Seasonal Changes of Moderately Wet Days

Figure 1:

Figure 1: The modeled track of Hurricane Joaquin as of Friday (10/2/2015) morning. credit

This past week, everyone on the U.S. East Coast has been following Hurricane Joaquin. Luckily, it is expected to go out the sea and not make landfall, but this hurricane got my state (Maryland) to think about storm preparedness.

This cause for concern is completely understandable! These massive precipitation events, such as Irene, cause damage that threatens both life and property, not to mention damage to coastal ecosystems!

But this post is not about these massive, hurricane-level, storms. It’s about the smaller rain events you probably don’t think twice about.

Moderately Wet Day Index

Figure 2:

Figure 2: Our new Moderately Wet Day index counts the frequency of days when 0.4 to 0.8 inches of precipitation fell. That’s less than a weekly watering of your strawberry plant!

For this project, we created a new “extreme” climate index. I put extreme in quotation marks since this new index is not describing extreme weather; rather, it’s determining fairly common weather! The Moderately Wet Days index determines the annual count of days when 10 to 20 mm of liquid precipitation fell.

That is between 0.4 and 0.8 inches of precipitation in one day. To put this to scale: most home gardeners (like myself) know that strawberries should get ~1 inch of water per week. Thus, this new index is less than what you water your home garden weekly!

I have deemed this index Moderately Wet Days. You can calculate it by subtracting the R10mm index (count of days when > 10 mm of precipitation fell) from the R20mm index (count of days when > 20 mm of precipitation fell).

Moderately Rainy Days = R10mm-R20mm

I am happy to hear out a better marketing name!

Have Moderately Wet Day change in Chesapeake Bay?

Figure 3:

Figure 3: The 21-year moving mean for the Moderately Wet Day index in the Northern Chesapeake region.

As defined, Moderately Wet Days is derived as an annual index. Figure 3 is the 21-year moving mean for the Northern Chesapeake region. While there is a cyclic pattern to the time series, it is apparent that these Moderately Wet days have historically increased. We can loosely say that the Northern Chesapeake has observed 4 to 5 more moderately wet days over the last century.

Think of this as 5 additional gloomy days when you’re likely to stay inside and binge on Netflixs or perhaps have to bring out the umbrella and raincoat for your trip to work.

Since we created our own time series by aggregating 9 NCDC-Daily weather stations in the Northern Chesapeake, I was able to use R to ask the question: How are Moderately Wet Days changing by season?

Figure 4:

Figure 4: The seasonal patterns of Moderately Wet Days in the Northern Chesapeake region.

It’s ironic that a few posts ago, I conducted this exercise specifically on the summer season and saw no change over time. Figure 4 shows this “no trend” in a new way. But since the annual Moderately Wet Days time series shows an increase, we should see a trend somewhere!

All seasons except summer appear to have increased over the last century. We can use the slope to estimate historic rate of change: Winter (0.1 days per decade), Spring (1.7 days per decade), and Fall (2.1 days per decade).

That’s pretty fascinating!

To bring this full circle: these smaller precipitation events may not be as damaging or media-worthy as a hurricane, but they still matter! Small storm events carried nitrogen to the tributaries in Chesapeake Bay. Even though we have begun reducing our nutrient footprint in the Chesapeake, it will be interesting to see how increases  in wet day frequency will affect this ecological issue!

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
This entry was posted in Ecosystem Stories, Precipitation and tagged , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *