Margaret A LeMone

... Bradley F. Smull, 2009: On How Hot Towers Fuel the Hadley Cell: An Observational and Modeling Study of Line-Organized Convection in the Equatorial Trough from TOGA COARE. J. Atmos. ... School of Meteorology, University of Oklahoma, Norman, Oklahoma. Bradley F. Smull ...

ABSTRACT Cited By (since 1996): 18, Export Date: 16 March 2012, Source: Scopus, doi: 10.1175/2008MWR2354.1, Language of Original Document: English, Correspondence Address: LeMone, M.A.; NCAR, Foothills Laboratory, 3450 Mitchell Lane, Boulder, CO 80301, United States; email: lemone@ucar.edu, References: Alfieri, J.G., Blanken, P.D., Yates, D.N., Steffen, K., Variability in the environmental factors driving evapotranspiration from a grazed rangeland during severe drought conditions (2007) J. Hydrometeor, 8, pp. 207-220;

The GLOBE Program (www.globe.gov) involves students and scientists in a worldwide environmental data collection effort. The GLOBE ONE field campaign (www.globe.gov/globeone) represents a model for a focused implementation of GLOBE via a geographically-specific project. The campaign, which occurred in Black Hawk County, Iowa from February 2004 to February 2006, was developed by GLOBE Principal Investigators (PIs), the GLOBE Program Office,

Horizontal roll vortices influence the distribution of turbulence, with turbulence variances and fluxes concentrated in regions of positive roll vertical velocity r. This `modulation' of turbulence can be explained simply in terms of the advection of turbulence-generating elements by rolls.A budget equation is derived for the roll-modulated turbulence energy. Evaluations of various terms in the equation shows that the modulation of turbulence variance is accounted for primarily by a similar modulation in mechanical and buoyancy production near the surface and by vertical transport at higher levels (100 m). Energy exchange between rolls and turbulence is relatively unimportant. That is, the rolls modulate, turbulence energy mainly by redistributing turbulence and turbulence-producing elements, rather than by exchanging energy.Similarly, it is shown that the exchange of energy between rolls and roll-modulated turbulence contributes considerably less to the energy equation of rolls than does the major term, buoyancy.

The CASES-97 dataset, supplemented by data from the surrounding area and from satellite, will enable us to isolate the effects of soil moisture on boundary layer evolution.Our initial approach will be to use the integrated dataset to determine (a) the factors that contribute to PBL growth, and (b) the factors that determine the wind, temperature, and wind profiles in the

The goal of this study is to examine the horizontal scale dependence of vertical eddy flux in the tropical marine surface boundary layer and how this scale dependence of flux relates to the bulk aerodynamic relationship and the parameterization of subgrid-scale flux. The fluxes of heat, moisture, and momentum are computed from data collected from 27 NCAR Electra flight legs in TOGA COARE (The Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment) with flight elevations lower than 40 m and flight runs longer than 60 km. The dependence of the fluxes on two length scales are studied: the cutoff length scale, defining the averaging length over which mean components are obtained in order to partition field variables into mean and perturbation components; and the flux averaging length scale, defining the length over which products of perturbations are averaged in order to estimate vertical fluxes. Based on the characteristics of the scale dependence of fluxes, the total flux of each flight leg is partitioned into `turbulent,' `large eddy,' and `mesoscale' fluxes due to motions smaller than 1 km, between 1 and 5 km, and between 5 km and the flight leg length, respectively.The results show that fluxes are sensitive to the choice of cutoff length scale in the presence of significant mesoscale activity and in the weak wind case where the turbulent fluxes are small. The turbulent momentum flux decreases with increasing flux averaging length scale due to mesoscale modulation of the turbulent stress vector.Mesoscale heat, moisture, and momentum fluxes for individual flight legs can reach 20% of the turbulent fluxes in the presence of well-organized convective cloud systems even at 35 m above the sea surface. The mesoscale flux is less correlated to the wind speed and bulk air-sea difference than turbulent fluxes. The local mesoscale flux can be upward or downward, and therefore, its average value is reduced when averaging over a single flight leg and reduced further when compositing over all of the legs. The mesoscale momentum flux is less systematic than the turbulent stress and is more sensitive to the flux averaging scale than the turbulent stress. Sampling and instrumentation problems are briefly discussed, particularly with respect to mesoscale motions.

