Hi, my name is Anna Wilson. I moved to Asheville, NC in the summer of 2004, just before several floods caused by tropical storms left me without power for more than a week. This spurred my previously casual interest in the weather to become an obsession that eventually prompted me to go back to school. Here I am with a PARSIVEL disdrometer (an instrument that measures the size and velocity of particles that pass through its sampling area), after I completed my bachelor’s degree in Atmospheric Science and started working towards my PhD in Environmental Engineering:
(photo credit: Daniel Moraff)
Starting when I was an undergraduate at the University of North Carolina at Asheville, I have been collecting data from tipping bucket rain gauges deployed in the Southern Appalachian Mountains in support of the GPM mission. During Intensive Observation Periods conducted during shorter, large scale field campaigns, I was able to work with many different types of instruments. Some of them are pictured here:
(photo credit: Gregory Cutrell)
From the left: a weather station, Micro Rain Radar, 3 channel microwave radiometer, tipping bucket rain gauge, and PARSIVEL disdrometer.
In the Southern Appalachians as well as in other mountainous regions worldwide, it is a big challenge to collect enough observations to fully understand events that are highly spatially heterogeneous (which means the conditions vary a lot from place to place within the storm). This heterogeneity is a common feature of weather events in complex terrain. Radars can’t see inside valleys, and often have to be set up to point relatively high up from the ground in order for the signal not to be blocked by the mountains. Unfortunately, there can be high variability in the reflectivity throughout the vertical column, and so what the radar sees at high altitude does not necessarily reflect the precipitation intensity as experienced at the ground surface. Satellites are a really exciting solution since they can see from the top of the atmosphere all the way down to the ground. At this point there are still limitations, such as ground clutter issues (radar bouncing off things that aren’t rain, like trees or the ground, and therefore not what scientists are interested in) and the problem of spatial heterogeneity in the terrain height and land surface within the resolution that is observed by the satellite (land features of different heights within the area the satellite is looking at). Part of the goal addressed by ground validation in the mountains is to help figure out how to be able to interpret those satellite observations to obtain the maximum information available. For ground validation, particularly in remote sites and for long time periods, rain gauges are the most simple and affordable solution. However, it is not possible to place enough rain gauges on the ground to be able to get a full sense of the precipitation event structure. Have you seen the GPM introductory video yet? Check out the part about all of the rain gauges in the world. Before you watch, how much space do you think all those rain gauges might take up if they were all collected in one place?
As I’m sure the students working on collecting rain and snow data at school know, a lot of interesting things can happen to the rain gauges! It is very important to keep the rain gauges well maintained and to keep good records of any problems you may have had, so that the data used for ground validation of the satellites have minimal uncertainty. My research group has maintained a network of remote field sites since 2007 and we have learned many lessons from this network. I will share some of them here.
First: While the rain gauges in our network are very self-sufficient and run on lithium batteries that can last for several years, we can’t leave them unattended for very long. In particular, they need to be visited pretty frequently during the summer, because of fast growing vegetation, and the fall, because of falling leaves. See below for an image from our first year of a rain gauge in the Pisgah National Forest with the overgrown vegetation, including before and after our maintenance visit (I hope you can tell which is which!)
(photo credits: University of North Carolina at Asheville Precipitation Measurement Mission Group)
Next, we learned that depending on the location, sometimes birds found our rain gauges to be a perfect place to perch! Before they left, they often left droppings on the gauge top. These need to be cleaned out so that they don’t impair the measurements. In some places, we had to install a circular set of spikes all along the edge of the rain gauge catch area! This worked really well to dissuade the birds from landing on the gauge top. At one of our sites, the Appalachian Highlands Science Learning Center in the National Park, we have an area where we test all of the different configurations and models of the rain gauges that we use in our network side by side so that we can make sure that any adjustments we make to the gauges (such as the bird protection spikes) don’t affect the measurements.
(photo credit: University of North Carolina at Asheville Precipitation Measurement Mission Group)
Birds aren’t the only creatures who find these instruments attractive! Especially during the fall and winter in the first year, we often had to evict insects from the inside of the gauges. They can climb up through the screens below the tipping buckets. We have found spiders, ants (as seen here), ladybugs, and sometimes wasps’ nests! One method we figured out to alleviate this problem was to add dryer sheets with fragrance to the inside of the gauges, as it repels the insects. We duct tape it to the inside of the gauge so that it is below the cover but outside the section with the tipping bucket, so that our measurements will not be compromised.
(photo credit: Daniel Martin)
Last, particularly in the Great Smoky Mountains National Park, we find that black bears are very curious about the gauges. We saw evidence of the bears during visits, such as scratches on the outside of the gauge, or bear hairs caught in the top. We were worried that the bears might be causing spurious tips on the rain gauges, affecting their accuracy, so we installed motion-detecting cameras in nearby trees to capture the animals when they approached the rain gauges. Now, with the animal cams, we can remove any extraneous tips! Luckily, although the bears seem to like using the gauges as scratching posts and scent marking them, they rarely cause tips, and even more rarely cause serious damage to the gauges such as knocking them over.
(photo is a still from a video captured on our animal cam)
Although it is quite a challenge to take robust precipitation measurements in these remote locations, I hope that you agree with me that it is worth it! Thank you all for being a part of this important project. Also, if you are reading this post, live in the United States or Canada, are not already taking rain gauge measurements, please consider being a part of CoCoRaHS (http://www.cocorahs.org/). That stands for Community Collaborative Rain, Hail & Snow Network, and is a great way to be a citizen scientist!