Convective Regions Research Articles

Spatial Radiative Feedbacks from Internal Variability Using Multiple Regression

AbstractThe sensitivity of the climate to CO2 forcing depends on spatially varying radiative feedbacks that act both locally and nonlocally. We assess whether a method employing multiple regression can be used to estimate local and nonlocal radiative feedbacks from internal variability. We test this method on millennial-length simulations performed with six coupled atmosphere–ocean general circulation models (AOGCMs). Given the spatial pattern of warming, the method does quite well at recreating the top-of-atmosphere flux response for most regions of Earth, except over the Southern Ocean where it consistently overestimates the change, leading to an overestimate of the sensitivity. For five of the six models, the method finds that local feedbacks are positive due to cloud processes, balanced by negative nonlocal shortwave cloud feedbacks associated with regions of tropical convection. For four of these models, the magnitudes of both are comparable to the Planck feedback, so that changes in the ratio between them could lead to large changes in climate sensitivity. The positive local feedback explains why observational studies that estimate spatial feedbacks using only local regressions predict an unstable climate. The method implies that sensitivity in these AOGCMs increases over time due to a reduction in the share of warming occurring in tropical convecting regions and the resulting weakening of associated shortwave cloud and longwave clear-sky feedbacks. Our results provide a step toward an observational estimate of time-varying climate sensitivity by demonstrating that many aspects of spatial feedbacks appear to be the same between internal variability and the forced response.

A Study on Assimilation of CYGNSS Wind Speed Data for Tropical Convection during 2018 January MJO

The National Aeronautics and Space Administration (NASA) Cyclone Global Navigation Satellite System (CYGNSS) mission was launched in December 2016. CYGNSS provides ocean surface wind speed retrieval along specular reflection tracks at an interval resolution of approximately 25 km. With a median revisit time of 2.8 h covering a ±35° latitude, CYGNSS can provide more frequent and accurate measurements of surface wind over the tropical oceans under heavy precipitation, especially within tropical cyclone cores and deep convection regions, than traditional scatterometers. In this study, CYGNSS v2.1 Level 2 wind speed data were assimilated into the Weather Research and Forecasting (WRF) model using the WRF Data Assimilation (WRFDA) system with hybrid 3- and 4-dimensional variational ensemble technology. Case studies were conducted to examine the impact of the CYGNSS data on forecasts of tropical cyclone (TC) Irving and a westerly wind burst (WWB) during the Madden–Julian oscillation (MJO) event over the Indian Ocean in early January 2018. The results indicate a positive impact of the CYGNSS data on the wind field. However, the impact from the CYGNSS data decreases rapidly within 4 h after data assimilation. Also, the influence of CYGNSS data only on precipitation forecast is found to be limited. The assimilation of CYGNSS data was further explored with an additional experiment in which CYGNSS data was combined with Global Precipitation Mission (GPM) Integrated Multi-satellitE Retrievals for GPM (IMERG) hourly precipitation and Advanced Scatterometer (ASCAT) wind vector and were assimilated into the WRF model. A significant positive impact was found on the tropical cyclone intensity and track forecasts. The short-term forecast of wind and precipitation fields were also improved for both TC Irving and the WWB event when the combined satellite data was assimilated.

Thunderstorm Cloud-Type Classification from Space-Based Lightning Imagers.

The organization and structure of thunderstorms determines the extent and severity of their hazards to the general public and their consequences for the Earth system. Distinguishing vigorous convective regions that produce heavy rain and hail from adjacent regions of stratiform or overhanging anvil clouds that produce light-to-no rainfall is valuable in operations and physical research. Cloud type algorithms that partition convection from stratiform regions have been developed for space-based radar, passive microwave, and now Geostationary Operational Environmental Satellites (GOES) Advanced Baseline Imager (ABI) multi-spectral products. However, there are limitations for each of these products including temporal availability, spatial coverage, and the degree to which they based on cloud microphysics. We have developed a cloud type algorithm for GOES Geostationary Lightning Mapper (GLM) observations that identifies convective / non-convective regions in thunderstorms based on signatures of interactions with non-convective charge structures in the lightning flash data. The GLM sensor permits a rapid (20-s) update cycle over the combined GOES-16 / GOES-17 domain across all hours of the day. Storm regions that do not produce lightning will not be classified by our algorithm, however. The GLM cloud type product is intended to provide situational awareness of electrified anvils and to complement other cloud type retrievals by providing a contemporary assessment tied to lightning physics. We propose that a future combined ABI / GLM cloud type algorithm would be a valuable product that could draw from the strengths of each instrument and approach.

