ERA5与ERA5-Land月降水数据提取值偏低问题求助

数据来源 #

使用哥白尼气候变化服务（CDS）提供的两类降水月均值数据：

• ERA5月均值数据（0.25×0.25°分辨率）
• ERA5-Land月均值数据（0.1×0.1°分辨率）

已完成的预处理流程 #

1. 通过R语言将原始.nc格式文件转换为GeoTIFF格式
2. 完成降水单位从米（m）到毫米（mm）的转换
3. 基于研究区边界裁剪得到目标GeoTIFF文件

问题 #

编写R代码提取研究区月降水均值时，得到的数值范围仅在0.1-3.7mm之间，不符合预期，恳请分析原因并提供解决建议。

降水提取代码 #

library(raster)
library(stringr)

# Set the directory containing the monthly raster files
raster_dir <- "D:/Datasets/ERA5-Land_Monthly_PRE/ERA5-Land/"

# Set the directory where you want to export the CSV file
output_dir <- "D:/Datasets/ERA5-Land_Monthly_PRE/"

# Get a list of all GeoTIFF files in the directory
raster_files <- list.files(raster_dir, pattern = ".tif$", full.names = TRUE)

# Create empty vectors to store the dates and mean values
dates <- character()
mean_values <- numeric()

# Iterate through the raster files
for (file in raster_files) {
  # Extract the year and month from the file name
  date <- str_extract(file, "\\d{4}_\\d{2}")
 
  # Read the raster file
  raster_data <- raster(file)
 
  # Calculate the mean of the pixel values
  mean_val <- mean(values(raster_data), na.rm = TRUE)
 
  # Append the date and mean value to the vectors
  dates <- c(dates, date)
  mean_values <- c(mean_values, mean_val)
}

# Create a data frame with the dates and mean values
mean_df <- data.frame(Date = dates, Mean_Value = mean_values, stringsAsFactors = FALSE)

# Export the data frame to a CSV file in the specified output directory
output_csv <- file.path(output_dir, "PRE.csv")
write.csv(mean_df, file = output_csv, row.names = FALSE)

.nc转GeoTIFF代码 #

library(ncdf4)
library(raster)

# Set the working directory to the parent directory of the output folder
setwd("D:/ERA5-Land_Monthly_PRE/ERA5-Land")

# Specify output folder name
output_dir <- "D:/ERA5-Land_Monthly_PRE/ERA5-Land"

if(!dir.exists(output_dir)) dir.create(output_dir)

fname <- file.choose()
nc <- nc_open(fname)

GRACE <- brick(fname, varname = "tp")

GRACE_array <- getValues(GRACE)
timeL <- colnames(GRACE_array)

for(i in timeL) {
  # Specify the output file path
  output_path <- file.path(output_dir, paste0(i, ".tif"))
  writeRaster(GRACE[[i]], filename = output_path, format = "GTiff", overwrite = TRUE)
}

单位转换（m转mm）代码 #

library(raster)

# Set the directory containing the input raster files
input_directory <- "D:/ERA5-Land_Monthly_PRE/ERA5-Land"

# Set the directory for saving the output raster files
output_directory <- "D:/ERA5-Land_Monthly_PRE/ERA5-Land"

# Get a list of all raster files in the input directory
raster_files <- list.files(input_directory, pattern = ".tif$", full.names = TRUE)

# Loop through each raster file
for (file in raster_files) {
  # Read the raster file
  r <- raster(file)
  
  # Convert the values from m to mm by multiplying by 1000
  r_mm <- r * 1000
  
  # Set the output file name by appending "_mm" to the original file name
  output_file <- file.path(output_directory, paste0(tools::file_path_sans_ext(basename(file)), "_mm.tif"))
  
  # Save the output raster file
  writeRaster(r_mm, filename = output_file, format = "GTiff", overwrite = TRUE)
  
  cat("Converted", basename(file), "and saved as", basename(output_file), "\n")
}

内容的提问来源于stack exchange，提问作者user20664990