Where Did That Water Vapor Come From?

ARM Alaskan cloud radar data are used to evaluate a new method for identifying the water vapor source regions behind precipitation events.

The Science

When it rains at a particular location, the atmospheric water vapor that ends up as rainfall may have evaporated and been transported from a long distance away or may have come from a nearby lake or ocean. Understanding the origins and transport patterns of water vapor in the atmosphere are important for understanding the response of the hydrologic cycle to changes in atmospheric circulation and thermal structure.  One method that scientists use to determine water vapor source regions, known as back trajectory analysis, involves tracking air parcels backwards in time using measured or modeled wind fields.  An important error source in these analyses is the initial altitude of the air parcel used to initiate the back trajectory since both wind speed and direction can vary significantly with height.  Scientists use cloud radar data from the Atmospheric Radiation Measurement (ARM) site in Barrow (Utqiavik), AK to evaluate a new approach for determining the initial altitude for back trajectory analyses.

The Impact

Vapor sources identified by back trajectories are sensitive to the choice of arrival heights, and so are the conclusions (e.g., meteorological conditions at the source) drawn from the analysis. By incorporating meteorological and precipitation record information, it is possible to retain the simplicity and efficiency of using a small number of back trajectories yet have the representation of whole air column tracking. Compared with a comprehensive air column tracking method, the new method minimizes the computation time required to run back trajectories and the post-processing time to extract the subset of trajectories relevant to an event, with no loss in useful information.  Comparison of the initial heights used in the new method with those derived from the ARM cloud radar measurements identifies cloud and precipitation conditions under which the new method is most applicable.


Lagrangian air parcel tracking is a powerful tool for estimating vapor source locations, particularly for isotope hydrology applications. Identified vapor source regions may be sensitive to the distribution of altitudes at which back trajectories are initiated. Ideally, those initial altitudes should reflect the altitudes where precipitation forms. This paper introduces a novel method for estimating these heights from reanalysis data and an air parcel lofting routine, which is referred to as the “Reanalysis” method. Using Barrow, Alaska (now known as Utqiagvik), as a test site, the study compares the distribution of air parcel initiation heights and vapor source conditions from back trajectories initiated at 1) heights determined by the Reanalysis method and 2) heights acquired from the ARM 35-GHz vertically resolved cloud radar data, termed the “Cloud Radar” method. Only 2 of the 70 events failed to produce condensation at any elevation. The distribution of air parcels generated by each method was compared based on the median height and the median-adjusted overlap. The excellent and good category events produced similar estimates of vapor source conditions, indicating that the median height of the back-trajectory initialization was a more important predictor of the vapor source location than the shape of the condensation profile. Poorly matched events tended to result from rain events where the Reanalysis method yielded much higher median heights than the Cloud Radar method. Many of these events were characterized by liquid precipitation from thin low-elevation clouds. The large positive bias of the Reanalysis method may be due to attenuation of the radar, precipitation localized to Barrow, or lower-resolution reanalysis data not representing thin cloud layers.

Based on the comparison with ARM radar data, the Reanalysis method can represent a point within a pixel when precipitation results from cloud layers thick enough to be resolved by the Reanalysis and the average conditions of the pixel are conducive to precipitation. The method tends to be less accurate with complex temperature and humidity profiles. This method should be used with caution when the meteorology indicates precipitation from intense convection, or very localized conditions. For studies of vapor source characteristics, this method will produce accurate and spatially robust results. It is an improvement over methods that track a small number of air parcels from fixed heights, yet it remains relatively computationally inexpensive.

Principal Investigator(s)

Annie Putman
University of Utah


The U.S. Department of Energy Office of Science resource, the Atmospheric Radiation Measurement User Facility (ARM) within the Biological and Environmental Research program, was critical to this research. This project was supported by NSF Grant 1022032.


Putman A, X. Feng, E. Posmentier, A. Faiia, and L. Sonder. “Testing a Novel Method for Initializing Air Parcel Back Trajectories in Precipitating Clouds Using Reanalysis Data.” Journal of Atmospheric and Oceanic Technology, 34(11) (2017). [DOI:10.1175/JTECH-D-17-0053.1]