North Atlantic subsurface temperature response controlled by effective freshwater input in “Heinrich” events

The northern North Atlantic (NNA) subsurface temperature in response to the slowdown of the Atlantic meridional overturning circulation (AMOC) is crucial for ice sheet calving and recovery of the AMOC in Heinrich events. Paleoclimate proxies and modeling studies suggest that the NNA subsurface exhibits a robust warming during Heinrich 1, but with a less clear response during Younger Dryas (YD). The mechanism for the potentially different subsurface responses has remained not well understood. Previous studies show different NNA subsurface response depending on hosing at different locations. Here, by examining a suite of “water-hosing” experiments with different hosing regions and intensities, we show that, regardless of the hosing location, NNA subsurface temperature response is determined by the effective freshening over the NNA deep convective regions through the competition between the warming associated with the suppressed vertical mixing and the cooling associated with the weakened AMOC heat transport. A weak effective freshening favors advective effect and, in turn, cooling, while a strong effective freshening strengthens the mixing effect and leads to a warming. Our results suggest that a cooling may occur in the NNA subsurface during YD.

Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics

Abstract. Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones. They can influence large-scale flow evolution by modifying the potential vorticity (PV) distribution during their cross-isentropic ascent. Although WCBs are typically described as slantwise-ascending and stratiform-cloud-producing airstreams, recent studies identified convective activity embedded within the large-scale WCB cloud band. However, the impacts of this WCB-embedded convection have not been investigated in detail. In this study, we systematically analyze the influence of embedded convection in an eastern North Atlantic WCB on the cloud and precipitation structure, on the PV distribution, and on larger-scale flow. For this reason, we apply online trajectories in a high-resolution convection-permitting simulation and perform a composite analysis to compare quasi-vertically ascending convective WCB trajectories with typical slantwise-ascending WCB trajectories. We find that the convective WCB ascent leads to substantially stronger surface precipitation and the formation of graupel in the middle to upper troposphere, which is absent for the slantwise WCB category, indicating the key role of WCB-embedded convection for precipitation extremes. Compared to the slantwise WCB trajectories, the initial equivalent potential temperature of the convective WCB trajectories is higher, and the convective WCB trajectories originate from a region of larger potential instability, which gives rise to more intense cloud diabatic heating and stronger cross-isentropic ascent. Moreover, the signature of embedded convection is distinctly imprinted in the PV structure. The diabatically generated low-level positive PV anomalies, associated with a cyclonic circulation anomaly, are substantially stronger for the convective WCB trajectories. The slantwise WCB trajectories lead to the formation of a widespread region of low-PV air (that still have weakly positive PV values) in the upper troposphere, in agreement with previous studies. In contrast, the convective WCB trajectories form mesoscale horizontal PV dipoles at upper levels, with one pole reaching negative PV values. On a larger scale, these individual mesoscale PV anomalies can aggregate to elongated PV dipole bands extending from the convective updraft region, which are associated with coherent larger-scale circulation anomalies. An illustrative example of such a convectively generated PV dipole band shows that within around 10 h the negative PV pole is advected closer to the upper-level waveguide, where it strengthens the isentropic PV gradient and contributes to the formation of a jet streak. This suggests that the mesoscale PV anomalies produced by embedded convection upstream organize and persist for several hours and therefore can influence the synoptic-scale circulation. They thus can be dynamically relevant, influence the jet stream and (potentially) the downstream flow evolution, which are highly relevant aspects for medium-range weather forecast. Finally, our results imply that a distinction between slantwise and convective WCB trajectories is meaningful because the convective WCB trajectories are characterized by distinct properties.

