Factor Separation Analysis Research Articles

Factor analysis of recent major heatwaves in East Asia

Heatwaves (HWs) present a major hazard to our society and more extreme heatwaves are expected with future climatic changes. Hence, it is important to improve our understanding of the underlying processes that drive HWs, in order to boost our socioeconomic–ecological resilience. In this study, we quantified the influences of key driving factors (large-scale atmospheric circulation, soil moisture, and sea surface temperature) and their synergies on recent heatwaves in East Asia. We conducted a factor separation analysis for three recent HW events by constraining the key factors in the regional Weather Research and Forecasting model with their climatologies or pseudo-observations in different combinations. Our study showed distinct spatial variations in the HW-controlling factors in East Asia. The synergistic interaction of large-scale circulation and soil moisture was the most important factors in the 2013 Chinese HW. During the 2018 HWs in Korea and Japan, the same stagnant large-scale atmospheric circulation played a dominant role in driving the HW events. The land-atmosphere coupling via soil moisture, its interaction with circulation, and SST exhibited stronger influences during the Korean HW than the Japanese HW. Our analysis also revealed temporal variations in the factors driving Korean and Chinese HWs due to typhoon passage and other multiple processes over heterogeneous surfaces (i.e., topographically induced Foehn winds, large-scale warm advection from the warm ocean, spatial differences in soil moisture). Our findings suggest that future heatwave-related studies should consider interactive contributions of key factors, their interplay with surface heterogeneities of complex terrain.

Designing a gradient structure in a Ni-based superalloy to improve fretting fatigue resistance at elevated temperatures through an ultrasonic surface rolling process

In this study, a gradient nanostructured GH4169 superalloy was prepared by ultrasonic surface rolling process (USRP), and the fretting fatigue resistance was investigated at elevated temperatures. After USRP, a high density of deformation twins, stacking faults and dislocations as well as precipitates were observed in the surface of the gradient nanostructured GH4169 superalloy when examined using transmission electron microscopy. Factor separation analysis was carried out to determine the leading factor for the improvement of the fretting fatigue properties of GH4169 at elevated temperatures, and the compressive residual stress was found to be the leading factor. The mechanism was determined to be dislocation movement that can be pinned by deformation twins, stacking faults and precipitates and, thus, is able to stabilize the compressive residual stress at elevated temperatures. As a result, the USRP-treated GH4169 with gradient nanostructure was found to have the highest fretting fatigue resistance at elevated temperatures.

Effects of orbital forcing, greenhouse gases and ice sheets on Saharan greening in past and future multi-millennia

Abstract. Climate archives reveal alternating arid and humid conditions in North Africa during the last several million years. Most likely the dry phases resembled current hyper-arid landscapes, whereas the wet phases known as African humid periods (AHPs) sustained much more surface water and greater vegetated areas that “greened” large parts of the Sahara region. Previous analyses of sediment cores from the Mediterranean Sea showed the last five AHPs differed in strength, duration and rate of change. To understand the causes of such differences we perform transient simulations of the past 190 000 years with the Earth system model of intermediate complexity CLIMBER-2. We analyse the amplitude and rate of change of the modelled AHP responses to changes in orbital parameters, greenhouse gases (GHGs) and ice sheets. In agreement with estimates from Mediterranean Sea sapropels, we find the model predicts a threshold in orbital forcing for Sahara greening and occurrence of AHPs. Maximum rates of change in simulated vegetation extent at AHP onset and termination correlate strongly with the rate of change of the orbital forcing. As suggested by available data for the Holocene AHP, the onset of modelled AHPs usually happens faster than termination. A factor separation analysis confirms the dominant role of the orbital forcing in driving the amplitude of precipitation and vegetation extent for past AHPs. Forcing due to changes in GHGs and ice sheets is only of secondary importance, with a small contribution from synergies with the orbital forcing. Via the factor separation we detect that the threshold in orbital forcing for AHP onset varies with GHG levels. To explore the implication of our finding from the palaeoclimate simulations for the AHPs that might occur in a greenhouse-gas-induced warmer climate, we extend the palaeoclimate simulations into the future. For the next 100 000 years the variations in orbital forcing will be smaller than during the last 100 millennia, and the insolation threshold for the onset of late Quaternary AHPs will not be crossed. However, with higher GHG concentrations the predicted threshold drops considerably. Thereby, the occurrence of AHPs in upcoming millennia appears to crucially depend on future concentrations of GHGs.

