Converting ERA5 Specific Humidity: From kg/kg to g/kg or mm/day

ERA5 is a global climate reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). It provides hourly meteorological data on a global grid with a resolution of about 31 km for the period from 1979 to the present. Specific humidity is one of the key variables provided by ERA5, defined as the mass of water vapor per unit mass of air. It is usually expressed in kg/kg, but in some cases it may be necessary to convert it to g/kg or mm/day for specific applications. This article provides a step-by-step guide to converting ERA5 specific humidity in kg/kg to g/kg or mm/day.

Conversion of Specific Humidity from kg/kg to g/kg

The conversion from kg/kg to g/kg is simple as it involves multiplying the specific humidity value by 1000. For example, if the specific gravity is 0.005 kg/kg, the corresponding value in g/kg would be 5 g/kg. This conversion is useful when working with data that requires specific humidity to be expressed in grams per kilogram, such as when calculating dew point temperature.
It is important to note that when specific humidity is converted from kg/kg to g/kg, the relative humidity values remain unchanged. This is because specific humidity and relative humidity are two different variables that are related but not equivalent. Specific humidity represents the actual amount of water vapor in the air, while relative humidity is the ratio of the actual amount of water vapor to the maximum amount of water vapor the air can hold at a given temperature and pressure. Therefore, converting specific humidity from kg/kg to g/kg does not affect relative humidity values.

Another thing to consider when converting specific humidity from kg/kg to g/kg is the units of other variables that may be used in conjunction with specific humidity. For example, if you are working with temperature data in Celsius, it is important to convert the temperature to Kelvin before calculating the dew point temperature using specific humidity in g/kg units. This is because the dew point temperature formula requires the temperature to be expressed in Kelvin.

Convert Specific Humidity from kg/kg to mm/day

The conversion from specific humidity in kg/kg to mm/day is more complex and involves several steps. This conversion is useful when working with data that requires specific humidity to be expressed in terms of precipitation or evapotranspiration. The conversion involves the following steps:

1. Convert specific gravity from kg/kg to g/kg as described in the previous section.
2. Calculate the saturation vapor pressure using the temperature and pressure data provided by ERA5. This can be done using the formula for the Clausius-Clapeyron equation:

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

Where es is the saturation vapor pressure in hPa and T is the temperature in Celsius.

1. Calculate the actual vapor pressure using the specific humidity and pressure data provided by ERA5. This can be done using the following formula:

e = q x P / (0.622 + 0.378 x q)

Where e is the actual vapor pressure in hPa, q is the specific gravity in g/kg, and P is the pressure in hPa.

1. Using the saturation vapor pressure and actual vapor pressure values, calculate the dew point temperature. This can be done using the following formula:

Td = (243.5 x ln(e/es)) / (17.67 – ln(e/es))

Where Td is the dew point temperature in Celsius, e is the actual vapor pressure in hPa, and es is the saturation vapor pressure in hPa.

1. Calculate the specific humidity deficit using the dew point temperature and the actual temperature values. This can be done using the following formula:

D = (1.2 x 10-6 x P x (100 – RH) x T) + (0.00063 x es x (Td – T))

Where D is the specific humidity deficit in g/kg, P is the pressure in hPa, RH is the relative humidity in percent, T is the temperature in Celsius, es is the saturation vapor pressure in hPa, and Td is the dew point temperature in Celsius.

1. Calculate the precipitation or evapotranspiration rate using the specific humidity deficit value. This can be done using the following formula:

P or ET = D x A x 86400Where P or ET is the precipitation or evapotranspiration rate in mm/day, D is the specific humidity deficit in g/kg, A is the area in square meters, and 86400 is the number of seconds in a day.

It is important to note that the conversion from specific humidity in kg/kg to mm/day is an approximation and may be affected by factors such as variability in atmospheric conditions and topography. Therefore, it is recommended to use caution when interpreting the results of this conversion and to consider other factors that may affect precipitation or evapotranspiration rates.

Conclusion

Conversion of ERA5 specific humidity from kg/kg to g/kg or mm/day can be a useful tool for various applications in Earth science. The conversion from kg/kg to g/kg is straightforward and involves multiplying the specific humidity value by 1000. However, the conversion from kg/kg to mm/day is more complex and involves several steps, including the calculation of saturation vapor pressure, actual vapor pressure, dew point temperature, specific humidity deficit, and precipitation or evapotranspiration rate.

When working with ERA5 specific humidity data, it is important to consider the units of other variables that may be used in conjunction with specific humidity. It is also recommended to use caution when interpreting the results of the conversion from specific humidity in kg/kg to mm/day, as it is an approximation that can be affected by various factors.

Overall, understanding how to convert ERA5 specific humidity from kg/kg to g/kg or mm/day can help Earth science researchers and practitioners better analyze and interpret climate data and make more informed decisions related to water resources management, agriculture, and other areas affected by precipitation and evapotranspiration.

FAQs

Q1: What is ERA5?

ERA5 is a global climate reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). It provides hourly meteorological data on a global grid at a resolution of approximately 31 km for the period from 1979 to the present.

Q2: What is specific humidity and how is it expressed in ERA5?

Specific humidity is the mass of water vapor per unit mass of air. It is expressed in kg/kg in ERA5.

Q3: How do you convert specific humidity from kg/kg to g/kg?

The conversion from kg/kg to g/kg involves multiplying the specific humidity value by 1000. For example, if the specific humidity is 0.005 kg/kg, the corresponding value in g/kg would be 5 g/kg.

Q4: Why is it important to consider other variables when converting specific humidity from kg/kg to g/kg?

It is important to consider other variables when converting specific humidity from kg/kg to g/kg because some formulas, such as the one used to calculate dew point temperature, require temperature to be expressed in Kelvin rather than Celsius. Therefore, it is important to ensure that all variables are expressed in the appropriate units.

Q5: How do you convert specific humidity fromkg/kg to mm/day?

The conversion from specific humidity in kg/kg to mm/day is more complex and involves several steps, including calculating the saturation vapor pressure, actual vapor pressure, dew point temperature, specific humidity deficit, and precipitation or evapotranspiration rate. These steps are outlined in detail in the article.

Q6: Is the conversion from specific humidity in kg/kg to mm/day an exact calculation?

No, the conversion from specific humidity in kg/kg to mm/day is an approximation and may be affected by factors such as variability in atmospheric conditions and topography. Therefore, it is recommended to use caution when interpreting the results of this conversion and to consider other factors that may affect precipitation or evapotranspiration rates.

Q7: What are some applications of converting specific humidity ERA5 from kg/kg to g/kg or mm/day?

Converting specific humidity ERA5 from kg/kg to g/kg or mm/day can be useful for various applications in Earth science, such as water resources management, agriculture, and climate modeling. For example, it can help researchers and practitioners to better analyze and interpret climate data and make more informed decisions related to these areas.