Break Spells Research Articles

Modulation of summer monsoon sub-seasonal surface air temperature over India by soil moisture-temperature coupling

The influence of soil moisture on the sub-seasonal warmer surface air temperature anomalies during drier soil conditions associated with break spells in the Indian summer monsoon precipitation is explored using observations. The multi-model analysis of land surface states and fluxes available from the Second Global Soil Wetness Project (GSWP-2) are found useful in understanding the mechanism for this soil moisture-temperature coupling on sub-seasonal timescales. The analysis uses a soil moisture-temperature coupling diagnostic computed based on linear correlations of daily fields. It is shown that the summer surface air temperature variations are linked to intraseasonal variations of the Indian monsoon precipitation, which control the land-climate coupling by modulating the soil moisture variations. Strong coupling mainly occurs during dry soil states within the summer monsoon season over the transition zones between wet and dry climates of central to north-west India. In contrast, the coupling is weak for constantly wet and energy-limited evaporative regimes over eastern India during the entire summer monsoon season. This observational based analysis provided a better understanding of the linkages between the sub-seasonal dry soil states and warm conditions during the Indian summer monsoon season. A proper representation of these aspects of land-atmosphere interactions in weather and climate models used for sub-seasonal and seasonal monsoon forecasting could be critical for several applications, in particular agriculture. The soil moisture-temperature coupling diagnostic used in this study will be a useful metric for evaluating the performance of weather and climate models.

Unraveling the missing link of ENSO control over the Indian monsoon rainfall

AbstractUsing long daily rainfall (113 years) data, clear evidence of modulation of the statistics of subseasonal active and break spells of the Indian summer monsoon rainfall (ISMR) by the El Niño–Southern Oscillation (ENSO) is unraveled. We show that during the El Niño (La Nina) years, the frequency of longer break (active) spells and shorter active (break) spells increases substantially, while the frequency of shorter breaks (active) and longer active (break) spells decreases compared to ENSO neutral years. It is shown that the large‐scale teleconnection through which ENSO weakens (strengthens) the ISMR through shortening (lengthening) of the length of rainy season (LRS) also creates a background for modulation of the statistics of subseasonal spells leading to a further reduction (increase) in the ISMR during El Niño (La Niña) years. Our findings indicate that the “statistics of subseasonal spells” is predictable, brightening the prospect of seasonal prediction of the ISMR.

Impact of aerosols and cloud parameters on Indian summer monsoon rain at intraseasonal scale: a diagnostic study

Aerosol and cloud parameters are known to be the influencing factors of the Indian summer monsoon rainfall (ISMR) variability at interannual and intraseasonal scales. In this study, we investigate the impact of remotely sensed aerosol optical depth and associated parameters (cloud fraction, cloud optical depth, cloud effective radii, cloud top pressure, and single-scattering albedo) on the individual active (break) spells of the Indian summer monsoon (ISM) season. Active and break spells are identified using satellite-derived data sets over the central Indian (CI) region. The present analysis suggests that the CI region is loaded with higher aerosol concentration and that rainfall is significantly negatively correlated with aerosol optical depth (significant at 1 % significance level) over CI. Contrary to the composite-based previous studies, it has been observed that the aerosol loading and cloud properties are considerably different during the individual active and break events. For break events, composite representation shows that aerosols are stacked along the Himalayan region while all individual break events do not portray this type of aerosol dispensation. It appears from the present analysis that the aerosols may impact the intraseasonal variability of ISMR through its indirect effect by altering the cloud properties and consequently the rainfall. Therefore, aerosols are supposed to be a regional contributor in affecting the intraseasonal variability of summer monsoon rainfall.