Modeling Large‐Scale Heatwave by Incorporating Enhanced Urban Representation

AbstractThis study evaluates the impact of land surface models (LSMs) and urban heterogeneity [using local climate zones (LCZs)] on air temperature simulated by the Weather Research and Forecasting model (WRF) during a regional extreme event. We simulated the 2017 heatwave over Europe considering four scenarios, using WRF coupled with two LSMs (i.e., Noah and Noah‐MP) with default land use/land cover (LULC) and with LCZs from the World Urban Database and Access Portal Tools (WUDAPT). The results showed that implementing the LCZs significantly improves the WRF simulations of the daily temperature regardless of the LSMs. Implementing the LCZs altered the surface energy balance partitioning in the simulations (i.e., the sensible heat flux was reduced and latent heat flux was increased) primarily due to a higher vegetation feedback in the LCZs. The changes in the surface flux translated into an increase in the simulated 2‐m relative humidity and 10‐m wind speed as well as changed air temperature within cities section and generated a temperature gradient that affected the temperatures beyond the urban regions. Despite these changes, the factor separation analysis indicated that the impact of LSM selection was more significant than the inclusion of LCZs. Interestingly, the lowest bias in temperature simulations was achieved when WRF was coupled with the Noah as the LSM and used WUDAPT as the LULC/urban representation.

Urban Effect on Sea-Breeze-Initiated Rainfall: A Case Study for Seoul Metropolitan Area

We examined the sea-breeze-initiated rainfall in the Seoul Metropolitan area (SMA) on 6 July 2017 using the weather research and forecasting (WRF) model. The model captures the arrival of the sea breeze front (SBF), the development of afternoon rainfall in the SMA, and the location of the sea-breeze-initiated maximum rainfall in the northeastern SMA reasonably well but overestimates the subsequent rainfall. We conducted sensitivity tests to better understand the urban effect on the sea-breeze-initiated rainfall event. Through factor separation analysis, we first examined the explicit role of sea and urban effect on sea-breeze-initiated rainfall. The results show that the interaction of sea and urban effects cause rainfall in the northwest and northeast of the SMA, indicating that both urban heat island circulation (UHIC) and sea breeze play an important role in the study case’s rainfall. We further examined the relative role of urban roughness and anthropogenic heat on the sea-breeze-initiated rainfall through factor separation analysis. Both anthropogenic heat and urban roughness play a role in increasing precipitation in the northeastern area of the SMA, with a larger contribution of anthropogenic heat than urban roughness. The relationship between low-level convergence at the SBF and urban factors is discussed.

A Factor Separation Study of the Effect of Synoptic‐Scale Wind, Atmospheric Moisture and of Their Synergy on the Diurnal Temperature Range During the Israeli Summer

AbstractThe trend of the decrease in the global average diurnal temperature range (DTR) is one of the clearest signals in twentieth century climate data. Regionally, DTR trends exhibit large variations. A factor separation analysis was conducted to investigate the impact of atmospheric moisture, synoptic‐scale winds, and their synergy on the DTR during the Israeli summer. The Weather Research and Forecasting (WRF) Single Column Model was run for summer representative days, at four locations in Israel. In almost all cases, the contribution of the factors and of their synergy to the DTR was dominated by their contribution to the diurnal minimum temperature, Tmin. The largest contribution resulted from atmospheric moisture, reducing the DTR. The contribution of synoptic‐scale winds showed more variability, with significant differences in both magnitude and sign on different days. The sign of the effect on Tmin depended on the relative direction and magnitude of the nocturnal synoptic‐scale wind with respect to the local wind, which in turn determined the effect on the low‐level jet (LLJ) and vertical mixing. The contribution from synergy between the two factors depended on the effect of moisture on the LLJ and on the effect of the synoptic‐scale winds on moisture advection in or out of the atmospheric column. All cases were classified into groups depending on the sign of contributions of the single factors and of their synergy when analyzing the DTR observed trends. These results highlight the importance of the synoptic wind, of its synergy with atmospheric moisture and of the feedback mechanisms.