Evolutionary modelling of subdwarf B stars using mesa with the predictive mixing and convective pre-mixing schemes

ABSTRACT The results of the evolutionary modelling of subdwarf B stars are presented. For the first time, we explore the core and near-core mixing in subdwarf B stars using new algorithms available in the mesa code: the predictive mixing scheme and the convective pre-mixing scheme. We show how both methods handle problems related to the determination of the convective boundary and the discrepancy between the core masses obtained from asteroseismology and evolutionary models, and long-standing problems related to the core-helium-burning phase, such as the splitting of the convective core and the occurrence of breathing pulses. We find that the convective pre-mixing scheme is the preferable algorithm. The masses of the convective core in the case of the predictive mixing and the combined convective and semiconvective regions in the case of the convective pre-mixing scheme are higher than in the models with only the Ledoux criterion, but they are still lower than the seismic-derived values. Both algorithms are promising and alternative methods of studying models of subdwarf B stars.

Magnitudes and Spatial Patterns of Interdecadal Temperature Variability in CMIP6

AbstractAttribution and prediction of global and regional warming requires a better understanding of the magnitude and spatial characteristics of internal global mean surface air temperature (GMST) variability. We examine interdecadal GMST variability in Coupled Modeling Intercomparison Projects, Phases 3, 5, and 6 (CMIP3, CMIP5, and CMIP6) preindustrial control (piControl), last millennium, and historical simulations and in observational data. We find that several CMIP6 simulations show more GMST interdecadal variability than the previous generations of model simulations. Nonetheless, we find that 100‐year trends in CMIP6 piControl simulations never exceed the maximum observed warming trend. Furthermore, interdecadal GMST variability in the unforced piControl simulations is associated with regional variability in the high latitudes and the east Pacific, whereas interdecadal GMST variability in instrumental data and in historical simulations with external forcing is more globally coherent and is associated with variability in tropical deep convective regions.

Impact of Subgrid Variation of Water Vapor on Longwave Radiation in a General Circulation Model

AbstractMost general circulation models compute radiation fluxes by assuming that water vapor is uniform within individual grid layers, which leads to an underestimation of satellite‐observed longwave (LW) cloud radiative forcing (LWCF). To fix this problem, we calculated water vapor content separately for clear and cloudy portions and used them to compute LW radiation. The impacts of this modification were examined by comparing two global simulations with and without the modification (NEW and OLD, respectively). Global‐annual mean LWCF from NEW was 1.8 W m−2 higher than that of OLD, thus remedying a long‐standing negative bias of LWCF. This improvement is a combined result of more clear‐sky and less all‐sky upward LW flux at the top of the atmosphere than OLD. Large increases in LWCF and clear‐sky LW flux occurred in the tropical deep convection and midlatitude storm track regions where upper‐ and middle‐level clouds are abundant. Although only the LW radiation scheme was modified, global‐annual mean shortwave cloud radiative forcing also increased, particularly in the vicinity of the eastern subtropical marine stratocumulus decks through radiative feedback processes. With this improved treatment, it may be possible to tune general circulation models in a more flexible and physical way without introducing compensating errors.

Thermally Driven Angular Momentum Transport in Hot Jupiters

Abstract We study the angular momentum transport inside hot Jupiters under the influence of gravitational and thermal forcing. Due to the strong stellar irradiation, a radiative region develops on top of the convective region. Internal gravity waves are launched at the radiative–convective boundaries (RCBs). The thermal response is dynamical and plays an important role in the angular momentum transport. By separating the gravitational and thermal forcing terms, we identify the thermal effects of increasing the angular momentum transport. For the low-frequency (in the corotating frame with planets) prograde (retrograde) tidal frequency, the angular momentum flux is positive (negative). The tidal interactions tend to drive the planet to the synchronous state. We find that the angular momentum transport associated with the internal gravity wave is very sensitive to relative position between the RCB and the penetration depth of the thermal forcing. If the RCB is in the vicinity of the thermal forcing penetration depth, even with small amplitude thermal forcing, the thermally driven angular momentum flux could be much larger than the flux induced by gravitational forcing. The thermally enhanced torque could drive the planet to the synchronous state in as short as a few 104 yr.

