Thara V. Prabha

Microphysics of Premonsoon and Monsoon Clouds as Seen from In Situ Measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX)

AbstractAnalysis of the microphysical structure of deep convective clouds using in situ measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the Indian peninsular region is presented. It is shown that droplet size distributions (DSDs) in highly polluted premonsoon clouds are substantially narrower than DSDs in less polluted monsoon clouds. High values of DSD dispersion (0.3–0.6) and its vertical variation in the transient and monsoon clouds are related largely to the existence of small cloud droplets with diameters less than 10 μm, which were found at nearly all levels. This finding indicates the existence of a continuous generation of the smallest droplets at different heights. In some cases this generation of small droplets leads to the formation of bimodal and even multimodal DSDs. The formation of bimodal DSDs is especially pronounced in monsoon clouds. Observational evidence is presented to suggest that in-cloud nucleation at elevated layers is a f...

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.

Aerosol effect on droplet spectral dispersion in warm continental cumuli

In situ aircraft measurements of cloud microphysical properties and aerosol during the 1st phase of the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX-I) over the Indian sub-continent provided initial opportunities to investigate the dispersion effect and its implications for estimating aerosol indirect effects in continental cumuli. In contrast to earlier studies on continental shallow cumuli, it is found that not only the cloud droplet number concentration but also the relative dispersion increases with the aerosol number concentration in continental cumuli. The first aerosol indirect effect estimated from the relative changes in droplet concentration and effective radius with aerosol number concentration are 0.13 and 0.07, respectively. In-depth analysis reveals that the dispersion effect could offset the cooling by enhanced droplet concentration by 39 in these continental cumuli. Adiabaticity analysis revealed aerosol indirect effect is lesser in subadiabatic clouds possibly due to inhomogeneous mixing processes. This study shows that adequate representation of the dispersion effect would help in accurately estimating the cloud albedo effect for continental cumuli and can reduce uncertainty in aerosol indirect effect estimates.

Spectral width of premonsoon and monsoon clouds over Indo‐Gangetic valley

[1] The combined effect of humidity and aerosol on cloud droplet spectral width (s) in continental monsoon clouds is a topic of significant relevance for precipitation and radiation budgets over monsoon regions. The droplet spectral width in polluted, dry premonsoon conditions and moist monsoon conditions observed near the Himalayan Foothills region during Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) is the focus of this study. Here s is small in premonsoon clouds developing from dry boundary layers. This is attributed to numerous aerosol particles and the absence/suppression of collision-coalescence during premonsoon. For polluted and dry premonsoon clouds, s is constant with height. In contrast to premonsoon clouds, s in monsoon clouds increases with height irrespective of whether they are polluted or clean. The mean radius of polluted monsoon clouds is half that of clean monsoon clouds. In monsoon clouds, both mean radius and s decreased with total cloud droplet number concentration (CDNC). The spectral widths of premonsoon clouds were independent of total droplet number concentrations, but both s and mean radius decreased with small droplet (diameter < 20 mm) number concentrations in the diluted part of the cloud. Observational evidence is provided for the formation of large droplets in the adiabatic regions of monsoon clouds. The number concentration of small droplets is found to decrease in the diluted cloud volumes that may be characterized by various spectral widths or mean droplet radii.

Soil thermal properties at Kalpakkam in coastal south India

Time series of soil surface and subsurface temperatures, soil heat flux, net radiation, air temperature and wind speed were measured at two locations in Kalpakkam, coastal southeast India. The data were analysed to estimate soil thermal diffusivity, thermal conductivity, volumetric heat capacity and soil heat flux. This paper describes the results and discusses their implications.

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...

Thermal internal boundary layer characteristics at a tropical coastal site as observed by a mini-SODAR under varying synoptic conditions

Atmospheric boundary layer observations are conducted at a coastal site during a transition phase from winter to summer season over the Indian peninsula. Thermal Internal Boundary Layer (TIBL) characteristics in presence of an off-shore and a weakly influenced on-shore synoptic wind are examined with the help of measurements carried out with a mini-SODAR (SOund Detection And Ranging), tethered balloon, and tower-based micrometeorological measurements. Influence of the changing synoptic scale conditions on turbulent characteristics of TIBL is discussed.Mini-SODAR data showed the development and decay of sea and land breeze. It is seen that the characteristics of TIBL over the coastal land after sea breeze onset are similar to that of a shallow convective boundary layer (CBL) commonly found over plain land. Inside the TIBL, a maximum wind speed was noted close to the surface due to the penetration of sea breeze. In the off-shore case, a distinct sea breeze circulation was observed unlike in the case of on-shore flow. In the presence of weak on-shore case, a ‘minor sea’ breeze is noted before the establishment of sea breeze and a reduction in the momentum fluxes gives rise to decrease in the turbulence intensity. Updraft in the sea breeze front was stronger during weak synoptic conditions. Influence of synoptic changes on the sea breeze-land breeze circulation such as onset, strength and duration of the sea-land breeze are also examined.