On the determining factors in cases of moderate to heavy precipitation in Tehran

Investigating the factors responsible for heavy precipitation events is important because of the financial and non-financial losses they often cause. Based on the ERA-Interim reanalysis and a numerical mesoscale forecasting model (WRF), the effects of four factors on significant precipitations in Tehran are studied. The four factors are upper-level potential vorticity (PV) anomalies, and mid-tropospheric PV anomalies as two meteorological factors; and the Alborz and Zagros mountains as two external factors. A factor separation analysis is applied to six days that are representative (most similar) of the six circulation types (CTs) previously identified for moderate to heavy precipitations in Tehran. The independent contribution of the upper-level PV anomaly to precipitation in each of the selected days of the second to the sixth CT is higher than 60%; hence, the upper-level tropospheric flow provided the main precipitation forcing factor, with much less contribution from the thermodynamic factor of the mid-level troposphere. Regarding the two orographic external factors, the higher positive impact on Tehran precipitation is found for the Alborz mountains. This is related to synoptic-scale weather systems that induce a northerly low-level flow towards the Alborz mountains. Moreover, in all the studied cases, except the fifth CT, the independent impact of the Zagros mountains on Tehran precipitation is positive. In all of the selected days except that of the first CT, the main factor responsible for precipitation is the intrusion of dry and cold stratospheric air associated with a large positive PV anomaly (streamer) at upper-tropospheric levels. For the first CT, further quest is necessary to find out the main factor.

Impacts of extremely asymmetrical polar ice sheets on the East Asian summer monsoon during the MIS-13 interglacial

Marine Isotope Stage (MIS) 13 (∼500 ka ago) was a relatively weak interglacial according to the benthic δ18O and Antarctic ice core records. However, proxy records from the Northern Hemisphere indicate that MIS-13 was at least as warm as or even warmer than more recent interglacials, with an extremely strong summer monsoon and possible melting of the Greenland ice sheet. In this study, assuming an asymmetric hemispheric climate, we investigate the response of the East Asian summer monsoon to various Greenland and Antarctic ice sheet sizes using a set of sensitivity experiments with the HadCM3 model through factor separation analysis. Results show that with no Greenland ice sheet and a large Antarctic ice sheet there is an enhancement of summer monsoon precipitation in the East Asian monsoon region of up to ∼7%, in agreement with the direction of change from proxy reconstructions for China. This enhancement is associated with a stronger land–sea thermal contrast, more water vapor transport to East Asia and a northward shift of the Intertropical Convergence Zone. The response to the Greenland ice sheet removal is related to thermodynamic and topographical effects that influence the East Asian monsoon through an orographically induced wave train. The response to the larger Antarctic ice sheet involves stronger upwelling of circumpolar deep water in the Southern Ocean melts the sea ice, leading to a warmer SST. This warming propagates to the North, affecting the East Asian summer monsoon.

A physical subgrid‐scale information exchange (PSIE) system for parametrization schemes in numerical weather prediction models

The choice and interaction of the parametrization schemes in a numerical weather prediction (NWP) model are crucial for the forecast quality. Because, classically, the parametrization schemes can only communicate to each other via the tendencies of the averaged grid‐scale prognostic variables, a considerable amount of subgrid‐scale information available within the schemes is lost.To overcome this problem, subgrid‐scale information should be directly exchanged between the parametrization schemes. In the present paper already existing subgrid‐scale communication paths, which have been successfully used in operational forecasts in the past, are extended to a physical subgrid‐scale information exchange (PSIE) system including all atmospheric parametrization schemes. For our studies we have chosen the parametrization schemes of microphysics, convection, turbulence, fractional cloud cover and radiation. The COSMO model of Deutscher Wetterdienst (DWD) with a grid size of Δx = 7 km serves as the hosting model for the implementation of the PSIE system. However, the couplings are formulated in a way that also allows the PSIE system to be applied to other NWP models. Simulations of real case‐studies of different weather situations are performed and compared to DWD observational precipitation data from RADOLAN (radar measurements calibrated by station data).Visual inspection of precipitation maps reveals that precipitation patterns look more realistic with the full PSIE system, especially during air‐mass convection situations. This is confirmed by fractional skill score (FSS) analyses, which also give an insight into the temporal development of the PSIE effects. The relative impact of the PSIE paths and their interaction is also studied in conjunction with a factor separation analysis.