Assessing the Effects of Microphysical Scheme on Convective and Stratiform Characteristics in a Mei-Yu Rainfall Combining WRF Simulation and Field Campaign Observations

Microphysics parameterization becomes increasingly important as the model grid spacing increases toward convection-resolving scales. Using observations from a field campaign for Mei-Yu rainfall in China, four bulk cloud microphysics schemes in the Weather Research and Forecasting (WRF) model were evaluated with respect to their ability to simulate precipitation, structure, and cloud microphysical properties over convective and stratiform regimes. These are the Thompson (THOM), Morrison graupel/hail (MOR_G/H), Stony Brook University (SBU_YLIN), and WRF double-moment six-class microphysics graupel/hail (WDM6_G/H). All schemes were able to predict the rain band but underestimated the total precipitation by 23%–35%. This is mainly attributed to the underestimation of stratiform precipitation and overestimation of convective rain. For the vertical distribution of radar reflectivity, many problems remain, such as lower reflectivity values aloft in both convective and stratiform regions and higher reflectivity values at middle level. Each bulk scheme has its advantages and shortcomings for different cloud regimes. Overall, the discrepancies between model output and observations mostly exist in the midlevel to upper level, which results from the inability of the model to accurately represent the particle size distribution, ice processes, and storm dynamics. Further observations from major field campaigns and more detailed evaluation are still necessary.

Impacts of Humidity Adjustment Through Radar Data Assimilation Using Cloud Analysis on the Analysis and Prediction of a Squall Line in Southern China

AbstractThis study examines the impacts of humidity adjustment in a cloud analysis system on the analysis and forecast of a squall line that occurred in southeast China on 23–24 April 2007. Radial velocity data are assimilated using the ARPS three‐dimensional variational system while reflectivity data are assimilated by a cloud analysis system. Experiments with two different humidity adjustment schemes are performed, with the original and enhanced versions. Another experiment does not adjust moisture. Both schemes generally decrease the humidity in front of the convective line and increase the humidity within the convective and stratiform regions of squall line compared to no humidity adjustment, and the original scheme produces the higher humidity within precipitation regions, especially the stratiform region. Both schemes improve the forecast of squall line structure, including the leading convective line, a transition zone, and a trailing stratiform region. Among the three experiments, the enhanced scheme produces the highest precipitation forecast skill. The latent heating rates are also diagnosed to investigate the microphysical responses to the humidity adjustment. The cooling outside of the observed precipitation regions corresponding to the humidity reduction also acts to suppress spurious precipitation. Water vapor condensation into cloud water and cloud water evaporation generally dominate the latent heating/cooling below the freezing level. Compared to the enhanced scheme, the original scheme releases much more latent heat in the middle troposphere, causing more warming. This is linked to the higher cloud water condensation rate, due to the higher amount of moisture addition/adjustment by the original scheme.

Constraining sources and relative flow rates of bottom waters in the Late Cretaceous Pacific Ocean

Abstract Geochemical data suggest that ocean circulation patterns changed over a period of long-term cooling during the last 10 m.y. of the Cretaceous (late Campanian–Maastrichtian). Proposed changes include enhanced deep-water formation in the South Atlantic and/or Indian sectors of the Southern Ocean, initiation or enhanced deep-water formation in the North Atlantic, and alternating regions of deep convection in the North and South Pacific. Existing geochemical data do not allow simple confirmation or rejection of any of these scenarios. To test Pacific circulation during the Maastrichtian, we measured neodymium isotopic (εNd) values from four Pacific Deep Sea Drilling Project and Ocean Drilling Program sites and compare results both to Earth system model simulations using Maastrichtian paleogeography and to previous studies. Pacific εNd results consistently show a small negative εNd excursion during a well-documented, ∼1–3 m.y. early Maastrichtian cooling pulse (EMCP) but no other consistent trends across the late Campanian–late Maastrichtian interval (∼10 m.y.). Model results show that different CO2 forcings lead to changes in rates, but not patterns, of circulation. These combined results support the existence of a sustained source region for intermediate and deep waters in the southwestern Pacific throughout the late Campanian–Maastrichtian and indicate that changes in εNd values during the EMCP reflect an increased rate of overturning in the Pacific rather than changes in the source area of Pacific bottom waters.