An examination of the properties of updraft and downdraft cores using Electra data from TOGA COARE shows that they have diameters and vertical velocities similar to cores observed over other parts of the tropical and subtropical oceans. As in previous studies, a core is defined as having vertical velocity of the same sign and greater than an absolute value of 1 m s1 for at least 500 m. A requirement that the core contain either cloud or precipitation throughout is added, but this should not affect the results significantly.Since the Electra was equipped with the Ophir III radiometric temperature sensor, it was also possible to make estimates of core buoyancies. As in TAMEX and EMEX, where core temperatures were estimated using the modified side-looking Barnes radiometer on the NOAA P3s, a significant fraction of both updraft and downdraft cores had apparent virtual temperatures greater than their environments. In fact, the average virtual temperature deviation from the environment for downdraft cores was +0.4 K.Sixteen of the strongest downdraft cores were examined, all of which had positive virtual-temperature deviations, to find the source of this surprising result. It is concluded that the downdraft cores are artificially warm because 100% relative humidity was assumed in calculating virtual temperature. However, reducing core mixing ratios to more physically realistic values does not eliminate warm virtual potential temperature downdraft cores, nor does water loading make all cores negatively buoyant. Thus positively buoyant convective downdrafts do exist, though probably in smaller numbers than previously suggested.

This is the first part of a two-part paper defining the nature of the vertical air motion in and around GATE cumulonimbus clouds. The statistics are from a total of 104 km of flight legs, flown on six days in GATE, at altitudes from near the surface to 8100 m. The basic data sets analyzed are time series of vertical velocity at a frequency of 1 Hz. For the purpose of study, convective events are divided into two categories: drafts, requiring only that vertical velocity be continuously positive (negative) for 500 m and exceed an absolute value of 0.5 m s1 for 1 s; and cores, the stronger portions of the stronger drafts, requiring that upward (downward) vertical velocity be continuously greater than an absolute value of 1 m s1 for 500 m. The distributions of average vertical velocity, maximum vertical velocity, diameter and mass flux are given for drafts and cores at five altitude intervals between 150 m and 8 km. In all cases, the distributions are approximately log-normal.Above cloud base, updrafts tend to be smaller but more intense than downdrafts. Updrafts and down-drafts near cloud base are comparable in size and intensity. Downdraft cores are smaller than updraft cores at all attitudes. They also are weaker, except near cloud base, where updraft and downdraft cores have comparable intensity. In the middle troposphere, only 10% of the updraft cores have mean vertical velocities greater than 5 m s1, and only 10% have diameters in excess of 2 km.

The wind and temperature fields of the Planetary boundary layer (PBL) are investigated during periods in which horizontal roll vortices are present. Measurements from a 444 m tower and from inertially-stabilized aircraft indicate the rolls are maintained primarily by 1) production of energy from the cross-roll component of the mean PBL wind spiral (inflectional instability and 2) buoyancy. Complicating a simple picture of two-dimensional rolls are other kilometer-scale eddies whose energy exchanges with the tolls may be important.The importance of inflectional instability is indicated by the similarity of roll structure to that predicted by models based on the formation of the rolls as a result of instability in the cross-wind (V component of the Ekman spiral. Rolls observed are generally oriented from 10° to 20° to the left of the wind at inversion base, with maximum roll vertical velocity at 0.33zi(where zi is inversion height) and maximum lateral velocity at 0.07zi Atmospheric roll magnitude compare favorably to those predicted by Brown, but predictions are consistently low with a maximum underestimate of 40%.Both tower and aircraft measurements indicate substantial heat flux by rolls. It is shown that including positive roll heat flux into Brown's neutral equilibrium energy budget will lead to rolls of larger magnitude.

Mechanisms responsible for meso- and convective-scale organization within a large tropical squall line that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment are investigated using a three-dimensional numerical cloud model. The squall line occurred in an environment typical of fast-moving tropical squall lines, characterized by moderate convective available potential energy and moderate-to-strong vertical shear beneath a low-level jet with weak reverse vertical shear above.A well-simulated aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi-linear to a three-dimensional bow-shaped structure over a 2-h period. This transition is accompanied by the development of both a prominent mesoscale vortex along the northern edge of the 40-60-km long bow-shaped feature and elongated bands of weaker reflectivity situated rearward and oriented transverse to the leading edge, within enhanced front-to-rear system relative midlevel flow, near the southern end of the bow. The vertical wind shear that arises from the convectively induced mesoscale flow within the squall-line system is found to be a critical factor influencing 1) the development of the vortex and 2) through its associated vertical pressure gradients, the pronounced along-line variability of the convective updraft and precipitation structure. The environmental wind profile is also critical to system organization since the orientation of its vertical shear (in layers both above and below the environmental jet height) relative to the local orientation of the incipient storm-induced subcloud cold pool directly influences the onset of the convectively induced mesoscale flow.

