Summer Monsoon Experiment Research Articles

A brief summary of Dr. G. V. Rao’s research interests

A brief summary of Dr. G. V. Rao's research interests is presented. Many of his earlier studies were in conjunction with the summer Monsoon Experiment of 1979 (MONEX-79). These included: 1) the structure of the Somali jet based on aerial observations; 2) sea-level air trajectories over the equatorial Indian Ocean; 3) structural features of the east African low-level flow; 4) effects of Indian Ocean surface temperature anomaly patterns on the summer monsoon circulations; 5) structures of the monsoon low-level flow over the Arabian Sea; 6) characteristics and momentum-flux budgets of the Arabian Sea convective bands; and 7) evaporation and precipitation over the Arabian Sea during the monsoon seasons. Dr. Rao's research efforts in recent years had focused on case studies of mesocyclones spawned by tropical cyclones (TCs) in Florida using Doppler radar data and a mesoscale numerical model. These included: 1) research on tornadic mesocyclones spawned by TC Earl in 1998; 2) documentation of subtle differences between tornadic and non-tornadic mesocyclones in TC Floyd in 1999; and 3) numerical simulation of the tornadic environment observed in peninsular Florida during TC Earl in 1998. Preliminary findings show that the supercells' cold pools interacted with an existing boundary resulting in increased baroclinicity and horizontal vorticity, and a maximization of the tornado production potential by the updrafts. The model successfully simulated the mesoscale features of the mesocyclones and the tornadic environment observed during TC Earl. A 24 h simulation of accumulated rainfall within the inner domain agreed well with the observed precipitation pattern over the region.

Vertical circulation structure,interannual variation features and variation mechanism of western pacific subtropical high

The paper investigates the vertical circulation structure of the western Pacific subtropical high (STH) and its interannual variation features in relation to East Asian subtropical summer monsoon and external thermal forcing by using the high-resolution and good-quality observations from the 1998 South China Sea Summer Monsoon Experiment (SCSMEX), the NCEP 40-year reanalysis data and relevant SST and the STH parameters. It is found that the vertical circulation structures differ greatly in features between quasi-stationary and transient components of the western Pacific STH. When rainstorms happen in the rainband of East Asian subtropical monsoon on the STH north side, the downdrafts are distinct around the ridge at a related meridian. The sinking at high (low) levels comes from the north (south) side of the STH, thereby revealing that the high is a tie between tropical and extratropical systems. The analyses of this paper suggest that the latent heat release associated with subtropical monsoon precipitation, the offshore SST and East Asian land-sea thermal contrast have a significant effect on the STH interannual anomaly. Our numer-ical experiment shows that the offshore SSTA-caused sensible heating may excite an anomalous anticyclonic circulation on the west side, which affects the intensity (area) and meridional position of the western Pacific STH.

Spatial Convergence of Bidirectional Reflectance Models

Abstract Analyses of bidirectional reflectance data are presented with implications regarding the spatial scales appropriate for inferring irradiances from radiances reflected by various surface–atmosphere scenes. Multiple-angle radiance data collected in a nearly simultaneous manner during the 1979 Summer Monsoon Experiment are analyzed using the squared coherency statistic to suggest a method to deduce the minimum spatial scale appropriate for irradiance inferences. Spatial convergence of the irradiances inferred from the component radiances is presented as a function of averaging distance to imply magnitudes of errors that may result from use of“similar scene” bidirectional reflectance models. The reduction in the inference errors with an increasing number of angular viewing positions is also presented. The data are analyzed in search of preferred viewing directions with the result that little improvement is imparted to the inference by viewing the scenes from any specific view direction.