Aerosol and cloud feedbacks on surface energy balance over selected regions of the Indian subcontinent

We investigate aerosol and cloud forcing on the surface energy balance over selected regions in India. Four regions were selected with different surface characteristics and have considerable differences in the long-term trends and seasonal distribution of clouds and aerosols. These regions are described as (1) northern semiarid, (2) humid subtropical, (3) populated central peninsula, and (4) northeast monsoon impacted. Modern Era Retrospective-analysis for Research and Applications (MERRA) data and Climate Forecast System Reanalysis version 2 (CFSR) data are used in this study. An intercomparison of cloud fractions from both data sets shows that CFSR systematically underestimates high-cloud fraction during premonsoon and monsoon seasons. However, there are fewer low-cloud fraction biases. The positive temporal trend over 31years (1979-2009) from MERRA in high clouds is greater than that of low clouds. This is due to positive anomalies in the cloud ice and supercooled liquid water content in MERRA. Biases in the radiative fluxes and surface fluxes show a strong relationship (correlations exceeding 0.8) with cloud fraction biases, more so for the high clouds. During the premonsoon season, aerosol forcing causes a change in surface shortwave radiation of -24.5, -25, -19, and -16Wm -2 over regions 1 -4, respectively. The corresponding longwave radiation decrease is -9.8, -6.8, -4.5, and -1.9Wm -2 over these same regions, respectively. The maximum surface shortwave reduction due to clouds, which is observed during the monsoon season, is -86, -113, -101, and -97Wm -2 for these same regions, respectively. A decreasing trend in the boundary layer height is noticed both in MERRA and CFSR. The variation in the Bowen ratio and its relation to aerosol and cloud effect anomalies are also discussed.

Comparison of In-Canopy Flux Footprints between Large-Eddy Simulation and the Lagrangian Simulation

Abstract Flux footprints for neutral shear-driven canopy flows are evaluated using large-eddy simulation (LES) and a Lagrangian stochastic (LS) model. The Lagrangian stochastic model is driven by flow statistics derived from the large-eddy simulation. LES results suggest that both surface and elevated sources inside the canopy contribute equally to the cumulative flux from an upwind distance of 4 times the canopy height. LES flux footprints are more contracted than those obtained using the Lagrangian stochastic model. This is attributed to an enhanced vertical diffusion and reduced horizontal diffusion. The ejection and sweep contributions to momentum exchange in the Lagrangian stochastic model are weaker than those in the large-eddy simulation. Ejections of low-momentum air dominate at all levels in the canopy modeled by the LES. In contrast, high-momentum sweep events are dominant within the LES canopy and low-momentum ejection events are dominant above the canopy. Dispersion parameters for the first- a...

The Weather Research and Forecasting (WRF) model: application in prediction of TSWV-vectors populations

In the southeastern United States, millions of dollars worth of crop damage associated with tobacco thrips (Frankliniella fusca Hinds) and western flower thrips (Frankliniella occidentalis Pergande) are reported annually for economically important crops such as cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.). Tobacco thrips and western flower thrips are also important vectors of Tomato spotted wilt virus (TSWV). The populations and spread of both thrips species relies on favourable weather conditions, especially prolonged temperatures above a minimum developmental threshold. Accurate local weather information is, therefore, crucial for early prediction of thrips’ population dynamics during the spring when thrips’ population information could assist farmers in mitigating damage to crops. The objective of this study was to demonstrate the application of the Weather Research and Forecasting (WRF) model predictions for developing high resolution spatial and temporal forecast maps of favourable conditions for thrips’ development even for locations where weather stations are not available. Tobacco thrips and western flower thrips were evaluated based on degree‐day models. The results showed that southwestern Georgia is more favourable for thrips development during the early part of the growing season, although the rate of development varied according to the thrips species. The maps that were produced using WRF output provided high‐resolution forecasts for favourable conditions and scouting guidance in places where weather information was limited. An accurate prediction of pest development based on forecasts of favourable conditions may assist growers in pest management decisions and in timely application of insecticides.