Total column ozone variations over oceanic region around Indian sub-continent during pre-monsoon of 2006

International audienceSpecial campaign mode ship-based sun photometric observations of total column ozone over the oceanic regions around the Indian sub-continent (56° E?6° E, 4° N?° N) during the pre-monsoon period (18 March?11 May) of 2006 have been used to investigate the spatial and temporal distributions. The overall mean ozone content over the sea region during this period is 298 DU with a variability of ±10 DU. There is a well defined diurnal (daytime) variation in total column ozone with maximum content around the noon-time hours. The amplitude of diurnal variation is higher over the Arabian Sea compared to that over Bay of Bengal. Spatial distribution of total ozone shows higher values over the Head Bay (North Bay of Bengal) and all along the west coast of India strongly pointing to continental origin of possible anthropogenic source. This is further corroborated from the spatial distribution of simultaneously measured aerosol optical thickness (AOT, at 1020 nm) and precipitable water. The overall mean AOT over the oceanic region is 0.09 and mean precipitable water (water vapor) over Indian Ocean region was 3.25 cm which is almost 1 cm higher than that observed over Bay of Bengal and Arabian Sea during the above pre-monsoon period

Wind profiler observations of a monsoon low-level jet over a tropical Indian station

Three-year high-resolution wind observations of the wind profiler have been utilized to characterize the diurnal and seasonal features of the monsoon Low-Level Jet (LLJ) over a tropical station, Gadanki (13.5° N, 79.2° E), with a focus on the diurnal variability of low-level winds. The Boreal summer monsoon winds show a conspicuously strong westerly LLJ with average wind speed exceeding 20 m s<sup>−1</sup>. The L-band wind profiler measurements have shown an advantage of better height and time resolutions over the conventional radiosonde method for diurnal wind measurements. An interesting diurnal oscillation of LLJ core has been observed. It is varying in the height range of 1.8±0.6 km with the maximum and minimum intensity noticed during the early morning and afternoon hours, respectively. The jet core (wind maxima) height is observed to coincide with the inversion height. Strong wind shears are normally located beneath the LLJ core. The sole wind profiler observations are capable of identifying the monsoon phases, such as onset, break and active spells, etc. The mutual influence between the LLJ and the boundary layer has been discussed. One notices that the observed LLJ diurnal structures depend on the local convective activity, wind shears and turbulence activity associated with boundary layer winds. The day-to-day change in the LLJ structure depends on the latitudinal position of the LLJ core

Heterogeneity in pre-monsoon aerosol types over the Arabian Sea deduced from ship-borne measurements of spectral AODs

Ship-borne sunphotometer measurements obtained in the Arabian Sea (AS) in the pre-monsoon season (18 April–10 May 2006) during a cruise campaign (ICARB) have been used to retrieve the Aerosol Optical Depth (AOD; τ) and the Ångström wavelength exponent (α). The continents surrounding the AS produce natural and anthropogenic aerosols that have distinctive influences on α and its spectral distribution. The α values were estimated by means of the least-squares method over the spectral bands 340–1020 nm and 340–870 nm. The spectral distribution of AOD in logarithmic co-ordinates could be fit using a 2nd order polynomial with higher accuracy in the wavelength band 340–1020 nm than in the 340–870 nm band. A polynomial fit analytically parameterizes the observed wavelength dependencies of AOD with least errors in spectral variation of α and yields accurate estimates of the coefficients (<i>a</i><sub>1</sub> and <i>a</i><sub>2</sub>). The coarse-mode (positive curvature in the lnτ<sub>λ</sub> vs. lnλ) aerosols are mainly depicted in the Northern part of the AS closely associated with the nearby arid areas while fine-mode aerosols are mainly observed over the far and coastal AS regions. In the study period the mean AOD at 500 nm is 0.25±0.11 and the α<sub>340-1020</sub> is 0.90±0.19. The α<sub>340-870</sub> exhibits similar values (0.92±0.18), while significant differences revealed for the constant terms of the polynomial fit (<i>a</i><sub>1</sub> and <i>a</i><sub>2</sub>) proportionally to the wavelength band used for their determination. Observed day-to-day variability in the aerosol load and optical properties are direct consequence of the local winds and air-mass trajectories along with the position of the ship

Characteristics of Rain Integral Parameters during Tropical Convective, Transition, and Stratiform Rain at Gadanki and Its Application in Rain Retrieval

In the present study the characteristics of rain integral parameters during tropical convective (C), transition (T), and stratiform (S) types of rain are studied with the help of Joss–Waldvogel disdrometer (JWD), L-band, and very-high-frequency wind profilers at Gadanki (13.5°N, 79.20°E). The classifications of three regimes are made with the help of an L-band wind profiler. For rain rate R C > S rain. During the three types of rain, correlations are found in the order of Z/Dm–R > Z–R > Dm–Z > Dm–R. Significant improvement is observed in rain retrieval by using the Z/Dm–R relation relative to the conventional Z–R relation. By utilizing the Z/Dm–R relations, the root-mean-square error was reduced by 19%–46%