Microphysical characteristics of monsoon clouds and cloud sublayers as revealed by the MONEX aircraft observations

The summer monsoon atmosphere was subjected to aerial cloud microphysical measurements by the National Center for Atmospheric Research (NCAR)'s Electra as part of the Arabian Sea component of the Summer Monsoon Experiment (SMONEX). The Particle Measuring System (PMS )'s probes were mounted on these flights. This study documents vertical profiles and concentration counts of aerosols and precipitation size particles for the flights on 20 and 24 June 1979. These flights were selected because the meteorology associated with them was analyzed in detail by several investigators who were interested in the turbulence flux measurements which were unique to these flights. The samples on 20 June were collected mostly underneath clouds except once within a cloud. In contrast, they were gathered exclusively within clouds on 24 June. The observations on these two days were collected in four different locations over the Arabian Sea when organized convection was presented. Results show that the aerosol concentration counts as measured by the ASAS-probe were higher than those expected from measurements in suppressed convective conditions. The ASAS and FSSP readings were nearly constant in the boundary layer. The FSSP readings increased more than ASAS readings with height passing them at elevations greater than 1500 m. Some young clouds with a depth of 1600 m showed maximum updraft values occurring at a certain level (e.g., 1000 m). At this level, a maximum concentration of droplets belonging to a particular size group ( ≈ 15 μm) was found. Thus a positive association was seen between these droplets and the vertical velocity distribution.

Characteristics of convection over the Arabian Sea during a period of monsoon onset

The organization of convection during the pre-onset, onset, and post-onset phases of monsoon has been studied using a linear stability model of convection which parameterized the latent heat released in preexisting cumulus convection, and the special thermodynamic and wind soundings collected during the Summer Monsoon Experiment (SMONEX) of 1979. The motivation for this study came from several satellite and research aircraft observations during SMONEX, which revealed that convection was organized into mesoscale bands over the Eastern (EAS), Central (CAS), and Western (WAS) Arabian seas. These bands varied in size from 20 to 100 km in length. Some (called longitudinal) were oriented parallel to the vertical shear of the base state winds; some (called transverse) were perpendicular. The linear stability model performed a normal mode analysis of convection and examined the factors governing the orientation of bands; longitudinal bands resulted when precipitation efficiency was high, and transverse, when the efficiency was low. The phase propagation of bands was reduced when more moisture was available. The linear stability model utilized a One Dimensional Steady State cloud (1DSS) model to estimate the latent heat released in preexisting cumulus convection. The 1DSS model developed Cumulus (Cu) clouds based on the observed temperature and humidity soundings in various parts of the Arabian Sea during different phases of the monsoon. Although this simple 1DSS model cannot reproduce faithfully all the characteristics of cumulus convection in the monsoon, some of the results, such as estimates of cloud tops, agreed well with those observed. Based on the 1DSS results and the kinematically computed mesoscale vertical velocities, three composite vertical cross sections (one for each): pre-onset, onset, and post-onset phases of the monsoon were constructed. Highlights of the cross sections were (1) during the pre-onset stage, WAS developed moderately strong Cu convection while EAS was inversion dominated; (2) during the onset stage, both CAS and EAS developed vigorous Cu convection while WAS did not show strong convection; and (3) during the post-onset stage, both CAS and EAS continued to develop Cu convection while WAS was inversion controlled. The growth of Cu convection seems to be in accord with satellite and research aircraft observations.

Observing Systems Experiments: Relative Model Response to Various FGGE Datasets in the Tropics

Abstract The successful deployment of many different observing systems during the summer Monsoon Experiment of 1979 provides a unique opportunity to perform extensive observing system experiments. These numerical studies, accomplished here with a ten-level, limited-area primitive equation model, allow the assessment of the value of individual or combined observing systems to the model's four-dimensional data assimilation system as well as to its subsequent forecasts. The specific objectives of this work include the investigation of (i) the relative merit of ten different data platforms, (ii) the relative role of wind and mass field data, (iii) the effect of different vertical distributions of single-level wind data, and (iv) the dynamical response of the model to different modes of data insertion. Eight experiments are summarized, all of which involved a 12-h data assimilation period based on the Newtonian relaxation procedure followed by a 36-h forecast. Predictions using all of the data produced very go...

