Calculating Relative Humidity at Single Pressure Level using ERA5 Data: An Earth Science Perspective

The European Centre for Medium-Range Weather Forecasts (ECMWF) is a leading provider of global weather forecasts and climate models. The fifth generation of their atmospheric reanalysis data is called ERA5 and is widely used by the scientific community for various applications. One of the most important parameters in atmospheric science is relative humidity, which is calculated based on temperature and pressure. In this article we will explore how to calculate relative humidity at a single pressure level using ERA5 data.

What is ERA5 data?

ERA5 is an atmospheric reanalysis dataset covering the period from 1979 to the present with a spatial resolution of 31 km and a temporal resolution of 1 hour. It includes various atmospheric variables such as temperature, pressure, wind, humidity, and precipitation. The data are produced by assimilating observations from various sources such as satellites, radiosondes, aircraft, and surface stations into a numerical weather prediction model.

The ERA5 data are freely available to the scientific community through the Copernicus Climate Change Service (C3S) Climate Data Store (CDS), which provides an easy-to-use interface for downloading and manipulating the data. The ERA5 data are stored in the netCDF format, which is a self-describing binary format containing metadata about the variables, dimensions and attributes.

Relative Humidity Calculation

Relative humidity is a measure of the amount of water vapor in the air relative to the maximum amount of water vapor the air can hold at a given temperature and pressure. It is expressed as a percentage and is an important parameter in weather forecasting, climate modeling, and air quality studies. The calculation of relative humidity requires knowledge of temperature and pressure, which are available in the ERA5 data.

To calculate relative humidity at a single pressure level using ERA5 data, the following steps must be taken:

1. Extract temperature and specific humidity data at the desired pressure level.
2. Calculate saturation vapor pressure from temperature data.
3. Calculate actual vapor pressure using specific humidity data.
4. Calculate relative humidity from saturation and actual vapor pressures.

Specific humidity is the mass of water vapor per unit mass of air and is available in ERA5 data. The saturation vapor pressure is the maximum amount of water vapor that the air can hold at a given temperature and is calculated using the following equation:

es = 6.112 × exp(17.67 × T)/(T + 243.5)

Where es is the saturation vapor pressure in hPa and T is the temperature in degrees Celsius. The actual vapor pressure is the amount of water vapor in the air and is calculated using the following equation:
e = q × p/(0.622 + 0.378 × q)

Where e is the actual vapor pressure in hPa, q is the specific humidity in kg/kg, and p is the pressure in hPa.

Once we have calculated the saturation and actual vapor pressures, we can calculate the relative humidity using the following equation:

RH = (e/es) × 100%.

Where RH is the relative humidity in percent.

Applications of Relative Humidity Data

Relative humidity data is used in a variety of atmospheric science applications, including weather forecasting, climate modeling, and air quality studies. In weather forecasting, relative humidity is used to predict the likelihood of precipitation, fog, and other weather phenomena. In climate modeling, relative humidity is an important parameter for understanding the water cycle and atmospheric-surface feedback mechanisms. In air quality studies, relative humidity is used to understand the formation and transport of pollutants in the atmosphere.
ERA5 data provide a valuable resource for atmospheric science researchers and practitioners to study the dynamics of the atmosphere and its impact on the environment. By calculating relative humidity at a single pressure level using ERA5 data, researchers can gain a better understanding of the moisture content of the atmosphere and its impact on weather and climate. This can lead to improved weather forecasts, better climate models, and better air quality predictions.

Conclusion

In this article, we have explored the calculation of relative humidity at a single pressure level using ERA5 data. We discussed the steps involved in the calculation of relative humidity and its importance in atmospheric science. We also discussed the applications of relative humidity data in weather forecasting, climate modeling, and air quality studies. ERA5 data provide a valuable resource for atmospheric science researchers and practitioners to study the dynamics of the atmosphere and its impact on the environment. By using ERA5 data, researchers can gain a better understanding of the atmosphere and its impact on weather, climate and air quality.
If you are interested in using ERA5 data for your research, the Copernicus Climate Change Service (C3S) Climate Data Store (CDS) provides an easy-to-use interface for downloading and manipulating the data. With a wide range of atmospheric variables available, including temperature, pressure, wind, humidity, and precipitation, there is a wealth of information that can be extracted from the ERA5 data to improve our understanding of the atmosphere and its impact on the environment.

FAQs

What is ERA5 data?

ERA5 is an atmospheric reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). It covers the period from 1979 to present with a spatial resolution of 31 km and a temporal resolution of 1 hour. It includes various atmospheric variables such as temperature, pressure, wind, humidity, and precipitation.

What is relative humidity?

Relative humidity is a measure of the amount of water vapor in the air relative to the maximum amount of water vapor that the air can hold at a given temperature and pressure. It is expressed as a percentage and is an important parameter in weather forecasting, climate modeling, and air quality studies.

How is relative humidity calculated using ERA5 data?

To calculate relative humidity at a single pressure level using ERA5 data, we need to extract temperature and specific humidity data at the desired pressure level. Then, we calculate the saturation vapor pressure using the temperature data and the actual vapor pressure using the specific humidity data. Finally, we calculate the relative humidity using the saturation and actual vapor pressures.

What are the applications of relative humidity data?

Relative humidity data is used in various applications in atmospheric science, including weather forecasting, climate modeling, and air quality studies. In weather forecasting, relative humidity is used to predict the likelihood of precipitation,fog, and other weather phenomena. In climate modeling, relative humidity is an important parameter for understanding the water cycle and the feedback mechanisms between the atmosphere and the surface. In air quality studies, relative humidity is used to understand the formation and transport of pollutants in the atmosphere.

Where can I access ERA5 data?

ERA5 data is freely available to the scientific community through the Copernicus Climate Change Service (C3S) Climate Data Store (CDS), which provides an easy-to-use interface to download and manipulate the data.

What is specific humidity?

Specific humidity is the mass of water vapor per unit mass of air, and it is available in the ERA5 data. It is an important parameter in atmospheric science for understanding the moisture content of the atmosphere.

What is the difference between actual vapor pressure and saturation vapor pressure?

Actual vapor pressure is the amount of water vapor in the air, while saturation vapor pressure is the maximum amount of water vapor that the air can hold at a given temperature. The difference between the two is an indication of the moisture content of the air and is expressed as relative humidity.