Kinematic and Moisture Characteristics of a Nonprecipitating Cold Front Observed during IHOP. Part I: Across-Front Structures

Abstract

A wide array of ground-based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP). This study, the first of two parts, is focused on describing structures in the across-front dimension. Coarsely resolved observations from the operational network and dropsondes deployed over a 200-km distance centered on the front are combined with higher-resolution observations from in situ sensors, Doppler radars, a microwave radiometer, and a differential absorption lidar that were collected across a ?40-km swath that straddled a ?100-km segment of the front. The northeast?southwest-oriented cold front moved toward the southeast at ?8?10 m s?1 during the morning hours, but its motion slowed to less than 1 m s?1 in the afternoon. In the early afternoon, the cold front separated cool air with a northerly component flow of 2?4 m s?1 from a 10-km-wide band of hot, dry air with 5 m s?1 winds out of the south-southwest. The average updraft at the frontal interface was ?0.5 m s?1 and slightly tilted back toward the cool air. A dryline was located to the southeast of the front, separating the hot, dry air mass from a warm, moist air mass composed of 10 m s?1 southerly winds. Later in the afternoon, the warm, moister air moved farther to the northwest, approaching the cold front. The dryline was still well observed in the southwestern part of the observational domain while it vanished almost completely in the northeastern part. Low-level convergence (?1 ? 10?3 s?1), vertical vorticity (?0.5 ? 10?3 s?1), and vertical velocity (?1 m s?1) increased. The strong stable layer located at ?2.0?2.5 km MSL weakened in the course of the afternoon, providing a basis for the development of isolated thunderstorms. The applicability of gravity current theory to the cold front was studied. There was evidence of certain gravity current characteristics, such as Froude numbers between 0.7 and 1.4, a pronounced feeder flow toward the leading edge, and a rotor circulation. Other characteristics, such as a sharp change in pressure and lobe and cleft structures, remain uncertain due to the temporally and spatially variable nature of the phenomenon and the coarse resolution of the measurements.