scholarly journals Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part II: Hurricane Mitch

2006 ◽  
Vol 63 (9) ◽  
pp. 2194-2211 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Surface Wind ◽  
Radial Profile ◽  
Surface Friction ◽  
Reduction Factor ◽  
Large Radius ◽  
Hurricane Mitch ◽  
Near Surface ◽  
Upper Boundary ◽  
Good Agreement

Abstract Part I of this paper presented a detailed analysis of the boundary layer of Hurricane Georges (1998), based mainly on the newly available high-resolution GPS dropsonde data. Here, similar techniques and data are used to study Hurricane Mitch (1998). In contrast to Hurricane Georges, the flow in the middle to upper boundary layer near the eyewall is found to be strongly supergradient, with the imbalance being statistically significant. The reason for the difference is shown to be the different radial structure of the storms, in that outside of the radius of maximum winds, the wind decreases much more quickly in Mitch than in Georges. Hurricane Mitch was close to inertially neutral at large radius, with a strong angular momentum gradient near the radius of maximum winds. Kepert and Wang predict strongly supergradient flow in the upper boundary layer near the radius of maximum winds in this situation; the observational analysis is thus in good agreement with their theory. The wind reduction factor (i.e., ratio of a near-surface wind speed to that at some level further aloft) is found to increase inward toward the radius of maximum winds, in accordance with theoretical predictions and the analysis by Franklin et al. Marked asymmetries in the boundary layer wind field and in the eyewall convection are shown to be consistent with asymmetric surface friction due to the storm’s proximity to land, rather than to motion. The boundary layer flow was simulated using Kepert and Wang’s model, forced by the observed storm motion, radial profile of gradient wind, and coastline position; and good agreement with the observations was obtained.

scholarly journals Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part I: Hurricane Georges

2006 ◽  
Vol 63 (9) ◽  
pp. 2169-2193 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Radial Profile ◽  
Lower Altitude ◽  
Near Surface ◽  
Wind Structure ◽  
The Right ◽  
Tropical Cyclone Boundary Layer ◽  
Upper Boundary

Abstract The GPS dropsonde allows observations at unprecedentedly high horizontal and vertical resolution, and of very high accuracy, within the tropical cyclone boundary layer. These data are used to document the boundary layer wind field of the core of Hurricane Georges (1998) when it was close to its maximum intensity. The spatial variability of the boundary layer wind structure is found to agree very well with the theoretical predictions in the works of Kepert and Wang. In particular, the ratio of the near-surface wind speed to that above the boundary layer is found to increase inward toward the radius of maximum winds and to be larger to the left of the track than to the right, while the low-level wind maximum is both more marked and at lower altitude on the left of the storm track than on the right. However, the expected supergradient flow in the upper boundary layer is not found, with the winds being diagnosed as close to gradient balance. The tropical cyclone boundary layer model of Kepert and Wang is used to simulate the boundary layer flow in Hurricane Georges. The simulated wind profiles are in good agreement with the observations, and the asymmetries are well captured. In addition, it is found that the modeled flow in the upper boundary layer at the eyewall is barely supergradient, in contrast to previously studied cases. It is argued that this lack of supergradient flow is a consequence of the particular radial structure in Georges, which had a comparatively slow decrease of wind speed with radius outside the eyewall. This radial profile leads to a relatively weak gradient of inertial stability near the eyewall and a strong gradient at larger radii, and hence the tropical cyclone boundary layer dynamics described by Kepert and Wang can produce only marginally supergradient flow near the radius of maximum winds. The lack of supergradient flow, diagnosed from the observational analysis, is thus attributed to the large-scale structure of this particular storm. A companion paper presents a similar analysis for Hurricane Mitch (1998), with contrasting results.