Structure and Dynamics of Two Monsoon Depressions. Part I: Observed Structure

Part I of a two-part paper describes the wind, thickness, and cloud-field evolution of two monsoon depressions observed during the 1979 Summer Monsoon Experiment (SMONEX). One depression was associated with the June Arabian Sea monsoon onset; the other formed over the Bay of Bengal in early July. The wind-field analyses showed the southwesterly tilt with height of both depressions. Using the subjectively produced wind-field analyses, height and thickness fields were derived using the balance equation. The thickness fields clearly showed the evolution of key features of both depressions, such as the development of a cold lower troposphere southwest of the depression's surface center and the upper-tropospheric warm-air pool above the region of maximum rainfall, west of the surface center. East-west cross sections showed that the thermal anomalies associated with both depressions tilted westward with height. Mesoscale analyses using research aircraft data showed that key structures evident from the synoptic-scale analyses were also reflected in the mesoscale structure of the depressions. Notable was a feature resembling a weak tropical cyclone eye during the mature stage of both depressions.

Microphysical Observations in Summer MONEX Convective and Stratiform Clouds

Two-dimensional images of ice particles observed by a NOAA WP-3D research aircraft during the Summer Monsoon Experiment (SMONEX) are examined. These images were obtained in the temperature interval from −25° to 0°C. The particle structures and size distributions found in convective and stratiform clouds are compared. Branched crystals were located predominantly in stratiform clouds while column-shaped crystals were located commonly in both stratiform and convective clouds. Stratiform clouds, particularly those observed at temperature warmer than −7°C, had a much greater percentage concentration of large ice particles (>0.8 mm in diameter), and many of these ice particles were aggregates or branched crystals. The importance of aggregation and deposition above the melting level in the stratiform clouds is strongly suggested by these findings. Ice particle number concentrations measured with the cloud probe were often very high in convective clouds, with a maximum value of approximately 800 L−1. The average convective-cloud concentration was approximately 230 L−1, while the average concentration in the stratiform clouds was approximately 20 L−1. Liquid water was almost completely absent in the convective updrafts, at temperatures between −10° and −22°C. This suggests that the convective updrafts may have been nearly completely glaciated, and the microphysics were dominated by deposition. The high particle concentrations in the convective updrafts suggest that the updrafts may provide most of the ice particles found in the stratiform cloud. Significant modification in particle structures and size distributions have occurred, however, by the time these suspended particles fall out of the stratiform clouds. These modifications appear to arise from aggregation and deposition.

Convective Bands as Structural Components of an Arabian Sea Convective Cloud Cluster

Abstract During the summer Monsoon Experiment 1979 (MONEX-79) a unique mix of meteorological data was obtained on 20 June to analyze a convective cloud cluster that formed over the extreme eastern Arabian Sea. An analysis of the available data revealed the existence of several convective bands (as well as other types of convection) in the cloud cluster. The convective bands formed and decayed as a result of complex interactions between synoptic-scale features [low-level westerly jet and upper-level tropical easterly jet (TEJ)], mesoscale features (mesoscale downdrafts and mesoscale wind variability), and convective-scale features (convective updrafts and downdrafts). The observed southeasterly propagation of the convective bands on 20 June was explained through an examination of the thermodynamics of the updrafts and downdrafts on various scales. A comparison was made between the Arabian Sea convective bands and those observed in the GATE (Global Atmospheric Research Program's Atlantic Tropical Experiment...