The International H2O Project 2002 (IHOP 2002), which was conducted during May and June 2002, sought to better understand spatial and temporal variations in the water vapor field in the Southern Great Plains (SGP) of the United States. One fundamental influence on this water vapor field is transpiration. Data collected at six IHOP 2002 surface sites, which are representative of the crop and grassland environments typical of the SPG as a whole, were used to validate a remote sensing based vegetation transpiration model (VTM). The model uses several vegetation indices derived from remotely sensed data in conjunction with surface observation data, for example air temperature and incident solar radiation, to estimate transpiration as a function of gross primary production and water use efficiency. While VTM has been tested over forest environments, this is the first evaluation of the model over crop and grassland environments. Since these land use types represent a significant proportion earth's terrestrial surface including nearly 40 percent of the land cover of the contiguous United States, this research marks an important step toward modeling transpiration over a region that plays a critical role in numerous biogeochemical cycles on both regional and global scales. By comparing model output with observations, it was found that the VTM represented temporal trends reasonable. It was found that the modeled values for transpiration were consistently less than the total observed moisture flux. This is to be expected since the VTM model currently considers only transpiration neglecting the moisture stream due to evaporation. Research is ongoing to develop an evaporation component for the VTM model so that it is able to accurately describe all streams of moisture transfer to the atmosphere.

... EDAS analyses; 4) 1-km horizontal resolution US Geological Survey 24-category land-use and 1-km horizontal resolution State Soil Geographic soil texture maps; and 5) 0.15° monthly satellite-derived green vegetation fraction based on 5-yr averages (Gutman and Ignatov 1997 ...

Time series of 1-Hz vertical velocity data collected during aircraft penetrations of oceanic cumulonimbus clouds over the western Pacific warm pool as part of the Equatorial Mesoscale Experiment (EMEX) are analyzed for updraft and downdraft events called cores. An updraft core is defined as occurring whenever the vertical velocity exceeds 1 m s1 for at least 500 m. A downdraft core is defined analogously. Over 19 000 km of straight and level flight legs are used in the analysis. Five hundred eleven updraft cores and 253 downdraft cores are included in the dataset.Core properties are summarized as distributions of average and maximum vertical velocity, diameter, and mass flux in four altitude intervals between 0.2 and 5.8 km. Distributions are approximately lognormal at all levels. Examination of the variation of the statistics with height suggests a maximum in vertical velocity between 2 and 3 km; slightly lower or equal vertical velocity is indicated at 5 km. Near the freezing level, virtual temperature deviations are found to be slightly positive for both updraft and downdraft cores. The excess in updraft cores is much smaller than that predicted by parcel theory.Comparisons with other studies that use the same analysis technique reveal that EMEX cores have approximately the same strength as cores of other oceanic areas, despite warmer sea surface temperatures. Diameter and mass flux are greater than those in GATE but smaller than those in hurricane rainbands. Oceanic cores are much weaker and appear to be slightly smaller than those observed over land during the Thunderstorm Project.The markedly weaker oceanic vertical velocities below 5.8 km (compared to the continental cores) cannot be attributed to smaller total convective available potential energy or to very high water loading. Rather, the authors suggest that water loading, although less than adiabatic, is more effective in reducing buoyancy of oceanic cores because of the smaller potential buoyancy below 5.8 km. Entrainment appears to be more effective in reducing buoyancy to well below adiabatic values in oceanic cores, a result consistent with the smaller oceanic core diameters in the lower cloud layer. It is speculated further that core diameters are related to boundary layer depth, which is clearly smaller over the oceans.

The vertical transport of horizontal momentum normal to a line of cumulonimbus observed during GATE on 14 September 1974 is against the vertical momentum gradient, contrary to the predictions of mixing-length theory. Data from repeated aircraft passes normal to the line's axis at heights from 0.15 to 5.5 km are used to document the flux and determine its source. The flux is concentrated in roughly a 25 km wide `active zone' just behind the leading edge of the line, in kilometer-scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces. The fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic, resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical, with the clouds at the surface well ahead of those aloft.Evaluation of the terms in the momentum-flux generation equation confirms that the above process, reflected by the velocity-buoyancy correlation term, is responsible for generating momentum flux of the observed sign. The component of momentum flux parallel to the axis of the convective band is generated much like `down-gradient' momentum flux within the fair-weather subcloud layer.

Results from a detailed three-dimensional model of the atmospheric boundary layer are compared with observational data in a case of nonprecipitating convection in a tropical boundary layer. The model is a slightly improved version of the one developed by Sommeria (1976) in collaboration with J.W. Deardorff. The experimental data come from the NCAR 1972 Puerto Rico experiment which provided a good set of aircraft turbulence measurements in the fair weather mixed layer over the tropical ocean. The comparison involves statistical properties of the turbulent field as well as some structural features in the presence of small clouds.