A New Time-dependent Theory of Tropical Cyclone Intensification

2021 ◽  
Author(s):  
Yuqing Wang ◽  
Yuanlong Li ◽  
Jing Xu
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Numerical Simulations ◽  
Strong Dependence ◽  
Surface Wind ◽  
Time Dependent ◽  
Quasi Equilibrium ◽  
Free Atmosphere ◽  
Near Surface ◽  
Tangential Wind

AbstractIn this study, the boundary-layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal wind balance with eyewall convection becoming in moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies as found recently based on idealized numerical simulations. An ad-hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic and eastern and western North Pacific. Compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as in observations.

scholarly journals Tropospheric BrO column densities in the Arctic derived from satellite: retrieval and comparison to ground-based measurements

2012 ◽  
Vol 5 (11) ◽  
pp. 2779-2807 ◽  
Author(s):  
H. Sihler ◽  
U. Platt ◽  
S. Beirle ◽  
T. Marbach ◽  
S. Kühl ◽  
...  
Keyword(s):  
Boundary Layer ◽  
The Arctic ◽  
Trace Gas ◽  
Natural Phenomenon ◽  
Satellite Measurements ◽  
Surface Layers ◽  
Vertical Column ◽  
Near Surface ◽  
The Vertical Distribution ◽  
Good Agreement

Abstract. During polar spring, halogen radicals like bromine monoxide (BrO) play an important role in the chemistry of tropospheric ozone destruction. Satellite measurements of the BrO distribution have become a particularly useful tool to investigate this probably natural phenomenon, but the separation of stratospheric and tropospheric partial columns of BrO is challenging. In this study, an algorithm was developed to retrieve tropospheric vertical column densities of BrO from data of high-resolution spectroscopic satellite instruments such as the second Global Ozone Monitoring Experiment (GOME-2). Unlike recently published approaches, the presented algorithm is capable of separating the fraction of BrO in the activated troposphere from the total BrO column solely based on remotely measured properties. The presented algorithm furthermore allows to estimate a realistic measurement error of the tropospheric BrO column. The sensitivity of each satellite pixel to BrO in the boundary layer is quantified using the measured UV radiance and the column density of the oxygen collision complex O4. A comparison of the sensitivities with CALIPSO LIDAR observations demonstrates that clouds shielding near-surface trace-gas columns can be reliably detected even over ice and snow. Retrieved tropospheric BrO columns are then compared to ground-based BrO measurements from two Arctic field campaigns in the Amundsen Gulf and at Barrow in 2008 and 2009, respectively. Our algorithm was found to be capable of retrieving enhanced near-surface BrO during both campaigns in good agreement with ground-based data. Some differences between ground-based and satellite measurements observed at Barrow can be explained by both elevated and shallow surface layers of BrO. The observations strongly suggest that surface release processes are the dominating source of BrO and that boundary layer meteorology influences the vertical distribution.

scholarly journals An Analysis of the Physical Characteristics of the Summer Low Atmosphere in the Gobi Desert Adjacent to Bosten Lake, Xinjiang, China

2018 ◽  
Vol 2018 ◽  
pp. 1-10 ◽  
Author(s):  
Yan Li ◽  
Xuejin Sun ◽  
Hui Ning ◽  
Hongcai Qin ◽  
Jiuquan Zhao
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Surface Wind ◽  
Physical Characteristics ◽  
Gobi Desert ◽  
Lake Area ◽  
Desert Area ◽  
Local Circulation ◽  
Bosten Lake ◽  
Near Surface

A month-long field observation campaign was conducted, which covered approximately 100 km2 of the Gobi Desert area on the southeast bank of Bosten Lake during the summer of 2016. The purpose of the study was to examine the physical characteristics of the low atmosphere over land-lake nonuniform underlying surfaces in the Gobi Desert of northwestern China. The results of the statistical analysis showed that, during the observational period, the average daytime surface horizontal thermal gradient reached up to −0.2°C/km from the lakeshore to southern Gobi Desert area. The near-surface wind field of the 7 km horizontal extent from the lakeshore was dominated by onshore breezes with average peak wind speeds above 5 m/s. In the atmospheric near-surface layer, an isohumidity layer at a height between 10 and 50 m a.g.l. was observed from 11:00 to 18:00 LST. Also, a case study for the atmospheric boundary layer and local circulation analyses was conducted. The onshore breezes were found to play a major role in the vertical structure of the local atmospheric boundary layer. The numerical simulation results indicated that there was an alternating day-night local circulation in the Bosten Lake area.