Mesoscale Organization and Cloud Microphysics in a Bay of Bengal Depression

Airborne radar and cloud microphysical data were obtained throughout a monsoon depression observed over the Bay of Bengal on 3–8 July 1979 during the Summer Monsoon Experiment of the Global Atmospheric Research Programme. The precipitation in the disturbance occurred in mesoscale rain areas 100–300 km in horizontal dimension, with the largest areas tending to be elongated in the east-west or northwest-southeast direction. These mesoscale precipitation features resembled the rain areas of equatorial cloud clusters. Each mesoscale precipitation feature contained intense convective cells but the area covered by rain was predominantly stratiform. The convective cells in the mesoscale features were sometimes arranged in rapidly moving, rather narrow arc-shaped east–west lines, with the stratiform precipitation trailing the line. In other cases, the convective cells were more randomly embedded in the stratiform rain. Ice particle images obtained at flight-level temperatures of +5 to −25°C indicated the precipitation mechanisms in the stratiform regions of the mesoscale precipitation features. Crystals were growing in habits determined by the ambient temperature and were drifting downward. Liquid water was virtually absent above the 0°C level. Pristine shapes, including needles, columns, plates and dendrites were observed in maximum concentration about a kilometer below the altitude where their growth habit was determined. Large dendritic crystals apparently aggregated to form large snowflakes. As ice particles fell below the 0°C level, they abruptly melted to form the drops that constituted the stratiform rain. These observations of precipitation growth and fallout imply that mesoscale updraft motion strong enough to promote ice particle growth, but not strong enough to prevent sedimentation of the particles or to maintain liquid water in the presence of ice, was prevalent at upper levels in the stratiform regions of the mesoscale precipitation features.

Vertical Structure of the Southwest Monsoon Low-Level Jet over the Central and Eastern Arabian Sea

The vertical structure of the low-level wind jet over the Arabian Sea during the southwest monsoon is modeled by using the geostrophic wind shear (thermal wind) and a two-layer boundary layer model. We show that geostrophic wind shear is dominant above the low-level wind maximum while turbulent momentum exchange is dominant below. Both effects combine to produce a low-level wind jet. Model results compare well with observations obtained from a research aircraft during the 1979 Summer Monsoon Experiment.

Analysis of remote measurements of tropospheric carbon monoxide concentrations made during the 1979 summer monsoon experiment (MONEX)

Mixing ratios of tropospheric CO as measured by an aircraft‐mounted radiometer over Saudi Arabia, the Arabian Sea, and northern India during May and June 1979 are reported. During early May, exceptionally high CO levels were detected over Saudi Arabia, and strong horizontal gradients in CO mixing ratios were seen to develop over a period of several days. Over the Arabian Sea, mixing ratios of the order of 150 parts per billion by volume were observed before the monsoon onset, and a pronounced decrease in CO was detected toward the equator. Subsequent measurements after the monsoon had become established revealed a consistent decrease in CO mixing ratio across this region. Analysis of aircraft dropsonde data and constant pressure daily streamline charts lend strong support to the hypothesis that this reduction is associated with the influx of CO‐poor southern hemisphere air in the monsoon southwesterlies.

Gas filter radiometer for carbon monoxide measurements during the 1979 summer monsoon experiment (MONEX)

The instrumental and data reduction techniques used in obtaining remote measurements of carbon monoxide during the 1979 Summer Monsoon Experiment are described. The form of the signal function (the variation of signal with altitude) and the impact of variations in the vertical distribution of carbon monoxide are discussed. Estimates of the experimental accuracy are made both by assessment of error sources through the use of numerical simulations and by comparison with concurrent measurements made by means of gas chromatography. It is found that the radiometric measurements tend to be about 9% lower than the direct measurements and to have a precision of about 8%.

Calibration of an Airborne Lyman-Alpha Hygrometer and Measurement of Water Vapor Flux Using a Thermoelectric Hygrometer

Abstract An improved calibration technique for an airborne Lyman-alpha hygrometer is presented. Like previous methods, it relies upon simultaneous measurement of absolute humidity determined from a slower response hygrometer. We show that a substantial improvement in the Lyman-alpha calibration is obtained by accounting for the time lag of the slower instrument. To show our technique we use data from Lyman-alpha and thermoelectric devices on the NCAR Electra during an investigation of the nearly neutral boundary layer over the Arabian Sea as part of the WMO/ICSU Summer Monsoon Experiment. We also show that for near-neutral conditions the eddy-correlation water vapor flux can be adequately estimated using the fast response vertical velocity data from a gust probe and slower response data from the thermoelectric device, which has been properly advanced to account for the time lag.