Aerosol Impacts on Mesoscale Convective Systems Forming Under Different Vertical Wind Shear Conditions

AbstractFollowing our previous study of wind shear effect on mesoscale convective system (MCS) organization under a clean atmospheric condition using the Weather Research and Forecasting model coupled with spectral‐bin microphysics, we conduct sensitivity simulations by increasing cloud condensation nuclei concentration to investigate aerosol impacts on MCSs forming under different wind shear conditions. We find that increased aerosols induce stronger updrafts and downdrafts in all MCSs. The stronger updrafts and enlarged convective core area contribute to larger vertical mass fluxes and enhance precipitation. The enhanced updrafts and vertical mass fluxes indicate convective invigoration. Increased updraft speed below 8‐km altitude is attributed to enhanced condensational heating, except for the weak wind shear and strong low‐level shear cases in which the enhanced low‐level convergence is another contributing factor. Interestingly, above 8‐km altitude, we see reduced updraft speed by the increased aerosols due to reduced vertical pressure perturbation gradient force. The accumulated rainfall and mean rain rate are increased with a greater occurrence frequency of heavy rain. Larger rain rate is seen in both convective and stratiform regions. In general, we see a higher frequency of deep clouds in the polluted condition because of invigorated convection, and more stratiform/anvil clouds, but a lower frequency of shallow warm clouds, with a larger significance for more organized MCSs. The consistently invigorated MCSs by aerosols under various wind shear conditions revealed by this study have important implications to weather and climate in warm and humid regions that are influenced by pollution.

Observational aspects of tropical mesoscale convective systems over southeast India

To enhance the knowledge of various physical mechanisms related to the evolution of Tropical Mesoscale Convective Systems (MCSs), detailed analysis has been performed using suite of observations (weather radar, electric field mill, surface weather station, flux tower, microwave radiometer and wind profilers) available at Gadanki (13.5°N/79.2°E), located over southeast India. Analysis suggests that these systems developed in warm, moist environment associated with large scale low level convergence. Significant variations in cloud to ground (CG) lightning activity indicate the storm electrification. Deep (shallow) vertical extents with high (low) reflectivity and cloud liquid water; dominant upward (downward) motion reveals variant distribution in convective (stratiform) portions. Existence of both +CG and –CG flashes in convective regions, dominant –CG in stratiform regions explains the relation between lightning polarity and rain and cloud type. Sharp changes in surface meteorological variables and variations in surface fluxes are noticed in connection to cold pool of the system. Increase (decrease) in temperature, moisture and equivalent potential temperature (θe) within the boundary layer in convective (stratiform) regions associated with latent heat warming (cooling) of air parcel are apparent. Presence of updrafts and downdrafts in convective region and dominant downdrafts in stratiform regions are evident from vertical velocity measurements. Isentropic upgliding (downgliding) illustrate the existence of isentropic ascents (descent) of air parcels in the storm vicinity. Veering (backing) of wind due to warm (cold) and moist (dry) air advections demonstrated the formation of θe ridge in storm environment. Blend of observations provided considerable insight of electrical, microphysical, thermodynamic, dynamic and kinematic features of MCS.

Wave Reflection and Transmission at the Interface of Convective and Stably Stratified Regions in a Rotating Star or Planet

Abstract We use a simplified model to study wave reflection and transmission at the interface of the convective region and stably stratified region (e.g., radiative zone in a star or stratification layer in a gaseous planet). The inertial wave in the convective region and gravito-inertial wave in the stably stratified region are considered. We begin with the polar area and then extend to any latitude. Six cases are discussed (see Table 1), and in Case 2 both waves co-exist in both regions. Four configurations are further discussed for Case 2. The angles and energy ratios of wave reflection and transmission are calculated. It is found that wave propagation and transmission depend on the ratio of buoyancy frequency to rotational frequency. In a rapidly rotating star or planet, the wave propagates across the interface and most of the energy of the incident wave is transmitted to the other region, but in a slowly rotating star or planet, wave transmission is inhibited.

