Quickstart guide
In varunayan: Download and Process 'ERA5', IMD, 'HadEX3' and 'CRU TS' Data

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

Varunayan = Varun + ayan. In Vedic mythology, Varun is the god of the sky, order, truth and water. In Sanskrit, ayan means path.

The varunayan package downloads climate variables for any region of the globe from ERA5. ERA5 is a reanalysis. It combines models and historical observations to estimate many atmospheric, land and ocean variables from 1940 to the present. The package gets the data from the Copernicus Climate Data Store (CDS).

library(varunayan)

CDS credentials

To get ERA5 data, you must have a CDS API key. Save the key one time only.

setup_cds_credentials(key = "your-personal-access-token")

Key arguments

The three download functions share these arguments.

| Argument | Meaning | |---|---| | request_id | Name of the request. Use a unique name for each request. | | variables | ERA5 variable names, for example "2m_temperature". | | start_date, end_date | Dates as "YYYY-MM-DD". | | frequency | "hourly" (default), "daily", "weekly", "monthly" or "yearly". | | resolution | Grid size in degrees (default 0.25). | | dataset_type, pressure_levels | Use "pressure" and a vector such as "850" for pressure-level data. |

Data for a bounding box

era5ify_bbox() gets the grid cells inside a box. Here we get daily temperature and precipitation for five days around Delhi.

The output is in long format: one row for each time, grid cell and variable. The variable column holds the ERA5 short name (t2m is 2 m temperature, tp is total precipitation). Temperatures are in °C. Precipitation is in mm and is the total for each day.

bbox <- era5ify_bbox(
  request_id = "quickstart_bbox",
  variables = c("2m_temperature", "total_precipitation"),
  start_date = "2024-01-01",
  end_date = "2024-01-05",
  north = 29, south = 28, east = 78, west = 76.5,
  frequency = "daily"
)
head(bbox)

Data for a GeoJSON region

era5ify_geojson() takes a path or URL of a GeoJSON file. Here we use the LGD state boundaries of India. With level = "feature", the output has one row for each state, with the mean of the grid cells inside it. The default level = "union" keeps each grid cell.

states <- era5ify_geojson(
  request_id = "quickstart_states",
  variables = "2m_temperature",
  start_date = "2024-01-01",
  end_date = "2024-01-01",
  json_file = "https://geo.bharatviz.org/geojsons/admin/India-geodata-lgd-states.geojson",
  frequency = "daily",
  level = "feature"
)
head(states)

Data for a point

era5ify_point() keeps the grid cell nearest to a latitude and longitude. Here we get the daily temperature and precipitation in Mumbai in the first week of August 2024, during the monsoon.

mumbai <- era5ify_point(
  request_id = "quickstart_point",
  variables = c("2m_temperature", "total_precipitation"),
  start_date = "2024-08-01",
  end_date = "2024-08-07",
  lat = 19.07, lon = 72.88,
  frequency = "daily"
)
mumbai

Data at pressure levels

ERA5 also gives the atmosphere at 37 pressure levels, from 1000 hPa (near the surface) to 1 hPa. A lower pressure is a higher altitude: 850 hPa is approximately 1.5 km and 200 hPa is approximately 12 km. Set dataset_type = "pressure" and give the levels in pressure_levels. The output has a pressure_level column (hPa).

Here we get the daily temperature at six levels over Delhi for three days. spatial_mean() with by = "pressure_level" gives the mean temperature at each level. The temperature decreases as the altitude increases.

profile <- era5ify_bbox(
  request_id = "quickstart_pressure",
  variables = "temperature",
  start_date = "2024-01-01",
  end_date = "2024-01-03",
  north = 29, south = 28, east = 78, west = 76.5,
  dataset_type = "pressure",
  pressure_levels = c("1000", "850", "700", "500", "300", "200"),
  frequency = "daily"
)
head(profile)

plot_series(spatial_mean(profile, "value", by = "pressure_level"),
  x = "pressure_level", y = "value",
  ylabel = "Temperature (°C)", title = "Temperature profile over Delhi, 1-3 January 2024"
)

Find a variable

ERA5 has hundreds of variables. search_variable() finds the variables that match a word, in the single-level and pressure-level datasets.

search_variable("temperature", dataset_type = "all")

Describe variables

describe_variables() gives a description and the unit of each variable.

describe_variables(
  c("2m_temperature", "total_precipitation", "surface_pressure"),
  dataset_type = "single"
)

IMD data

The India Meteorological Department (IMD) gives gridded rainfall and temperature for India. You do not need credentials for IMD data.

rain <- imd_rainfall_bbox(
  request_id = "quickstart_imd_rain",
  start_year = 2023, end_year = 2023,
  north = 22, south = 16, east = 80, west = 73,
  frequency = "monthly"
)
head(rain)