Wind profiler analysis of the African Easterly Jet in relation with the boundary layer and the Saharan heat-low

International audienceWind profiler measurements in Niger and Benin during the African Monsoon Multidisciplinary Analysis project are used to study the intraseasonal variability of the low and mid troposphere at several time-scales. We focus on the African Easterly Jet (AEJ) and its interaction with the Saharan Heat-Low (HL) and the planetary boundary layer (PBL). We find a pronounced diurnal cycle of the AEJ, characterised by a decrease of wind speed during the afternoon, reaching a minimum at 1800 UTC of about 15-20% of the daily average during the pre-onset period. This decrease is out of phase with the HL intensity, but in phase with the daytime turbulent mixing associated with the PBL. The interaction between the PBL and mid-troposphere is likely responsible for this daily decrease of the AEJ. During the transition periods (dry to wet or wet to dry), the HL seems to govern the AEJ; however, slightly before the monsoon onset, it has no direct influence on the jet. During that time, we find smaller AEJ wind speed for deeper PBL, as found at the diurnal time-scale. This is consistent with the still large surface heating at that time, which favours deep PBL growth, with a top inversion often higher than the shear layer between the monsoon and the easterlies. After the monsoon onset, deep convection, African Easterly Waves (AEWs) and dry intrusions make the synoptic environment complex and blur the interaction between AEJ and PBL. We still find weaker AEJ for deeper PBL, but likely without a direct connection between them

Multi-decadal variation of the net downward shortwave radiation over south Asia: the solar dimming effect

The solar radiation flux at the earth's surface has gone through decadal changes of decreasing and increasing trends over the globe. These phenomena known as dimming and brightening, respectively, have attracted the scientific interest in relation to the changes in radiative balance and climate. Despite the interest in the solar dimming/brightening phenomenon in various parts of the world, south Asia has not attracted great scientific attention so far. The present work uses the net downward shortwave radiation (NDSWR) values derived from satellites (Modern Era Retrospective-analysis for Research and Applications, MERRA 2D) in order to examine the multi-decadal variations in the incoming solar radiation over south Asia for the period of 1979–2004. From the analysis it is seen that solar dimming continues over south Asia with a trend of −0.54 Wm⁻² yr⁻¹. Assuming clear skies an average decrease of −0.05 Wm⁻² yr⁻¹ in NDSWR was observed, which is attributed to increased aerosol emissions over the region. There is evidence that the increase in cloud optical depth plays the major role for the solar dimming over the area. The cloud optical depth (MERRA retrievals) has increased by 10.7% during the study period, with the largest increase to be detected for the high-level (atmospheric pressure P < 400 hPa) clouds (31.2%). Nevertheless, the decrease in solar radiation and the role of aerosols and clouds exhibit large monthly and seasonal variations directly affected by the local monsoon system, the anthropogenic and natural aerosol emissions. All these aspects are examined in detail aiming at shedding light into the solar dimming phenomenon over a densely populated area

A simple biota removal algorithm for 35 GHz cloud radar measurements

Cloud radar reflectivity profiles can be an important measurement for the investigation of cloud vertical structure (CVS). However, extracting intended meteorological cloud content from the measurement often demands an effective technique or algorithm that can reduce error and observational uncertainties in the recorded data. In this work, a technique is proposed to identify and separate cloud and non-hydrometeor echoes using the radar Doppler spectral moments profile measurements. The point and volume target-based theoretical radar sensitivity curves are used for removing the receiver noise floor and identified radar echoes are scrutinized according to the signal decorrelation period. Here, it is hypothesized that cloud echoes are observed to be temporally more coherent and homogenous and have a longer correlation period than biota. That can be checked statistically using  ∼ 4 s sliding mean and standard deviation value of reflectivity profiles. The above step helps in screen out clouds critically by filtering out the biota. The final important step strives for the retrieval of cloud height. The proposed algorithm potentially identifies cloud height solely through the systematic characterization of Z variability using the local atmospheric vertical structure knowledge besides to the theoretical, statistical and echo tracing tools. Thus, characterization of high-resolution cloud radar reflectivity profile measurements has been done with the theoretical echo sensitivity curves and observed echo statistics for the true cloud height tracking (TEST). TEST showed superior performance in screening out clouds and filtering out isolated insects. TEST constrained with polarimetric measurements was found to be more promising under high-density biota whereas TEST combined with linear depolarization ratio and spectral width perform potentially to filter out biota within the highly turbulent shallow cumulus clouds in the convective boundary layer (CBL). This TEST technique is promisingly simple in realization but powerful in performance due to the flexibility in constraining, identifying and filtering out the biota and screening out the true cloud content, especially the CBL clouds. Therefore, the TEST algorithm is superior for screening out the low-level clouds that are strongly linked to the rainmaking mechanism associated with the Indian Summer Monsoon region's CVS