# Impacts of Compound Hot-Dry Events on Vegetation Productivity over Northern East Asia
## 2 Materials and Methods
### Experimental Design  
The hourly meteorological forcings obtained from the WATCH Forcing Data methodology applied to the ERA5 (WFDE5) dataset from 2003 to 2019 are used to drive CLM5. The WFDE5 dataset covers near-surface atmospheric variables, including precipitation, air temperature, specific humidity, incident shortwave and longwave radiation, surface air pressure, and wind speed at an hourly temporal resolution and 0.5\({}^{\circ}\) spatial resolution over the global land surface [35]. To acquire the equilibrium states of the simulated soil and vegetation carbon and nitrogen pools, we performed a 200-year spin-up simulation by cycling the 17 years of WFDE5 forcings in accelerated decomposition mode, followed by another 50 years of spin-up simulation with the accelerated mode turned off. Then, a control run (_Control_) from 2003 to 2019 was restarted from the state at the end of the spin-up simulation.  
To explore the combined and individual effects of drought and heat, eight sensitivity experiments were conducted, employing different combinations of atmospheric forcings. The first sensitivity experiment, _SE_Clim_, utilizes climate mean forcings in 2 m temperature, precipitation, incident radiation (including solar and longwave radiation), and 2 m specific humidity during CHD events. The involvement of climatological means in near-surface air pressure and wind basically does not change the simulated results we are concerned about. The difference between the _Control_ and _SE_Clim_ simulations during the CHD events reveals the combined effect of drought and heat.  
Figure 1: Study area and dominant plant functional type (PFT) distributions. The PFTs are represented by the following abbreviations: B for broadleaf, N for needleleaf, D for deciduous, E for evergreen, S for shrub, and T for tree.  
The second sensitivity experiment, \(SE\_PRQ\), focuses on the impact of drought by utilizing forcings with climate means in 2 m temperature. This experiment aims to quantify the influence of precipitation deficit accompanied by atmospheric dryness and enhanced solar radiation while comparing it with \(SE\_Clim\). The third sensitivity experiment (\(SE\_RQ\)) employs forcings with climate means in 2 m temperature and precipitation to examine the combined effect of incident radiation and air humidity. Additionally, the fourth sensitivity experiment (\(SE\_TR\)) explicitly investigates the combined effect of incident solar radiation and 2 m temperature due to their close relationship.  
Subsequent sensitivity experiments, namely \(SE\_R\), \(SE\_P\), \(SE\_T\), and \(SE\_Q\), are conducted to explore the individual effects of incident radiation, precipitation, 2 m temperature, and 2 m specific humidity, respectively. Each experiment was initiated on January 1 with the same initial conditions and continued for one year. Further details about these experiments can be found in Table 1.  
To obtain the climate means of precipitation, the hourly precipitation in every grid cell is derived by multiplying a scaling factor, \(f_{prcp}\)[36]. The scaling factor in year \(i\) is defined as \(f_{prcp}=P_{50 th}/P_{i}\). Here, \(P_{i}\) is the event average precipitation during the 30 th day before the date when the specified CHD event starts to its end date of the _ith_ year. And \(P_{50 th}\) is the 50 th percentile of precipitation of the 41 event averages of precipitation during 1979-2019 [36]. To obtain the climate means of 2 m temperature, the hourly 2 m temperature in every grid cell is calculated by multiplying the scaling factor, \(f_{fair}\), in which \(f_{fair}=P_{50 th}/T_{i}\). \(T_{i}\) is the event average of 2 m temperature from the date when the specified CHD event starts to the date when the event ends in the _ith_ year. And \(T_{50 th}\) is the 50 th percentile of the average 2 m temperature during 1979-2019. A similar approach to that of deriving the climate means of 2 m temperature was applied for acquiring the climate means in radiative forcings and specific humidity, respectively.