coherence between forecast deformation and precipitation was found using spatial correlation coefficients. Composite precipitation forecasts featured a broad area of rainfall stretched parallel to the composite front, whereas the composite observed precipitation covered a smaller area and had a WNW–ESE orientation relative to the front, consistent with mesoscale convective systems (MCSs) propagating at a slight right angle relative to the thermal gradient. Thus, deficiencies in the NAM precipitation forecasts may at least partially result from the inability to depict MCSs properly. It was observed that errors in the precipitation forecasts appeared to lag those of the kinematic fields, and so it seems likely that deficiencies in the precipitation forecasts are related to the overprediction of the kinematic fields such as deformation. However, no attempts were made to establish whether the overpredicted kinematic fields actually contributed to the errors in the precipitation forecasts or whether the overpredicted kinematic fields were simply an artifact of the precipitation errors. Regardless of the relationship between such errors, recognition of typical warm-season environments associated with these errors should be useful to operational forecasters.

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