Summer monsoon experiment— a review : Krishnamurti, T.N., 1985. Mon. Weath. Rev., 113(9): 1590–1626

Summer monsoon experiment— a review : Krishnamurti, T.N., 1985. Mon. Weath. Rev., 113(9): 1590–1626

Some synoptic aspects of the low level inversion over Arabian Sea during MONEX'79

Aerological observations over the Arabian Sea during the Summer Monsoon Experiment in 1979 show the presence of low level thermal inversion over the oceanic region prior to the onset, at the time of onset and at the established phase of the southwest monsoon. The thermal and moisture stratifications in the sub-inversion layer have been presented on the basis of thermodynamic chart. The spatial and temporal variations of the inversion layers as found from the dropsonde data coflected from the United States aircraft Convair 990, Electra and P-J, have been presented in the form of charts. The formation of inversion may be attributed mainly to the confluence of the cool moist air of oceanic origin and the dry warm air of continental origin.
 When a synoptic disturbance approached the area, a moist tongue was seen to form in the sub-inversion layer. The formation of this sharp moisture gradient may be related to the vertical upward motion below the inversion. The profiles of static stability during this period show that the height of the base of inversion becomes higher with the increase of static stability parameter in the sub-inversion layer.
 Vertical p-velocities have been calculated at different locations using the dropsonde data. The impact of the vertical p-velocity on the inversion layer at different phases of monsoon has been discussed.

Summer Monsoon Experiment—A Review

Abstract This paper presents a short summary of the Summer Monsoon Experiment (MONEX). The review is largely based on those papers that have made use of the summer MONEX observations during 1979. 0bservational aspects of this study emphasize the annual march of the monsoon rainfall belt from Indonesia to the foothills of the Himalayas, from the northern winter to the northern summer season and a reverse motion thereafter. The excellent FGGE/MONEX data sets have provided a detailed definition of the divergent wind; these are summarized with reference to the Hadley and the Walker circulations. The manner in which monsoonal circulations respond to the evolving differential heating fields are presented via the mutual interactions among the rotational and divergent wind components. Specific examples of heat sources from the studies of Luo and Yanai highlight their contrast over different regions of the monsoon including the Tibetan Plateau. A problem of considerable interest in this context is the cooling of t...

Structures of the Monsoon Low–Level Flow and the Monsoon Boundary Layer over the East Central Arabian Sea

Abstract The monsoon boundary layer was subjected to aerial measurements by the National Center for Atmospheric Research (NCAR) Electra on 20 and 24 June 1979 as part of the Arabian Sea component of the Summer Monsoon Experiment (MONEX). Gust probe (also called fast response or turbulence) measurements of u (eastward), v (northward), w, T and ρv, (absolute humidity), available from 80 to 550 m are used to compute turbulent fluxes, while slow response (one-second) measurements of u, v, T and Td available from 80 to 20 m, are used to relate thew fluxes to the low-level flow. The turbulence Rights on 20 June were made under an extensive synoptic scale cloud system. Within this cloud system variations in the cloud type and convective activity occurred between the two areas where intensive boundary layer (RL) measurements were made. The southern area, which had lower ρv values, was dominated by multiple cloud layers, downward virtual heat flux, weak upward moisture flux in the BL, and a sharply defined jet str...

Interaction of Low-Level Flow with the Western Ghat Mountains and Offshore Convection in the Summer Monsoon

Abstract Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure decelera...

Heat Low Over the Saudi Arabian Desert During May 1979 (Summer MONEX)

Abstract In May of 1979 a unique data set was obtained over the desert area in Saudi Arabia near the surface heat low as part of the GARP Monsoon Experiment (MONEX). Analysis of the data reveals that during the day a well-mixed layer extends up to 650 mb, while a stable layer forms above the desert surface during the night. Calculations of the divergence and kinematic vertical velocity show that the entire troposphere is dynamically active with strong descending motions during the day down to a 1 km deep surface layer which has ascending motion in it. There is daytime surface convergence, divergence around 850 mb, and convergence above 700 mb. During the pre-sunrise hours there is descending motion down to the surface with divergence at nearly all levels from the surface up to 550 mb and convergence above this level. The roles of vertical and horizontal advective, as well as radiative, processes in the thermal budget have been investigated. During the pre-sunrise hours the horizontal advective and longwav...