Internal gravity waves convectively forced in the atmospheric residual layer during the morning transition

Generation of internal gravity waves in the boundary layer is investigated from observations. Simultaneous measurements from a 2 µm Doppler lidar and a 0.5 µm backscatter lidar are combined to analyse the occurrence, or not, of internal gravity waves in the residual layer during the morning transition on two days, 10 and 14 June 2005. Three cases are studied for illustrating three different flow configurations in the residual layer: no wave, evanescent wave and propagating wave. Comparison of the three cases suggests two necessary conditions for the generation of gravity waves: a stably stratified residual layer and a convective boundary layer with mechanical forcing frequencies less than the Brunt–Vaisala frequency. The horizontal wind shear probably plays a role in the dynamics of the waves, but, in the cases analysed, it is not sufficient alone to generate the observed waves through the obstacle effect. In the case of wave propagation, the waves tilt upstream and against the wind shear, with a typical horizontal wavelength and a line phase direction with respect to the vertical of 2.4 km and 32°, respectively. Unexpectedly, we found that measurements of the wave-associated vertical velocity and the displacement of tracers (0.5 µm depolarization ratio or 2 µm backcatter, both indicative of relative humidity fluctuations) are in phase. Possible explanations include: (i) aerosol particles are not passive with respect to temperature or water vapour fluctuations; or (ii) a nonlinear wave-turbulence interaction is at work and needs further investigation. Copyright © 2011 Royal Meteorological Society

[1]  C. Nappo.  An introduction to atmospheric gravity waves , 2002 .

[2]  T. Böhme,et al.  Investigation of short‐period gravity waves with the Lindenberg 482 MHz tropospheric wind profiler , 2004 .

[3]  M. Alexander,et al.  Gravity wave dynamics and effects in the middle atmosphere , 2003 .

[4]  Gravity waves above a convective boundary layer: A comparison between wind‐profiler observations and numerical simulations , 2007 .

[5]  F. Hall,et al.  Observed generation of an atmospheric gravity wave by shear instability in the mean flow of the planetary boundary layer , 1973 .

[6]  A. E. Gill.  Atmosphere-Ocean Dynamics , 1982 .

[7]  S. Caughey,et al.  Some aspects of turbulence structure through the depth of the convective boundary layer , 1979 .

[8]  J. Cuesta,et al.  On the Correlation between Convective Plume Updrafts and Downdrafts, Lidar Reflectivity and Depolarization Ratio , 2007 .

[9]  J. C. Doran,et al.  The 2001 Phoenix Sunrise experiment: vertical mixing and chemistry during the morning transition in Phoenix , 2003 .

[10]  Kenneth J. Davis,et al.  Long-Term Observations of the Dynamics of the Continental Planetary Boundary Layer , 2001 .

[11]  P. Savov,et al.  Influence of the boundary layer development on the ozone concentration over an urban area , 2008 .

[12]  J. Finnigan,et al.  Wave-turbulence dynamics in the stably stratified boundary layer , 1993 .

[13]  J. Hunt.  Turbulence structure in thermal convection and shear-free boundary layers , 1984, Journal of Fluid Mechanics.

[14]  T. Clark,et al.  Three‐dimensional numerical experiments on convectively forced internal gravity waves , 1989 .

[15]  S. Gray,et al.  Analysis of convectively‐generated gravity waves in mesoscale model simulations and wind‐profiler observations , 2008 .

[16]  Turker Ince,et al.  Atmospheric Disturbances that Generate Intermittent Turbulence in Nocturnal Boundary Layers , 2004 .

[17]  R. Fovell,et al.  Numerical simulations of convectively generated stratospheric gravity waves , 1992 .

[18]  J. Holton,et al.  The Gravity Wave Response above Deep Convection in a Squall Line Simulation. , 1995 .

[19]  T. Hauf.  Aircraft Observation of Convection Waves over Southern Germany—A Case Study , 1993 .

[20]  K. Davis,et al.  A Case Study of CO2, CO and Particles Content Evolution in the Suburban Atmospheric Boundary Layer Using a 2-μm Doppler DIAL, a 1-μm Backscatter Lidar and an Array of In-situ Sensors , 2008 .

[21]  R. Kershaw.  Parametrization of momentum transport by convectively generated gravity waves , 1995 .

[22]  C. Flamant,et al.  Urban boundary-layer height determination from lidar measurements over the paris area. , 1999, Applied optics.

[23]  R. Stull.  An Introduction to Boundary Layer Meteorology , 1988 .

[24]  R. Garcia,et al.  Transient Response to Localized Episodic Heating in the Tropics. Part I: Excitation and Short-Time Near-Field Behavior. , 1985 .

[25]  J. W. Fitzgerald.  Model of the aerosol extinction profile in a well-mixed marine boundary layer. , 1989, Applied optics.

[26]  D. Carruthers,et al.  Waves in the Overlying inversion of the Convective Boundary Layer , 1987 .

[27]  Terry L. Clark,et al.  Convection waves: Observations of gravity wave systems over convectively active boundary layers , 2007 .

[28]  Joseph Sanak,et al.  Relative humidity impact on aerosol parameters in a Paris suburban area , 2005 .

[29]  B. Sutherland.  Internal wave reflection in uniform shear , 2000 .

[30]  Thomas Hauf,et al.  Convectively forced internal gravity waves: Results from two-dimensional numerical experiments , 1986 .

[31]  J. Kaimal,et al.  Turbulence Structure in the Convective Boundary Layer , 1976 .

[32]  Larry Mahrt,et al.  Stratified Atmospheric Boundary Layers , 1999 .

[33]  D. Carruthers,et al.  Velocity fluctuations near an interface between a turbulent region and a stably stratified layer , 1986, Journal of Fluid Mechanics.

[34]  T. Clark,et al.  Gravity waves generated by the dry convective boundary layer: Two‐dimensional scale selection and boundary‐layer feedback , 2002 .

[35]  G. J. Fochesatto,et al.  Evidence Of Dynamical Coupling Between The Residual Layer And The Developing Convective Boundary Layer , 2001 .

[36]  F. Lott.  The transient emission of propagating gravity waves by a stably stratified shear layer , 1997 .