Anandakumar Karipot

Impact of non-local advection on flux footprints over a tall forest canopy: a tracer flux experiment

Since the early 1990s, in the planning and execution of their flux measurement campaigns, micrometeorologists have used the footprint concept to determine site quality with respect to both fetch and surface homogeneity. While the usefulness of these models has been demonstrated time and time again, there are cases which clearly call for caution. For instance, over tall forest canopies, footprint models have been applied without prior experimental validation, thus causing uncertainties in the interpretation of flux data particularly close to sources and sinks. Another example is that, despite recent advances aimed at identifying both the spatial extent of upwind sources and their individual weight to a point flux measurement, little attention has been given to the possible contribution of sources well outside the footprint region to flux measurements. This paper evaluates footprint models applied to fluxes above tall forest canopies with a tracer experiment. This work further identifies the contribution of sources outside the footprint region to an in situ flux measurement. A field campaign was performed over an 1 l-year old slash pine canopy and a passive tracer (SF6) was released from a line source deployed near the treetop. Vertical SF6 fluxes were measured using the eddy covariance technique at two positions downwind from the source. Measured fluxes during near-neutral, unstable and very unstable conditions were compared against a Lagrangian simulation and an analytical solution to the diffusion equation, two methods commonly used to predict the footprints. Both the formulations compare favorably with SF6 flux measurements except in cases characterized by wind direction-specific advection. Results suggest that, in addition to local source inhomogeneities within the footprint, in some conditions, non-local, larger scale forcings originating hundreds of meters outside the footprint envelope can contribute significantly to flux measurements. This finding is supported by sodar measurements of the three-dimensional velocity field. This finding further suggests caution to experimentalists involved with flux measurements and illustrates the fact that flux measurements must be made with an awareness of landscape-wide surface properties.

Characteristics of Nocturnal Low-Level Jets Observed in the North Florida Area

The seasonal and interannual variability of the nocturnal low-level jets over the north Florida region are investigated using sodar measurements spanning 540 nights. On average, jets are present in 62% of the nocturnal periods examined. The observed jet speeds range between 3 and 21 m s 21 and heights are between 80 and 700 m. Observations show that the low-level jet occurs more frequently (70% of the nocturnal periods) during the colder months November‐February in contrast with the warmer months June‐August (;47%). The presence of southerly jets dominates the summer months, whereas northerly jets are more frequent during winter. Colder months frequently exhibit jets with speeds exceeding 14 m s 21 , often associated with the passage of frontal systems. The interannual variability observed using the North American Regional Reanalysis (NARR) wind profile data during a 4-yr period shows only minimal differences in jet characteristics.A comparison of jet heights with NARR planetary boundary layer heights suggests that jets at the north Florida location frequently occur within the planetary boundary layer. The occurrence and speed of observed low-level jets are linked to both the land‐ocean temperature contrast and to the strength and orientation of surface pressure gradients over the region. A high occurrence of large-amplitude oscillations with approximately a 24-h period near the jet height is shown using the Hilbert‐Huang transform analysis, suggesting that inertial oscillations are one possible cause of jet formation in north Florida.

Low-Frequency Effects on Eddy Covariance Fluxes under the Influence of a Low-Level Jet

Abstract Turbulent bursts observed over a tall forest canopy during the initiation of a nocturnal low-level jet (LLJ) are studied with the help of wavelet analysis. The burst of turbulence is observed in response to a shear instability associated with the initiation of LLJ. Turbulent kinetic energy, momentum, and CO2-rich cold air are transferred downward by large eddies with length scales that are higher than the LLJ height. Microfronts are observed over the canopy as a secondary instability that enhances the mixing processes within and above the canopy. The scale-dependent wavelet correlation analysis reveals that countergradient fluxes result from low frequencies, whereas cogradient flux is associated with high-frequency turbulent motions. The countergradient flux is initially noted at low frequencies, and, through coherent motions, it is transferred to smaller scales with a nearly 20-min delay. The countergradient flux dominates at the initiation of the event and reduces net flux, whereas enhanced cog...

Estimating gross primary productivity of a tropical forest ecosystem over north-east India using LAI and meteorological variables

Tropical forests act as a major sink of atmospheric carbon dioxide, and store large amounts of carbon in biomass. India is a tropical country with regions of dense vegetation and high biodiversity. However due to the paucity of observations, the carbon sequestration potential of these forests could not be assessed in detail so far. To address this gap, several flux towers were erected over different ecosystems in India by Indian Institute of Tropical Meteorology as part of the MetFlux India project funded by MoES (Ministry of Earth Sciences, Government of India). A 50 m tall tower was set up over a semi-evergreen moist deciduous forest named Kaziranga National Park in north-eastern part of India which houses a significant stretch of local forest cover. Climatically this region is identified to be humid sub-tropical. Here we report first generation of the in situ meteorological observations and leaf area index (LAI) measurements from this site. LAI obtained from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) is compared with the in situ measured LAI. We use these in situ measurements to calculate the total gross photosynthesis (or gross primary productivity, GPP) of the forest using a calibrated model. LAI and GPP show prominent seasonal variation. LAI ranges between 0.75 in winter to 3.25 in summer. Annual GPP is estimated to be

A Case Study of Turbulence in the Nocturnal Boundary Layer During the Indian Summer Monsoon

2.11\,\hbox {kg C m}^{-2} \, \hbox {year}^{-1}

Influence of nocturnal low‐level jet on turbulence structure and CO2 flux measurements over a forest canopy

2.11kg Cm-2year-1.

Role of low-level jets and boundary-layer properties on the NBL budget technique

An extensive field study has been undertaken to quantify the aerial release of spray material through the changes in meteorology as the day progresses. An important subset of these collected data is one-second interval data of the aircraft behavior and the mechanical release systems. These unique data provide an excellent source of information on bounding the variability in the expected deposition patterns, and how this variability might impact any error bounds established around the time-averaged predictions generated by the AGDISP model. This paper quantifies the variability in aerial application parameters and makes suggestions with regard to possible implications of this variability on the variability of deposition predictions in the flight line direction.

Scale interactions near the foothills of Himalayas during CAIPEEX

Observations from the Cloud-Aerosol Interaction and Precipitation Enhancement Experiment-Integrated Ground Observation Campaign (CAIPEEX-IGOC) provide a rare opportunity to investigate nocturnal atmospheric surface-layer processes and surface-layer turbulent characteristics associated with the low-level jet (LLJ). Here, an observational case study of the nocturnal boundary layer is presented during the peak monsoon season over Peninsular India using data collected over a single night representative of the synoptic conditions of the Indian summer monsoon. Datasets based on Doppler lidar and eddy-covariance are used for this purpose. The LLJ is found to generate nocturnal turbulence by introducing mechanical shear at higher levels within the boundary layer. Sporadic and intermittent turbulent events observed during this period are closely associated with large eddies at the scale of the height of the jet nose. Flux densities in the stable boundary layer are observed to become non-local under the influence of the LLJ. Different turbulence regimes are identified, along with transitions between turbulent periods and intermittency. Wavelet analysis is used to elucidate the presence of large-scale eddies and associated intermittency during nocturnal periods in the surface layer. Although the LLJ is a regional-scale phenomenon it has far reaching consequences with regard to surface-atmosphere exchange processes.