Water Vapor Data Helps Meteorologists Predict Whether a Storm Will Strengthen or Weaken

As autumn arrives, the peak of Atlantic hurricane season is upon us, and the amount of water vapor in the atmosphere plays a major role in the strength of tropical cyclones. To quantify water vapor, meteorologists use the term “precipitable water vapor” (PWV) to describe the amount of water vapor contained in a column of air extending from the surface to the top of the atmosphere. 

The most intuitive way to think about PWV is to imagine a wet towel that represents the water vapor in the atmosphere and then picture that wet towel as it is completely wrung out into a glass. The volume of the water in that glass would be the total amount of PWV, as if that water vapor had precipitated from the atmosphere to the ground. Total column water vapor (TCWV) is technically the correct term to describe this measurement; however, in practice, PWV and TCWV are used interchangeably since the total amount of PWV over an area is normally the quantity forecasters are interested in. 

Measuring PWV normally requires the use of radiosondes, which are launched by NASA, weather organizations, and meteorological offices around the world. These instruments measure the atmospheric humidity (in addition to many other variables) at different levels of the atmosphere, creating a sounding, or vertical profile, of the atmosphere. The sounding of atmospheric humidity can then be used to compute PWV. 

Though radiosondes are reliable, the spatial coverage (a single point) and temporal coverage (twice daily) leaves many gaps in the data around the world, especially over the oceans (NOAA, 2014). Instruments such as NASA’s Atmospheric Infrared Sounder (AIRS) have the capability to produce soundings worldwide twice a day (NASA, 2021). AIRS soundings have helped to provide PWV measurements worldwide for over two decades (Figure 1), filling in gaps to complement radiosonde measurements and providing vital information to weather models. 

Areas of high PWV alert forecasters to the potential for heavy precipitation rates and flash flooding. Tropical cyclones are fueled by the condensation of moisture, and areas of high PWV provide ample amounts, enabling the storms to rapidly strengthen and produce extremely heavy rain and wind. 

High sea surface temperatures (SSTs) are also widely known to strengthen tropical cyclones. This, however, is only half the story. Water evaporation increases in areas of high SSTs. The additional water vapor provided to the lower atmosphere from the ocean surface is then wrapped around into the cyclone (Figure 2), causing the storm to strengthen and, in some cases, rapidly intensify.

PWV is commonly reported as the total column value, but as previously mentioned, the link between SSTs and tropical cyclones is the water vapor evaporated from the ocean surface. Therefore, the lower-level PWV, and not the TCWV (in some cases), may be more representative of the moisture fueling a tropical cyclone. 

NASA instruments such as the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard the Terra and Aqua satellites provide lower-atmosphere PWV products such as MOD08_D3 and MY08_D3, which provide measurements from the surface to 680 millibars. These data are useful in quantifying the lower-level moisture, though users are limited to investigating a predetermined atmospheric layer depth.

The AIRS instrument provides soundings of water vapor mixing ratio (WVMR), which represents the ratio of the mass of water vapor to dry air, typically expressed in grams per kilogram (g/kg). WVMR is computed using variables such as dewpoint temperature and pressure. These variables are simpler to measure rather than directly measuring the amount water vapor itself, and therefore WVMR is the variable usually reported from a sounding (Figure 3). 

The vertical distribution of WVMR can then be integrated to approximate the PWV between any two pressure levels of the atmosphere through a short sequence of equations involving the assumption of atmospheric hydrostatic balance and dividing by the density of the air (The COMET Program/MetEd, 2018). 

For users interested in quickly obtaining PWV, the quantity can be estimated graphically from WVMR by using an average mixing ratio between two pressure levels. NASA’s Giovanni makes it simple to obtain soundings of WVMR from AIRS using its vertical profile function. 

Figure 3 shows an AIRS WVMR sounding of coastal Texas the day before Hurricane Harvey's devastating landfall in 2017. This sounding represents a snapshot of the water vapor available directly preceding Harvey's rapid intensification into Category 4 status. Note the high levels of WVMR near the surface: this is one factor that likely contributed to the strengthening of Harvey. 

Tutorial

Users of NASA’s Giovanni can plot their own AIRS WVMR vertical profile and follow this step-by-step tutorial to estimate the PWV at different levels of the atmosphere prior to Hurricane Harvey’s landfall, or they can use the same steps to estimate PWV for their own purposes. With hurricane season in full swing, information about PWV is critical, and instruments like NASA's AIRS can help provide forecasters with vital data that can assist in predicting the intensity and impact of tropical cyclones. 

References

Ann Arbor Earth Science. (2013, April 8). Atmospheric Water Vapor - Precipitable Water.

NASA. (2021, May 15). AIRS - Earth Coverage. NASA.

NOAA. (2014, June 13). Radiosondes | National Oceanic and Atmospheric Administration. JetStream.

Stull, Roland. (2016). Chapter 4 Water Vapor. In Practical Meteorology: An Algebra-Based Survey of Atmospheric Science (pp. 87–118). Book, UBC. 

The COMET Program/MetEd. (2018, June 22). Precipitable Water Definition [Video]. YouTube.