On the effect of orbital forcing on mid-Pliocene climate, vegetation and ice sheets

Abstract. We present results from modelling of the mid-Pliocene warm period (3.3–3 million years ago) using the Earth system model of intermediate complexity CLIMBER-2 analysing the effect of changes in boundary conditions as well as of orbital forcing on climate. First we performed equilibrium experiments following the PlioMIP (Pliocene Model Intercomparison Project) protocol with a CO2 concentration of 405 ppm, reconstructed mid-Pliocene orography and vegetation and a present-day orbital configuration. Simulated global Pliocene warming is about 2.5 °C, fully consistent with results of atmosphere–ocean general circulation model simulations performed for the same modelling setup. A factor separation analysis attributes 1.5 °C warming to CO2, 0.3 °C to orography, 0.2 °C to ice sheets and 0.4 °C to vegetation. Transient simulations for the entire mid-Pliocene warm period with time-dependent orbital forcing as well as interactive ice sheets and vegetation give a global warming varying within the range 1.9–2.8 °C. Ice sheet and vegetation feedbacks in synergy act as amplifiers of the orbital forcing, transforming seasonal insolation variations into an annual mean temperature signal. The effect of orbital forcing is more significant at high latitudes, especially during boreal summer, when the warming over land varies in the wide range from 0 to 10 °C. The modelled ice-sheet extent and vegetation distribution also show significant temporal variations. Modelled and reconstructed data for Northern Hemisphere sea-surface temperatures and vegetation distribution show the best agreement if the reconstructions are assumed to be representative for the warmest periods during the orbital cycles. This suggests that low-resolution Pliocene palaeoclimate reconstructions can reflect not only the impact of increased CO2 concentrations and topography changes but also the effect of orbital forcing. Therefore, the climate (Earth system) sensitivity estimates from Pliocene reconstructions which do not account for the effect of orbital forcing can be biased toward high values.

Impact of Flow Dependence, Column Covariance, and Forecast Model Type on Surface-Observation Assimilation for Probabilistic PBL Profile Nowcasts

Abstract A probabilistic verification and factor-separation analysis (FSA) elucidate skillful nowcasts of planetary boundary layer (PBL) temperature, moisture, and wind profiles with a single-column model (SCM) and ensemble filter (EF) assimilation of surface observations. Recently, an FSA showed the importance of surface assimilation versus advection and radiation on ensemble-mean skill. That work addressed the necessary complexity of the model and assimilation scheme for improving PBL nowcasts, relative to deterministic-mesoscale predictions. Here, probabilistic ensemble-based SCM forecasts are compared to a simple probabilistic postprocessing scheme termed climatological dressing (CD). CD adjusts a deterministic mesoscale forecast using surface-atmosphere 3D-climatological covariances, a 30-min persistence model, and surface-forecast errors. It also dresses the adjusted profile with an in-sample uncertainty distribution (obtained from archives) scaled by the 30-min forecast error. Superior deterministic skill from SCM/EF results during night when flow-dependent covariances are more accurate than climatological covariances. CD is deterministically more skillful for temperature and moisture profiles during daytime because SCM/PBL parameterization yields biased covariances. SCM/EF is most probabilistically skillful because (a) the EF covariances accommodate large seasonal variability, (b) the 30-min error persistence assumption fails during nighttime, and (c) vertical error covariance estimates from archived forecasts are generally poor estimates of actual error covariances. A probabilistic FSA of the SCM/EF shows the relative importance of surface assimilation, radiation parameterization, and advection during night. Results confirm surface assimilation as the most important factor. A factor can be deterministically beneficial and probabilistically detrimental, or vice versa, depending on its role in reducing mean error or improving sharpness. Assimilation results in notable probabilistic improvement for nowcasts of low-level jet structures.