ABSTRACT This investigation examines the meso- and microscale aspects of the 9 March 1992 cold front that passed through Kansas during the daylight hours. The principal feature of this front is the relatively rapid frontogenesis that occurred. The total change in the cross-frontal temperature is about 6 K, with most of the change occurring between about 0820 and 1400 local time and over a relatively small subsection of the total frontal width. The surface data are able to resolve a sharp horizontal transition zone of 1-2 km. The principal physical processes that produce this frontogenesis are shown to be the cross-frontal differential sensible heating, associated with differential cloud cover, and the convergence of warm and cold air toward the front. The former process is responsible for an increase in the magnitude of the differential temperature change across the front; the latter process concentrates the existing temperature differential across an ever-decreasing transitional zone until a near discontinuity in the horizontal temperature distribution is essentially established during the period of a few hours. Two approaches are taken to demonstrate that these processes control the observed frontogenesis. First, surface data from an enhanced array, set up during the Storm-scale Operational and Research Meteorology Fronts Experiment System Test, are used to evaluate the terms that contribute to the time rate of change of the gradient of potential temperature, d|Δθ|/dt, following the motion of the front. Then, the processes of differential sensible heating and convergence are incorporated into a simple two-dimensional nonlinear model that serves to provide a forecast of the surface temperature and velocity fields from given initial conditions that are appropriate at the onset of the surface heating. Verification of the model predictions by observed data confirms that both processes contribute to the observed daytime frontogenesis on 9 March 1992. A critique of the model does, however, suggest that the accuracy of some quantitative evaluations could be improved.

... Full Text . PDF (5334 KB). Tammy M. Weckwerth, Hanne V. Murphey, Cyrille Flamant, Janine Goldstein, Crystalyne R. Pettet. (2008) An Observational Study of Convection Initiation on 12 June 2002 during IHOP_2002. ... CrossRef. Terry L. Clark, Thomas Hauf, Joachim P. Kuettner. ...

A performance analysis of the three turbulence-measuring aircraft which participated in the GATE is presented. These aircraft were a Lockheed C-130 operated by the Meteorological Research Flight Centre of the U.K. Meteorological Office, a Douglas DC-6 operated by the Research Flight Facility of the National Oceanographic and Atmospheric Administration, and a Lockheed L-188 operated by the Research Aviation Facility of the National Center for Atmospheric Research.The results are based on formal intercomparison flights and analysis of fair weather days on which two or more of the aircraft were flying. In the formal intercomparison flights, two or more of the aircraft flew side by side in the fair weather atmospheric mixed layer. In both cases, the aircraft flew L-shaped patterns, consisting of 30 km legs along and normal to the mixed layer wind direction.Quantities compared include the variances of three wind components, potential temperature, moisture, and the vertical fluxes of horizontal momentum, temperature, and moisture. The analysis shows that when all components of the gust probe system are working properly, interaircraft biases are less than the expected atmospheric variability. Quirks of the three data sets are pointed out for the benefit of future GATE data users.

Previously published profiles of vertical velocity (w) skewness observed in the convective atmospheric boundary layer show deficits in the upper part of the layer, relative to large eddy simulations designed to apply to highly convective cloudless planetary boundary layers. Thus, we examine w-skewness profiles from data collected in other experiments. We find that skewness profiles in the three highly convective cases with the fewest and smallest clouds agree better with the large eddy simulation results than other profiles presented here and previously; however the deficit at the top of the boundary layer-though smaller-remains.We hypothesize that the remaining deficit for these three cases results from the presence of 10-km wavelength quasi two-dimensional sinusoidal structures, which have near-zero skewness. The small domain and periodic boundary conditions of a large eddy simulation may not allow such structures to develop fully. Removal of the effects of these structures by counting only flight legs nearly parallel to their axes, for two of the cases, improves agreement between the simulation and observations. We speculate that these structures result from gravity waves interacting with the boundary layer.

