Testing a Novel Method for Initializing Air Parcel Back Trajectories in Precipitating Clouds Using Reanalysis Data

Abstract

AbstractLagrangian 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 Utqiaġvik), 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 35-GHz vertically resolved cloud radar, 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 on the basis of the median height and the median-adjusted overlap, and yielded excellent (n = 28), good (n = 20), fair (n = 6), and poor (n = 14) matches between the two methods. The excellent and good category events tended to produce condensation profiles with lower median heights, which translated to more similar vapor source characteristics. Poorly matched events tended to result from rain events where the Reanalysis method yielded much higher median heights than the Cloud Radar method.