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May 2002

IHOP2002: The search for water vapor

If NCAR offices seem a bit empty this month and next, the reason may be the number of scientists who are taking part in one of the biggest weather studies in North American history. The International H2O Project (IHOP2002), based in Oklahoma, is assembling over 100 people, six aircraft, and a unique mix of instruments from the United States and Europe. Its main goals are to accurately determine how water vapor varies in three dimensions over time and to use that knowledge to improve warm-season precipitation forecasts.

IHOP2002 co-leads Tammy Weckwerth and David Parsons. (Photo by Carlye Calvin.)

"It’s the most demanding project we’ve seen here in a decade," says ATD director Dave Carlson. "It takes virtually all of our ground systems in the field."

Where, when, and how hard it will rain are the most difficult forecast elements to nail down, especially in the warm season (and the large overall gap in precipitation forecast accuracy between summer and winter appears to be growing across the lower 48 states). For that reason, the U.S. Weather Research Program has listed IHOP as a high priority, paving the way for multiagency support. IHOP will combine real-time forecasts with unprecedented high-speed, high-resolution sampling of water vapor from an all-star lineup of moisture sensors.

Unlike many experiments held on the plains during severe-weather season, IHOP won’t be examining showers and thunderstorms themselves as much as the hydrological backdrop that triggers them. "We’re focusing on the moisture content in preconvective situations, when there’s nothing out there," says IHOP co-lead Tammy Weckwerth of ATD. Too many clouds could actually impede some of the lidars and other instruments, so a few clear but humid mornings will be most welcome.

Peggy LeMone of MMM will be part of a team of scientists looking at the processes determining surface fluxes, which may be related to the initiation or evolution of storms. She is also looking at horizontal differences in boundary-layer structure and fluxes. "We don’t fully understand what creates those differences—terrain, or surface properties such as land use and soil moisture," she says. "We hope to elucidate the relative roles of these two factors."

Peggy is also working with a team of scientists to study the little-understood impact of soil moisture on water vapor in the atmosphere. RAP’s Fei Chen, one of the scientists looking into soil moisture, explains: "It’s not enough to look in the atmosphere to see what’s going on with water vapor. We also have to look in the soil and vegetation because they provide the atmosphere with water vapor through complex plant transpiration processes. This is a new front for us." In addition to Fei and Peggy, the soil moisture team includes Jeff Cole, Heather McIntyre, Hatim Sharif, Tom Warner, and David Yates (all in RAP), and Steven Semmer (ATD).

Virtually everyone at ATD is playing a role in IHOP. Dave Parsons is a co-lead for the whole project, and Brigitte Baeuerle and Melinda Tignor are helping to coordinate the division’s IHOP activities.

Finding the right mix

One reason so many different sensors are being trained on water vapor is that no single tool does the job. "Ideally, we’d like to have a radar-type display showing water vapor, but we’re far away from that goal," says Tammy.

Four of the aircraft in IHOP2002 will carry state-of-the-art remote sensing systems, including passive and microwave sounders and differential absorption lidars. The idea is to replicate current and future satellite systems designed to provide vertical profiles of water vapor, temperature, and winds.

NCAR's S-Pol radar will track water vapor above the Oklahoma Panhandle as part of IHOP2002. (Photo by Carlye Calvin.)

In mixing these and other sensors at IHOP, the hope is that each kind of instrument can help shed light on the others’ performance—and, in turn, point the way toward an optimal blend that can truly advance prediction. Researchers will assess how the benefits in forecast skill afforded by improved water vapor measurements stack up against potential improvements in other areas, such as better models or wind-field data.

The IHOP sensors will be located from northern Texas to southern Kansas, accross what is already one of the most heavily monitored patches of atmosphere on the planet. The result will be a feast for boundary layer scientists studying the lowest kilometer of the atmosphere. "We’ll also be looking at how surface and boundary layer variations scale upward to affect [thunderstorm] initiation and evolution," says Dave Parsons.