scholarly journals Parameterization of convective transport in the boundary layer and its impact on the representation of the diurnal cycle of wind and dust emissions

2015 ◽  
Vol 15 (12) ◽  
pp. 6775-6788 ◽  
Author(s):  
F. Hourdin ◽  
M. Gueye ◽  
B. Diallo ◽  
J.-L. Dufresne ◽  
J. Escribano ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
General Circulation ◽  
Surface Wind ◽  
Circulation Model ◽  
Convective Transport ◽  
Strong Nonlinearity ◽  
Thermal Plumes ◽  
Near Surface ◽  
Dust Emissions

Abstract. We investigate how the representation of the boundary layer in a climate model impacts the representation of the near-surface wind and dust emission, with a focus on the Sahel/Sahara region. We show that the combination of vertical turbulent diffusion with a representation of the thermal cells of the convective boundary layer by a mass flux scheme leads to realistic representation of the diurnal cycle of wind in spring, with a maximum near-surface wind in the morning. This maximum occurs when the thermal plumes reach the low-level jet that forms during the night at a few hundred meters above surface. The horizontal momentum in the jet is transported downward to the surface by compensating subsidence around thermal plumes in typically less than 1 h. This leads to a rapid increase of wind speed at surface and therefore of dust emissions owing to the strong nonlinearity of emission laws. The numerical experiments are performed with a zoomed and nudged configuration of the LMDZ general circulation model coupled to the emission module of the CHIMERE chemistry transport model, in which winds are relaxed toward that of the ERA-Interim reanalyses. The new set of parameterizations leads to a strong improvement of the representation of the diurnal cycle of wind when compared to a previous version of LMDZ as well as to the reanalyses used for nudging themselves. It also generates dust emissions in better agreement with current estimates, but the aerosol optical thickness is still significantly underestimated.

scholarly journals The climatology of planetary boundary layer height in China derived from radiosonde and reanalysis data

2016 ◽  
Author(s):  
Jianping Guo ◽  
Yucong Miao ◽  
Yong Zhang ◽  
Huan Liu ◽  
Zhanqing Li ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Reanalysis Data ◽  
Near Surface ◽  
Layer Height ◽  
Boundary Layer Height ◽  
Early Afternoon ◽  
Fine Resolution

Abstract. The important roles of planetary boundary layer (PBL) in climate, weather and air quality have long been recognized, but little has been known about the PBL climatology in China. Using the fine-resolution sounding observations made across China and a reanalysis data, we conducted a comprehensive investigation of the PBL in China from January 2011 to July 2015. The boundary layer height (BLH) is found to be generally higher in spring and summer than that in fall and winter. The comparison of seasonally averaged BLH derived from observations and reanalysis shows good agreement. The BLH derived from three- or four-times-daily soundings in summer tends to peak in the early afternoon, and the diurnal amplitude of BLH is higher in the northern and western sub-regions of China than other sub-regions. The meteorological influence on the annual cycle of BLH are investigated as well, showing that BLH at most sounding sites is negatively associated with the surface pressure and lower tropospheric stability, but positively associated with the near-surface wind speed and temperature. This indicates that meteorology plays a significant role in the PBL processes. Overall, the key findings obtained from this study lay a solid foundation for us to gain a deep insight into the fundamentals of PBL in China, which helps understand the roles of PBL playing in the air pollution, weather and climate of China.

Adaptation of the near-surface wind to the development of sand transport

1993 ◽  
Vol 252 ◽  
pp. 99-115 ◽  
Author(s):  
Ian K. Mcewan ◽  
Brian B. Willetts
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Wind ◽  
Internal Boundary Layer ◽  
Internal Boundary ◽  
Sand Transport ◽  
Temporal Response ◽  
Near Surface ◽  
Spatial Response ◽  
Aerodynamic Roughness

