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Ke Peng
,
Richard Rotunno
,
George H. Bryan
, and
Juan Fang

Abstract

In a previous study, the authors showed that the intensification process of a numerically simulated axisymmetric tropical cyclone (TC) can be divided into two periods denoted by “phase I” and “phase II.” The intensification process in phase II can be qualitatively described by Emanuel’s intensification theory in which the angular momentum (M) and saturated entropy (s*) surfaces are congruent in the TC interior. During phase I, however, the M and s* surfaces evolve from nearly orthogonal to almost congruent, and thus, the intensifying simulated TC has a different physical character as compared to that found in phase II. The present work uses a numerical simulation to investigate the evolution of an axisymmetric TC during phase I. The present results show that sporadic, deep convective annular rings play an important role in the simulated axisymmetric TC evolution in phase I. The convergence in low-level radial (Ekman) inflow in the boundary layer of the TC vortex, together with the increase of near-surface s* produced by sea surface fluxes, leads to episodes of convective rings around the TC center. These convective rings transport larger values of s* and M from the lower troposphere upward to the tropopause; the locally large values of M associated with the convective rings cause a radially outward bias in the upper-level radial velocity and an inward bias in the low-level radial velocity. Through a repetition of this process, the pattern (i.e., phase II) gradually emerges. The role of internal gravity waves related to the episodes of convection and the TC intensification process during phase I is also discussed.

Full access
Qingguo Fang
,
Kekuan Chu
,
Bowen Zhou
,
Xunlai Chen
,
Zhen Peng
,
Chunsheng Zhang
,
Ming Luo
, and
Chunyang Zhao

Abstract

Based on turbulence measurements from sonic anemometers instrumented at multiple levels on a 356-m-tall meteorological tower located on the south coast of China, an observation study of the turbulent dissipation rate (ε) in a landfalling tropical cyclone boundary layer (TCBL) is conducted. Three indirect methods (i.e., the power spectra, the second- and the third-order structure functions) are compared for the calculation of ε. The third-order structure function computes the smallest ε among the three methods, but shows the largest uncertainty. The second-order structure function gives similar ε estimates as the power spectra, and is adopted for its reduced uncertainty. The measured ε in the landfalling TCBL is O(10−1) m2 s−3, much greater than typical atmospheric boundary layer values as well as oceanic TCBL values. The value of ε is found to scale with the local friction velocity rather than the surface friction velocity, implying a highly localized nature of turbulence. Conventional parameterizations of ε are evaluated against observations. Process-based ε models assuming a local balance between shear production and dissipation prove inadequate, as shear production merely accounts for half of the dissipation away from the surface. In comparison, scaling-based ε models used by planetary boundary layer (PBL) schemes are more advantageous. With both tuning of the model coefficients and adjustment of the dissipation length scale, the performance of an ε model in a widely used PBL scheme is shown to produce similar values to the observations.

Significance Statement

Dissipation in a turbulent flow refers to the conversion of kinetic energy to heat through molecular viscosity, and is of key dynamic and thermal–dynamic importance in the tropical cyclone boundary layer (TCBL). Past observations of ε in the TCBL have been rare. Few existing ones are based on surface and flight measurements that are either constrained in height or limited in time. This study utilizes turbulence measurements taken from a 356-m meteorological tower to study dissipation of a landfalling TC. The tall tower platform offers a valuable turbulence dataset that extends beyond the surface layer of the TCBL. The observations are used to improve the physical understanding of dissipation, and to evaluate diagnostic ε models for numerical weather prediction.

Restricted access
J. Li
,
J. Steppeler
,
F. Fang
,
C. C. Pain
,
J. Zhu
,
X. Peng
,
L. Dong
,
Y. Li
,
L. Tao
,
W. Leng
,
Y. Wang
, and
J. Zheng
Open access
Jingping Duan
,
Michael Bevis
,
Peng Fang
,
Yehuda Bock
,
Steven Chiswell
,
Steven Businger
,
Christian Rocken
,
Frederick Solheim
,
Terasa van Hove
,
Randolph Ware
,
Simon McClusky
,
Thomas A. Herring
, and
Robert W. King

Abstract

A simple approach to estimating vertically integrated atmospheric water vapor, or precipitable water, from Global Positioning System (GPS) radio signals collected by a regional network of ground-based geodetic GPS receiver is illustrated and validated. Standard space geodetic methods are used to estimate the zenith delay caused by the neutral atmosphere, and surface pressure measurements are used to compute the hydrostatic (or “dry”) component of this delay. The zenith hydrostatic delay is subtracted from the zenith neutral delay to determine the zenith wet delay, which is then transformed into an estimate of precipitable water. By incorporating a few remote global tracking stations (and thus long baselines) into the geodetic analysis of a regional GPS network, it is possible to resolve the absolute (not merely the relative) value of the zenith neutral delay at each station in the augmented network. This approach eliminates any need for external comparisons with water vapor radiometer observations and delivers a pure GPS solution for precipitable water. Since the neutral delay is decomposed into its hydrostatic and wet components after the geodetic inversion, the geodetic analysis is not complicated by the fact that some GPS stations are equipped with barometers and some are not. This approach is taken to reduce observations collected in the field experiment GPS/STORM and recover precipitable water with an rms error of 1.0–1.5 mm.

Full access
Angelyn W. Moore
,
Ivory J. Small
,
Seth I. Gutman
,
Yehuda Bock
,
John L. Dumas
,
Peng Fang
,
Jennifer S. Haase
,
Mark E. Jackson
, and
Jayme L. Laber

Abstract

During the North American Monsoon, low-to-midlevel moisture is transported in surges from the Gulf of California and Eastern Pacific Ocean into Mexico and the American Southwest. As rising levels of precipitable water interact with the mountainous terrain, severe thunderstorms can develop, resulting in flash floods that threaten life and property. The rapid evolution of these storms, coupled with the relative lack of upper-air and surface weather observations in the region, make them difficult to predict and monitor, and guidance from numerical weather prediction models can vary greatly under these conditions. Precipitable water vapor (PW) estimates derived from continuously operating ground-based GPS receivers have been available for some time from NOAA’s GPS-Met program, but these observations have been of limited utility to operational forecasters in part due to poor spatial resolution. Under a NASA Advanced Information Systems Technology project, 37 real-time stations were added to NOAA’s GPS-Met analysis providing 30-min PW estimates, reducing station spacing from approximately 150 km to 30 km in Southern California. An 18–22 July 2013 North American Monsoon event provided an opportunity to evaluate the utility of the additional upper-air moisture observations to enhance National Weather Service (NWS) forecaster situational awareness during the rapidly developing event. NWS forecasters used these additional data to detect rapid moisture increases at intervals between the available 1–6-h model updates and approximately twice-daily radiosonde observations, and these contributed tangibly to the issuance of timely flood watches and warnings in advance of flash floods, debris flows, and related road closures.

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