Warm Core Structure of Hurricane Erin Diagnosed from High Altitude Dropsondes during CAMEX-4

Abstract

A combination of multiaircraft and several satellite sensors were used to examine the core of Hurricane Erin on 10 September 2001, as part of the Fourth Convection and Moisture Experiment (CAMEX-4) program. During the first set of aircraft passes, around 1700 UTC, Erin was still at its maximum intensity with a central pressure of 969 hPa and wind speed of 105 kt (54 m s−1). The storm was moving slowly northwestward at 4 m s−1, over an increasingly colder sea surface. Three instrumented aircraft, the National Oceanic and Atmospheric Administration (NOAA) P3 with radar, the National Aeronautics and Space Administration (NASA) ER-2 at 19 km, newly equipped with GPS dropwindsondes, and the NASA DC-8 with dropwindsondes flew in formation across the eye at about 1700 UTC and again 2.5 h later around 1930 UTC. The storm had weakened by 13 m s−1 between the first and second eye penetrations. The warm core had a maximum temperature anomaly of only 11°C, located at 500 hPa, much weaker and lower than active hurricanes. The core appeared to slant rearward above 400 hPa. Even on the first penetration, airborne radar showed that the eyewall cloud towers were dying. The tops fell short of reaching 15 km and a melting band was found throughout. The tropopause had a bulge to 15.8-km elevation (environment ∼14.4 km) above the dying convection. The paper presents a consistent picture of the vortex in shear interaction from a primarily thermodynamic perspective. A feature of Erin at this time was a pronounced wavenumber-1 convective asymmetry with all convective activity being confined to the forward quadrants on the left side of the shear vector as calculated from analyses. This is similar to that predicted by the mesoscale numerical models, which also predict that such small amounts of shear would not affect the storm intensity. In Erin, it is remarkable that relatively small shear produced such a pronounced asymmetry in the convection. From the three-dimensional analysis of dropsonde data, horizontal asymmetries in lower and middle tropospheric warming were identified. The warm anomalies are consistent with the pattern of mesoscale vertical motions inferred from the shear-induced wavenumber-1 asymmetry, dipole in rain intensity, and surface convergence.

Interactions between Simulated Tropical Cyclones and an Environment with a Variable Coriolis Parameter

“…Frank and Ritchie (2001) relate the weakening of the tropical cyclone under shear to a disruption in the tropical cyclone struc-ture by 1) the advection of the upper-level structure of the tropical cyclone (where inertial stability is small) downstream, 2) the effect of less optimal eye warming forced by asymmetrically organized convection, and 3) outward mixing of moist static energy from the eye and eyewall by transient eddies in the upper levels. While these results are similar to observational studies of tropical cyclones in shear (e.g., Black et al 2002;Corbosiero and Molinari 2002;Halverson et al 2006), the numerical studies listed above (other than Bender 1997) were limited to studies of tropical cyclones in unidirectional shear on a constant-f plane.…”

A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer

“…The feature is found downwind of a region of convergence that supports a spirallike convective asymmetry dominated by wave number 1. It was this latter convective asymmetry that was emphasised in Halverson et al (2006). Divergence at the surface implies, by mass continuity, downward motion above.…”

“…Arc cloud bands indicate persistent downdrafts. Halverson et al (2006), although focusing on the upper-level warm core structure of Hurricane Erin (2001) in vertical shear, present a map of surface divergence (their Fig. 4, here reproduced as Fig.…”

Passive millimeter-wave signatures of ice particles in Hurricane Erin

“…While CAMEX-4 had broad-based instrumentation on multiple platforms including several aircraft and ground locations, this work focuses on the observations from four instruments on the ER-2 aircraft. On board the ER-2, flying at an altitude of approximately 20 km, the instruments of interest for this work are the High Altitude Monolithic Microwave Integrated Circuit (MMIC) Scanning Radiometer (HAMSR) [3], the Advanced Microwave Precipitation Radiometer (AMPR) [4], the ER-2 Doppler Radar (EDOP) [5], and the ER-2 dropsonde system [6]. The first three instruments measure atmospheric hydrometeors in the microwave region of the electromagnetic spectrum, while the dropsondes are released from the ER-2 and measure temperature, relative humidity and wind speed as they fall toward the Earth's surface.…”

“…The second step involves obtaining appropriate temperature (T), pressure (P), and relative humidity (RH) profiles. Despite the historic data collection efforts of the CAMEX-4 experiment, which included the first stratospheric in-situ sampling [6] of a mature tropical cyclone, there remained significant gaps in the dropsonde data, with only three dropsondes released along the selected 430 km flight path. (See star symbols on Fig.…”