Rainfall

Storms Over the Ocean – CC-BY-NC by Lochaven

Day 10 of 30DayMapChallenge: « Air » (previously).

Where did it rain the most in a day last year?

Using ERA5 post-processed daily statistics on single levels from 1940 to present.

ERA5 is the fifth generation ECMWF reanalysis for the global climate and weather for the past 8 decades. Data is available from 1940 onwards. Reanalysis combines model data with observations from across the world into a globally complete and consistent dataset using the laws of physics.

library(terra)
library(dplyr)
library(ggplot2)
library(glue)
library(sf)
library(ggspatial)
library(rnaturalearth)

Data

First, you need to register for a (free) account. Once the query form is sent (with year 2024, total precipitation, all months, all days etc.), your job is queued and you’ll need to wait to get your link to a downloadable dataset. It took around 35 min and we obtain a 600 MB NetCDF file.

An API is also available to automate the queries.

The coordinates are in the [0, 360] degrees range (often the case for climate data, don’t ask me why), so we need to rotate the data to find our more familiar [-180, 180]. The total precipitation is in meters; we’ll convert to cm.

# It takes a few minutes to compute, so we cache it
if (!file.exists("max_rainfall_cm.tif")) {
  rainfall <- rast("~/data/era5/4d4e91553b16edf0e896d4511e028719.nc") 
  
  max_rainfall_cm <- max(rainfall, na.rm = TRUE) * 100
  max_rainfall_cm <- rotate(max_rainfall_cm)
  set.crs(max_rainfall_cm, "EPSG:4326")
  writeRaster(max_rainfall_cm, "max_rainfall_cm.tif",
              gdal = c("COMPRESS=DEFLATE"))
} else {
  max_rainfall_cm <- rast("max_rainfall_cm.tif")
}

# Get the world max value and its location
val_max_rainfall <- round(max(where.max(max_rainfall_cm)[[1, "value"]]), 1)
loc_max_rainfall <- xyFromCell(max_rainfall_cm, 
                               where.max(max_rainfall_cm)[2]) |> 
  st_point() |> 
  st_sfc(crs = "EPSG:4326") |> 
  st_sf() |> 
  mutate(val_max_rainfall = val_max_rainfall)

# map background
world <- ne_countries() 

Map

ggplot() +
  layer_spatial(data = max_rainfall_cm) +
  geom_sf(data = world, fill = NA, color = "white") +
  geom_sf(data = loc_max_rainfall, color = "red") +
  geom_sf_text(data = loc_max_rainfall, aes(label = val_max_rainfall), 
               fontface = "bold", vjust = -1, color = "red") +
  scale_fill_viridis_c(name = "height (cm)", trans = "sqrt") +
  coord_sf(crs = "EPSG:8857") +
  labs(title = "Maximum daily rainfall",
       subtitle = "2024",
       fill = "type", x ="", y = "",
       caption = glue("https://r.iresmi.net/ - {Sys.Date()}
                      data: ERA5 doi:10.24381/cds.4991cf48 / Natural Earth")) +
  theme_minimal() +
  theme(plot.caption = element_text(size = 6, 
                                    color = "darkgrey"),
        legend.position = "bottom")

Figure 1: Maximum daily rainfall in 2024

It seems the maximum rainfall in 24 hours was 48.7 cm, off the northern coast of The Philippines…

Finding the date it happened is an exercise left to the reader.