My PSU Meteorology graduate student, Matt Cann, and I have embarked on an exciting summer field project to learn more about how topography influences the spatial distribution of precipitation. We are using the Hubbard Brook Experimental Forest (HBEF) as our study location due to the network of 25 precipitation gauges that have been there for over 40 years. Matt’s inspection of this dataset has revealed that during precipitation events of >1.5 cm with a south wind, the precipitation amounts decrease with elevation – opposite of what typically occurs!

 

We were shocked when we saw this pattern. After some thinking and consulting mountain weather literature, we hypothesize that as the south wind blows over the southern ridge of HBEF, it develops a mountain wave and rotor on the leeside that focuses precipitation in the valley where the Hubbard Brook runs. The mountain wave starts to rise upward as the flow continues northward to the northern ridge of HBEF, but has not rose enough to increase precipitation amounts again.

 

Before we can truly test this hypothesis by taking wind measurements across the valley and numerically modeling it, we must first confirm that the highest precipitation amounts are occurring in the deepest part of the valley – there is a latitudinal gap in the precipitation gauge network around the brook. Matt and I decided we should deploy a series of rain gauges along the brook for this summer to capture at least six rain events >1.5cm with a southerly wind component.

 

After scouting the HBEF for places with a large enough canopy opening to place rain gauges, we determined we must place gauges IN the brook itself. We hiked along the brook in mid-May when it was very dry and the brook stage height was very low – the brook was easy to cross by hopping from stone-to-stone. We planned weeks in advance to install four rain gauges in the brook on Monday, June 1. If you live in central New England, you may recall the two inches of rain that fell from Saturday through Monday. Matt and I decided to see if the brook was still navigable with all the rain. We knew the brook would be higher, but we were still a bit surprised at just how high the water was – it was a raging torrent of water!

The brook was just navigable enough at the site for the most upstream gauge (see picture above) and Matt did a tremendous job of installing the first gauge. We also installed some gauges on land next to the existing network of gauges to measure any catchment differences due to our different type of gauge. At the other brook sites, the water was much less forgiving than the first site and we decided to install the other three brook gauges on Wednesday after the water level subsided. Sure enough, the brook was back to a more tame flow on Wednesday (see picture below) and we could easily reach each site by way of dry, exposed stones.

Now that the gauges have been installed, we will empty them the day prior to any 1.5 cm rainfall occurs and check the rainfall amounts the day after the rain. The weather models indicate the first such event could be as soon as this Monday. Matt and I are eager to measure the rainfall on Tuesday and find out if indeed the highest amounts fall at the low elevation of the brook.

 

Stay tuned for more updates as our research continues through this summer!