Large-eddy simulation of non-isothermal flow over a circular cylinder

In the present work, flow around a heated circular cylinder, at a Reynolds number of Re=3900, is investigated using large-eddy simulation (LES). Large differences in temperature, 25 ∘C, 100 ∘C, 200 ∘C, and 300 ∘C between the cylinder and the oncoming flow are considered, and its effect on the flow and thermal characteristics in the near wake region are studied. The numerical methodology employed is validated for both, the mean and second-order statistics, with the direct numerical simulation (DNS) data available in the literature. The results are analyzed using the mean temperature, velocity, Reynolds stresses, temperature variances, turbulent heat fluxes and energy spectra. The flow and thermal characteristics are studied along the center line in the wake, and in the transverse direction at two locations. The non-isothermal flow characteristics are compared with isothermal flow, to study the effect of temperature on the flow dynamics. Phase-averaging is performed to analyze the regions of turbulence production and convection of heat. It is observed that, the flow characteristics vary non-linearly with the temperature, and the effect is insignificant till a temperature difference of 100 ∘C, however, beyond this significant effect could be noticed. The effect of temperature difference is prominent in the thermal characteristics for all temperature differences, 25 ∘C to 300 ∘C, considered. The transverse component of shear stress fluctuations are observed to be dominant over the stream-wise components at both the locations downstream, thereby enhancing the local mixing of the fluid and hence, the heat transfer.

Probing the mid-layer structure of red giants

Context. The space-borne missions CoRoT and Kepler have already brought stringent constraints on the internal structure of low-mass evolved stars, a large part of which results from the detection of mixed modes. However, all the potential of these oscillation modes as a diagnosis of the stellar interior has not been fully exploited yet. In particular, the coupling factor or the gravity-offset of mixed modes, q and εg, are expected to provide additional constraints on the mid-layers of red giants, which are located between the hydrogen-burning shell and the neighborhood of the base of the convective zone. The link between these parameters and the properties of this region, nevertheless, still remains to be precisely established. Aims. In the present paper, we investigate the potential of the coupling factor in probing the mid-layer structure of evolved stars. Methods. Guided by typical stellar models and general physical considerations, we modeled the coupling region along with evolution. We subsequently obtained an analytical expression of q based on the asymptotic theory of mixed modes and compared it to observations. Results. We show that the value of q is degenerate with respect to the thickness of the coupling evanescent region and the local density scale height. On the subgiant branch and the beginning of the red giant branch (RGB), the model predicts that the peak in the observed value of q is necessarily associated with the important shrinking and the subsequent thickening of the coupling region, which is located in the radiative zone at these stages. The large spread in the measurement is interpreted as the result of the high sensitivity of q to the structure properties when the coupling region becomes very thin. Nevertheless, the important degeneracy of q in this regime prevents us from unambiguously concluding on the precise structural origin of the observed values. In later stages, the progressive migration of the coupling region toward the convective zone is expected to result in a slight and smooth decrease in q, which is in agreement with observations. At one point just before the end of the first-dredge up and the luminosity bump, the coupling region becomes entirely located in the convective region and its continuous thickening is shown to be responsible for the observed decrease in q. We demonstrate that q has the promising potential to probe the migration of the base of the convective region as well as convective extra-mixing during this stage. We also show that the frequency-dependence of q cannot be neglected in the oscillation spectra of such evolved RGB stars, which is in contrast with what is assumed in the current measurement methods. This fact can have an influence on the physical interpretation of the observed values. In red clump stars, in which the coupling regions are very thin and located in the radiative zone, the small variations and spread observed in q suggest that their mid-layer structure is very stable. Conclusions. A structural interpretation of the global observed variations in q was obtained and the potential of this parameter in probing the dynamics of the mid-layer properties of red giants is highlighted. This analytical study paves the way for a more quantitative exploration of the link of q with the internal properties of evolved stars using stellar models for a proper interpretation of the observations. This will be undertaken in the following papers of this series.

Real–time prediction of penetration depths of laser surface melting based on coaxial visual monitoring

Laser surface melting (LSM) is an important laser processing technique that can be used in laser conduction welding, laser remelting, and defect elimination. Penetration depths are essential to determine LSM qualities. To enhance the ability to predict penetration depths, a real-time prediction method based on coaxial visual characteristics and the convection state of a molten pool is reported. First, the coaxial visual characteristics of the molten pool are analyzed, and its image is divided into convection, transition, and boundary regions. Based on these region characteristics, the convection state can be indicated. Then, the relationships among process parameters, coaxial visual characteristics, and penetration depths are presented. Finally, we attempt to predict the penetration depths based on these coaxial visual characteristics. The results show that the convection region characteristics are more sensitive to the changes in process parameters. The oscillation amplitude of the convection region can be considered monitored objects to indicate the convection state. The convection region can predict the penetration depths better than the entire molten pool. Furthermore, this work will be beneficial to understanding the experimental phenomenon and enhance the monitoring ability of other laser processing techniques, such as laser alloying, laser cladding, and laser-based additive manufacturing.

Convective‐Stratiform Rainfall of Typhoon Fitow (2013): Sensitivity to Rainfall Partitioning Methods

AbstractRainfall separation methods are widely used in literatures, with some algorithms applied to surface rainfall and some applied to radar or other observations. How the effectiveness or practicability of these rainfall partitioning algorithms when applied to model results is not well known. In this study, three convective‐stratiform rainfall partitioning methods are compared and evaluated by analyzing cloud properties and associated precipitation budgets based on both observations and grid‐scale simulations from a three‐dimensional Weather Research and Forecasting modeling of Typhoon Fitow (2013). Results show that convective‐stratiform rainfall separation based on surface rain rate is generally more reasonable than that based on radar reflectivity in terms of statistics of vertical velocity and raindrop microphysical budgets. The radar reflectivity partition method does not conduct an exact demarcation between convective and stratiform regions, since the rainfall in stratiform region has similar characteristics as convective rainfall, whereas the rainfall classified as convective rainfall has similar characteristics as stratiform rainfall. Observational analysis based on satellite and lightning confirms these results and shows that the rainfall separation based on surface rain rate generally detects the cellular convective rainfall near eyewall and isolated convection in spiral rainbands, whereas the rainfall separation based on radar reflectivity not only fails to identify the scattered convective rainfall but also overestimates the convective rainfall embedded among the stratiform regions. Above all, the method based on surface rainfall produces more reasonable statistical and microphysical characteristics for both convective and stratiform regions of precipitation, which generally agree with the real rainfall distributions from satellite and lightning observations.

Change of the Rainfall Seasonality Over Central Peruvian Andes: Onset, End, Duration and Its Relationship With Large-Scale Atmospheric Circulation

Changes of the onset dates, end dates, and duration of the rainy season over central Peruvian Andes (Mantaro river basin, MRB) could severely affect water resources management and the main economic activities (e.g., rainfed agriculture, raising cattle, among others). Nonetheless, these changes have not been documented for the Tropical Andes. To asses that, we used daily datasets of observed rainfall during the 1965–2013 period. For this period, the average onset (end) date of the rainy season over the MRB occurs in the pentad 17 (19–23 September) [pentad 57 (7–11 April)]. The duration of the rainy season mainly is modulated by the onset dates due to it has higher variability than end dates. There is a reduction of 3 days/decade in the duration of wet season over the MRB for the last four decades due to the delay of the onset days. Furthermore, El Niño favors late-onset and early end of the rainy season, while La Niña favors early onset and late end of the rainy season in the MRB. Onset dates are related to the propagation of the convective region of the South American Monsoon System (SAMS), from the Caribbean region toward the central Amazon basin. Early (late)-onset days are associated with a southward (northward) shift of the South Atlantic Convergence Zone (SACZ) and weak (strong) convection over equatorial Atlantic that induces the southernmost propagation (eastward shift) of the SAMS.