A model of wind-blown sand transport is described with particular emphasis on the feedback between the grain cloud and the near-surface wind. The results from this model are used to develop Owen's (1964) hypothesis that ‘the grain layer behaves, so far as the flow outside it is concerned, as increased aerodynamic roughness whose height is proportional to the thickness of the layer’. The hypothesis is developed to show the influence this dynamic roughness has on the turbulent boundary layer above the saltation layer. Two processes are identified which influence the path of the system towards equilibrium. The first is the feedback between the near-surface wind and the grain cloud in which the quantity of sand transported is limited by the carrying capacity of the wind. The second is due to the temporal development of an internal boundary layer in response to the additional roughness imposed on the flow above the grain layer by the grain cloud. A similarity is noted between the temporal response of a turbulent boundary layer to sand transport and the spatial response of a turbulent boundary layer downstream of a step increase in surface roughness. Finally it is noted that the work may have important implications for transport rate prediction in unsteady winds.

scholarly journals Effect of wind speed on aerosol optical depth over remote oceans, based on data from the Maritime Aerosol Network

2012 ◽  
Vol 5 (2) ◽  
pp. 377-388 ◽  
Author(s):  
A. Smirnov ◽  
A. M. Sayer ◽  
B. N. Holben ◽  
N. C. Hsu ◽  
S. M. Sakerin ◽  
...  
Keyword(s):  
Wind Speed ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Surface Wind ◽  
Correlation Coefficients ◽  
Surface Wind Speed ◽  
Near Surface ◽  
Strong Winds ◽  
The Relationship ◽  
Good Agreement

Abstract. The Maritime Aerosol Network (MAN) has been collecting data over the oceans since November 2006. The MAN archive provides a valuable resource for aerosol studies in maritime environments. In the current paper we investigate correlations between ship-borne aerosol optical depth (AOD) and near-surface wind speed, either measured (onboard or from satellite) or modeled (NCEP). According to our analysis, wind speed influences columnar aerosol optical depth, although the slope of the linear regression between AOD and wind speed is not steep (~0.004–0.005), even for strong winds over 10 m s−1. The relationships show significant scatter (correlation coefficients typically in the range 0.3–0.5); the majority of this scatter can be explained by the uncertainty on the input data. The various wind speed sources considered yield similar patterns. Results are in good agreement with the majority of previously published relationships between surface wind speed and ship-based or satellite-based AOD measurements. The basic relationships are similar for all the wind speed sources considered; however, the gradient of the relationship varies by around a factor of two depending on the wind data used.

scholarly journals Strong Down-Valley Low-Level Jets over the Atacama Desert: Observational Characterization

2013 ◽  
Vol 52 (12) ◽  
pp. 2735-2752 ◽  
Author(s):  
Ricardo C. Muñoz ◽  
Mark J. Falvey ◽  
Marcelo Araya ◽  
Martin Jacques-Coper
Keyword(s):  
Surface Wind ◽  
Atacama Desert ◽  
Surface Friction ◽  
Gradient Force ◽  
Turbulence Structure ◽  
Pressure Gradient Force ◽  
Surface Cooling ◽  
Environmental Implications ◽  
Near Surface ◽  
Drainage Flows

AbstractThe near-surface wind and temperature regime at three points in the Atacama Desert of northern Chile is described using two years of multilevel measurements from 80-m towers located in an altitude range between 2100 and 2700 m MSL. The data reveal the frequent development of strong nocturnal drainage flows at all sites. Down-valley, nose-shaped wind speed profiles are observed, with maximum values occurring at heights between 20 and 60 m AGL. The flow intensity shows considerable interdaily variability and a seasonal modulation of maximum speeds, which in the cold season can attain hourly average values of more than 20 m s−1. Turbulent mixing appears to be important over the full tower layer, affecting the curvature of the nighttime temperature profile and possibly explaining the observed increase of surface temperatures in the down-valley direction. Nocturnal valley winds and temperatures are weakly controlled by upper-air conditions observed at the nearest aerological station. Estimates of terms in the momentum budget for the development and quasi-stationary phases of the down-valley flows suggest that the pressure gradient force due to the near-surface cooling along the sloping valley axes plays an important role in these drainage flows. A scale for the jet nose height of equilibrium turbulent down-slope jets is proposed that is based on surface friction velocity and surface inversion intensity. At one of the sites, this scale explains about 70% of the case-to-case observed variance of jet nose heights. Further modeling and observations are needed, however, to define better the dynamics, extent, and turbulence structure of this flow system, which has significant wind-energy, climatic, and environmental implications.

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