Quantitative Analysis of Factors Contributing to Urban Heat Island Intensity

AbstractThis study identifies causative factors of the urban heat island (UHI) and quantifies their relative contributions to the daytime and nighttime UHI intensities using a mesoscale atmospheric model that includes a single-layer urban canopy model. A midlatitude city and summertime conditions are considered. Three main causative factors are identified: anthropogenic heat, impervious surfaces, and three-dimensional (3D) urban geometry. Furthermore, the 3D urban geometry factor is subdivided into three subfactors: additional heat stored in vertical walls, radiation trapping, and wind speed reduction. To separate the contributions of the factors and interactions between the factors, a factor separation analysis is performed. In the daytime, the impervious surfaces contribute most to the UHI intensity. The anthropogenic heat contributes positively to the UHI intensity, whereas the 3D urban geometry contributes negatively. In the nighttime, the anthropogenic heat itself contributes most to the UHI intensity, although it interacts strongly with other factors. The factor that contributes the second most is the impervious-surfaces factor. The 3D urban geometry contributes positively to the nighttime UHI intensity. Among the 3D urban geometry subfactors, the additional heat stored in vertical walls contributes most to both the daytime and nighttime UHI intensities. Extensive sensitivity experiments to anthropogenic heat intensity and urban surface parameters show that the relative importance and ranking order of the contributions are similar to those in the control experiment.

The Roles of Surface-Observation Ensemble Assimilation and Model Complexity for Nowcasting of PBL Profiles: A Factor Separation Analysis

AbstractRecent results showed the ability of surface-observation assimilation with a single-column model (SCM) and an ensemble filter (EF) to skillfully estimate the vertical structure of the PBL when only climatological information is provided for initialization and forcing. The present study quantifies the relative benefits of model complexity, compared to surface-observation assimilation, for making 30-min SCM ensemble predictions (nowcasts). The SCM is initialized and forced by timely mesoscale forecasts, making it capable of providing flow-dependent probabilistic very short-range forecasts of PBL profiles wherever surface observations are available. Factor separation (FS) analysis measures the relative contributions to skill from EF surface assimilation compared to selected SCM components: parameterized radiation and objectively scaled horizontal advection. Here, the SCM–EF system is presented and its deterministic skill (as represented by ensemble-mean error) is analyzed with FS. Results show that surface assimilation can more meaningfully contribute to the skill levels of temperature, wind, and mixing-ratio nowcasts than model enhancements under a wide range of flow scenarios. However, in the convective PBL regime surface assimilation can enhance the moist bias often observed in parameterized PBL mixing ratio profiles due to poor covariances estimated from the ensemble. Then, the SCM–EF proves useful in revealing a model deficiency. Externally imposed horizontal advection is required to provide skillful ensemble-mean forecasts when not assimilating surface observations, but can offset the benefit realized from assimilation by quickly sweeping the updated state out of the domain. The radiation scheme has a minor effect on forecast performance. It improves the nowcast surface temperature at night, and can act synergistically with assimilation to improve low-level jet predictions, but the effect above the surface steeply decreases with height. The results suggest that an SCM–EF may be helpful in wind-power and pollutant dispersion applications.

Quantifying the sensitivity of simulated climate change to model configuration

This study used “factor separation” to quantify the sensitivity of simulated present and future surface temperatures and precipitation to alternative regional climate model physics components. The method enables a quantitative isolation of the effects of using each physical component as well as the combined effect of two or more components. Simulation results are presented from eight versions of the Mesoscale Modeling System Version 5 (MM5), one-way nested within one version of the Goddard Institute for Space Studies Atmosphere-Ocean Global Climate Model (GISS AOGCM). The MM5 simulations were made at 108 km grid spacing over the continental United States for five summers in the 1990s and 2050s. Results show that the choice of cumulus convection parameterization is the most important “factor” in the simulation of contemporary surface summer temperatures and precipitation over both the western and eastern USA. The choice of boundary layer scheme and radiation package also increases the range of model simulation results. Moreover, the alternative configurations give quite different results for surface temperature and precipitation in the 2050s. For example, simulated 2050s surface temperatures by the scheme with the coolest 1990s surface temperatures are comparable to 1990s temperatures produced by other schemes. The study analyzes the spatial distribution of 1990s to 2050s projected changes in the surface temperature for the eight MM5 versions. The predicted surface temperature change at a given grid point, averaged over all eight model configurations, is generally about twice the standard deviation of the eight predicted changes, indicating relative consensus among the different model projections. Factor separation analysis indicates that the choice of cumulus parameterization is the most important modeling factor amongst the three tested contributing to the computed 1990s to 2050s surface temperature change, although enhanced warming over many areas is also attributable to synergistic effects of changing all three model components. Simulated ensemble mean precipitation changes, however, are very small and generally smaller than the inter-model standard deviations. The MM5 versions therefore offer little consensus regarding 1990s to 2050s changes in precipitation rates.