Surface-station, radiosonde, and Doppler minisodar data from the Cooperative Atmosphere-Surface Exchange Study-1997 (CASES-97) field project, collected in a 60-km-wide array in the lower Walnut River watershed (terrain variation 150 m) southeast of Wichita, Kansas, are used to study the relationship of the change of the 2-m potential temperature 2m with station elevation ze, 2m/ze ,ze to the ambient wind and thermal stratification /z ,z during fair-weather nights. As in many previous studies, predawn 2m varies linearly with ze, and ,ze ,z over a depth h that represents the maximum elevation range of the stations. Departures from the linear 2m-elevation relationship (<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze line) are related to vegetation (cool for vegetation, warm for bare ground), local terrain (drainage flows from nearby hills, although a causal relationship is not established), and the formation of a cold pool at lower elevations on some days.<IMG SRC="/images/indent.gif" WIDTH=15 HEIGHT=10 BORDER=0 ALT="">The near-surface flow and its evolution are functions of the Froude number Fr = S/(Nh), where S is the mean wind speed from the surface to h, and N is the corresponding Brunt-Väisälä frequency. The near-surface wind is coupled to the ambient flow for Fr = 3.3, based on where the straight line relating <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze to ln Fr intersects the ln Fr axis. Under these conditions, <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">2m is constant horizontally even though <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,z > 0, suggesting that near-surface air moves up- and downslope dry adiabatically. However, <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">2m cools (or warms) everywhere at the same rate. The lowest Froude numbers are associated with drainage flows, while intermediate values characterize regimes with intermediate behavior. The evolution of <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">2m horizontal variability σ<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif"> through the night is also a function of the predawn Froude number. For the nights with the lowest Fr, the σ<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif"> maximum occurs in the last 1-3 h before sunrise. For nights with Fr <IMG border="0" src="/charent/ISO_CHARACTERS_MIXED/LOWERCASE/sim.gif"> 3.3 (<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze <IMG border="0" src="/charent/ISO_CHARACTERS_MIXED/LOWERCASE/ap.gif"> 0) and for intermediate values, σ<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif"> peaks 2-3 h after sunset. The standard deviations relative to the <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze line reach their lowest values in the last hours of darkness. Thus, it is not surprising that the relationships of <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze to Fr and <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,z based on data through the night show more scatter, and <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze <IMG border="0" src="/charent/ISO_CHARACTERS_MIXED/LOWERCASE/sim.gif"> 0.5<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,z in contrast to the predawn relationship. However, <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">,ze <IMG border="0" src="/charent/ISO_CHARACTERS_MIXED/LOWERCASE/ap.gif"> 0 for ln Fr = 3.7, a value similar to that just before sunrise.<IMG SRC="/images/indent.gif" WIDTH=15 HEIGHT=10 BORDER=0 ALT="">A heuristic Lagrangian parcel model is used to explain the horizontal uniformity of time-evolving <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">2m when the surface flow is coupled with the ambient wind, as well as both the linear variation of <IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif">2m with elevation and the time required to reach maximum values of σ<IMG border="0" src="/charent/GREEK/UNACCENTED/CAP/Theta.gif"> under drainage-flow conditions.

The refractive index structure parameter CN2 has contributions from the temperature and humidity structure parameters Cr2 and CQ2 and from the joint structure parameter CTQ. We briefly review the behavior of these structure parameters in the surface layer. We show that the surface-layer similarity expressions for Cr2, CTQ and CQ2 yield, in the unstable limit, mixed-layer scaling laws which are in good agreement with data at small z/zi, where zi is the mixed-layer depth. However, we show that entrainment effects cause large departures from these laws in mid and upper regions of the convective boundary layer.Using Deardorff's idealization of the structure of the interfacial region at the top of a convective boundary layer, we use a `mean-field closure' approach to develop scaling expressions for the structure parameters generated by the entrainment process there. The available data on CT2, CTQ and CQ2 near the convective boundary-layer top, from both steady and evolving cases, are shown to be consistent with these new scaling expressions.

... Variation diurne. ; Etats Unis. ; Front arctique. ; Amérique du Nord. ; Amérique. ; Mots-clés espagnols / Spanish Keywords. Mesoescala. ; Capa límite atmosférico. ; Temperatura potencial. ; Gradiente temperatura. ; Evolución. ; Variación diurna. ; Estados Unidos. ; Frente ártico. ; ...

ABSTRACT Cited By (since 1996): 18, Export Date: 16 March 2012, Source: Scopus, doi: 10.1175/2008MWR2354.1, Language of Original Document: English, Correspondence Address: LeMone, M.A.; NCAR, Foothills Laboratory, 3450 Mitchell Lane, Boulder, CO 80301, United States; email: lemone@ucar.edu, References: Alfieri, J.G., Blanken, P.D., Yates, D.N., Steffen, K., Variability in the environmental factors driving evapotranspiration from a grazed rangeland during severe drought conditions (2007) J. Hydrometeor, 8, pp. 207-220;

The relationship of subcloud layer structures and fluxes to cumulus distribution and depth of cumulus convection is re-examined in the light of recently obtained airborne data on wind, temperature, humidity, and overhead cloud occurrence. Three case studies are used, in ...