The IHOP aircraft and ground-based mobile units will monitor conditions in and near atmospheric boundaries, including the recurrent dry line, a frontal feature that often serves as a focus for spring storms. Wind data from profilers already in place across the network will be joined by special radiosonde launches and a variety of fixed measurement platforms added for the experiment, including soil moisture stations.

There’s a strong modeling and nowcasting element to IHOP. Investigators will be collocated in Norman, Oklahoma, with operational forecasters from NOAA’s Storm Prediction Center. They will examine a suite of models run in near–real time using a subset of readily available data. During the field phase, preliminary data products and operations reports will be available through the Web-based catalog that JOSS maintains. JOSS staff, including Dick Dirks, Jim Moore, and Steve Williams, will also take the lead in managing and archiving final data sets, in coordination with the U.S. Department of Energy’s Atmospheric Radiation Measurement project and other participants.

With so many types of water vapor sensors on hand from different sources, ATD investigators will focus on establishing relative accuracies and combining the diverse data into one integrated set. The end product should prove useful in testing and debugging a wide range of models on varying scales.

As for weather forecasts, the ultimate goal is to improve quantitative outlooks of rain and snow. Right now the downpours most threatening to society are still the hardest to predict with precision. When Tropical Storm Allison stalled over Houston in June 2001, some locations got as much as 25 inches of rain (635 millimeters) on a night when official guidance called for amounts on the order of 5 inches (127 millimeters).

Dave Parsons feels a comprehensive look at the water vapor that fuels heavy rain is "science waiting to happen in a number of research areas"—many of them to be addressed through NCAR’s new multiscale strategic initiative on water cycles (see On the Web).

Over a dozen university researchers will be participating in IHOP through NSF grants alone. "It’s probably the largest and most expensive project I’ve ever dealt with," says NSF project officer Stephan Nelson. "It’s also far and away the most complex, in that there are a lot of deployable systems that can be directed to different places depending on the weather. The coordination among those is going to be quite difficult. I think that’ll be one of the biggest challenges of this project." • Bob Henson

On the Web:


IHOP data management (JOSS/ARM)

IHOP/NCAR Soil Moisture, Soil Temperature, and Vegetation Observation Network

NCAR as an Integrator (NCAR strategic plan summary)

GPS goes to work at IHOP

As they slice through the Oklahoma sky, signals from the Global Positioning System will add an important element to IHOP2002. Techniques developed over the past few years allow scientists to infer the amount of water vapor along the slanted path from a GPS satellite to a ground-based receiver. At IHOP, a critical mass of these receivers and other sensors could lead to a breakthrough in the use of GPS to assess moisture overhead.

Over a dozen receivers in the IHOP area are part of SuomiNet, a collaboration established in the late 1990s in honor of satellite meteorology pioneer Verner Suomi. The still-evolving network, coordinated by UOP, has over 30 sites in operation, most at universities. Another 30 are on the way and dozens of others are registered for possible deployment. Each SuomiNet site gathers GPS-based data every 30 seconds and standard surface weather data every 3 minutes. The data are relayed to UCAR’s GPS Science and Technology (GST) program. Though now archived under password protection, SuomiNet data will soon be available through the GST Web site (see below).

Along with the SuomiNet sites, the IHOP region is studded with other GPS receivers from UCAR, DOE, and NOAA. A receiver near Haskell, Oklahoma, has already detected a slight increase in average water vapor since its installation in 1996. Six more receivers will be brought to IHOP by Météo-France.

After the field phase is complete, the investigators (led by GST director Randolph "Stick" Ware) hope to combine the GPS slant-path data with readings from profiling radiometers to produce three-dimensional portraits of water vapor. Another goal, according to GST’s Chris Rocken, is to assimilate the data into weather models. "We hope that in particular the forecasting of convective storms and heavy precipitation will benefit from the detailed water vapor fields we want to obtain from the GPS observations," Chris says. • BH

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Edited by David Hosansky, hosansky@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Tues May 28 17:08:40 MST 2001