Simulations of Mideast transboundary ozone transport: A source apportionment case study

The Regional Atmospheric Modeling System (RAMS), Comprehensive Air Quality Model with Extensions (CAMx), and a new emissions inventory were used to study O3 formation downwind from the Israeli coast during summer 1997. While RAMS reproduced the observed diurnal variation of surface wind speeds and temperatures reasonably well, directions were somewhat less correlated. CAMx generally also reproduced the timing, location, and magnitude of measured surface and 300-m O3 max over Jerusalem, but also showed an even larger concurrent max over western Jordan, an area without observations. Observed near-zero nighttime surface O3 values, however, were over predicted because the CAMx grid was too large to accurately account for local emissions and titration effects. CAMx also generally accurately predicted another observed 300m local max near the Dead Sea, but could not account for a unique O3 depletion mechanism from bromine monoxide released from its water surface. Factor-separation analysis showed that transportation emissions produced 66% and 29% of the Jerusalem and Jordan 300-m simulated maxima, respectively, while power plant emissions produced about 35% and 62%, respectively.

Secondary flows induced by wind forcing in the Rh�ne region of freshwater influence

Secondary flows induced by the blocking effect of a river plume on a transverse upwelling are investigated in a microtidal region of freshwater influence (ROFI). A nested version of the SYMPHONIE primitive-equation free-surface model for 3-D baroclinic coastal flows has been developed for the Rhône ROFI. The main characteristics of the model are a generalized sigma coordinate system in finite differences, using a time splitting for external and internal modes and high-order numerical advection schemes for density fields in combination with an modified turbulence closure scheme. The nesting system consists of two grids forced by the high-resolution ALADIN model atmospheric data. The coarse grid of 3 km resolution for the whole Gulf of Lions allows the forcing of the Liguro-Provençal large-scale current when the fine mesh of 1-km resolution is centred on the river mouth of the Grand Rhône. Documented field experiments from the Biodypar 3 field campaign performed during March 1999 are used for validation. Numerical results, CTD profiles and a SPOT TSM visible image are in good agreement concerning the shape and structure of the river plume. Other coastal flow features can be observed from satellite imagery. Computations of realistic situations recover these main secondary structures. Complementary process-oriented runs give an explanation of how the coastal upwelling induced by an inhomogeneous offshore wind is destabilized by the combination of the river plume and along-shelf current-blocking effects. In the end, a factor-separation analysis provides evidence that the locally non-linear effects in momentum contribute to the occurrence of secondary vortices.

Improved Simulation of Florida Summer Convection Using the PLACE Land Model and a 1.5-Order Turbulence Parameterization Coupled to the Penn State–NCAR Mesoscale Model

Three major modifications to the treatment of land surface processes in the Pennsylvania State University‐ National Center for Atmospheric Research mesoscale model MM5, are tested in a matrix of eight model experiments. Paired together in each dimension of the matrix are versions of the code with and without one of the changes. The three changes involve 1) a sophisticated land surface model [the Parameterization for Land‐ Atmosphere Convective Exchange (PLACE)], 2) the soil moisture and temperature initial conditions derived from running PLACE offline, and 3) a 1.5-order turbulent kinetic energy (TKE) turbulence boundary layer. The code without changes, defined as the control code, uses the most widely applied land surface, soil initialization, and boundary layer options found in the current MM5 community code. As an initial test of these modifications, a case was chosen in which they should have their greatest effect: conditions where heterogeneous surface forcing dominates over dynamic processes. The case chosen is one with widespread summertime moist convection, during the Convection and Precipitation Electrification Experiment (CaPE) in the middle of the Florida peninsula. Of the eight runs, the code with all three changes (labeled TKE-PLACE) demonstrates the best overall skill in terms of biases of the surface variables, rainfall, and percent and root-mean-square error of cloud cover fraction for this case. An early, isolated convective storm that formed near the east coast, at the downwind edge of a region of anomalous wet soil, and within the dense cluster of CaPE mesoscale observation stations, is correctly simulated only by TKE-PLACE. It does not develop in any of the other seven runs. A factor separation analysis shows that a successful simulation requires the inclusion of the more sophisticated land surface model, realistic initial soil moisture and temperature, and the higher-order closure of the planetary boundary layer (PBL) in order to better represent the effect of joint and synergistic (nonlinear) contributions from the land surface and PBL on the moist convection.