Active and break events of Indian summer monsoon during 1901–2014

The study lists active and break monsoon events over India over a very long period (1901–2014) identified using criteria based on a rainfall index derived over a critical high rainfall region called core monsoon zone. The break and active spells identified in this study were mostly comparable with that identified in the earlier studies based on similar rainfall criteria during the common data period (1951–2007). However, some noticeable differences were observed in the rainfall anomaly pattern associated with the break monsoon spells identified in this study and that identified based on the synoptic criteria in the earlier studies. The stringent rainfall criteria used in this study seems to be better criteria for identifying the breaks. During the study period, both the active and break spells of short duration were more frequent than the long duration with about 63.4 % of the break spells and 94.3 % of the active spells falling in the range of 3–6 days. There were no active spells of duration ≥13 days. Whereas, about 8 % of the break spells were of duration ≥13 days. During both the halves of the data period (1901–1957 and 1958–2014), there was no change in the distribution of the break events. However, the number of active spells showed an increase of about 12 % in the in the second half, which was mainly in the short duration (3–6 days) spells. During the data period, decadal variations of break days showed an out phase of relationship with the number of days of monsoon depression (MD). Relatively stronger in phase relationship was observed between the decadal variation of MD days and that of the active days till around early 1980s which reversed later due to sudden decrease in the MD days. During the same period, both the active and break days were in the increasing phase. This was also coincided with the sudden and significant increase in the number of days of monsoon lows (LOW). The LOWs, which generally have short life helped in the occurrence of active spells of short duration. Thus, post early 1980s, the increase in the active days covering short duration active spells caused by the significant increase in the LOW days compensated the decrease in the active days covering relatively long duration active spells caused by the MD days. This lead to the out of phase relationship between MD days and the active days post early 1980s.

Study of intraseasonal variability of Indian summer monsoon using a regional climate model

The Indian summer monsoon season is very heterogeneous over Indian land mass from precipitation point of view. The intraseasonal variability of the rainfall during summer is marked by the active and break spells of the rainfall. The regional climate model version 4.0 (RegCM4.0) forced with European centre of medium range weather forecast interim reanalysis (ERA-Int) is used to examine the intraseasonal variability and meteorological processes associated with it. The model rightly represents the climatology of different fields such as the surface temperature, sea level pressure, lower level wind and the precipitation for monsoon season. The model captures the different active and break spells and the results are in agreement with the observed value and previous studies. The major features of the active/break periods, such as the positive/negative rainfall anomaly over the monsoon core region (MCR) and negative/positive rainfall anomaly over the foothills of Himalayas and southern part of India is nicely represented in the model. The model rightly reproduces the evolution of the active and break phase and also the revival from the break period by the northward propagation of active rainfall anomaly. The heat trough type of circulation is analysed in detail along with the atmospheric condition during active and break spell over the MCR. The atmospheric condition over MCR resembles the heat trough type circulation during break spells. The moisture availability, moisture–precipitation relation and their transition during active and break period over the MCR is established.

Future projection of Indian summer monsoon variability under climate change scenario: An assessment from CMIP5 climate models

In this study, the impact of enhanced anthropogenic greenhouse gas emissions on the possible future changes in different aspects of daily-to-interannual variability of Indian summer monsoon (ISM) is systematically assessed using 20 coupled models participated in the Coupled Model Inter-comparison Project Phase 5. The historical (1951–1999) and future (2051–2099) simulations under the strongest Representative Concentration Pathway have been analyzed for this purpose. A few reliable models are selected based on their competence in simulating the basic features of present-climate ISM variability. The robust and consistent projections across the selected models suggest substantial changes in the ISM variability by the end of 21st century indicating strong sensitivity of ISM to global warming. On the seasonal scale, the all-India summer monsoon mean rainfall is likely to increase moderately in future, primarily governed by enhanced thermodynamic conditions due to atmospheric warming, but slightly offset by weakened large scale monsoon circulation. It is projected that the rainfall magnitude will increase over core monsoon zone in future climate, along with lengthening of the season due to late withdrawal. On interannual timescales, it is speculated that severity and frequency of both strong monsoon (SM) and weak monsoon (WM) might increase noticeably in future climate. Substantial changes in the daily variability of ISM are also projected, which are largely associated with the increase in heavy rainfall events and decrease in both low rain-rate and number of wet days during future monsoon. On the subseasonal scale, the model projections depict considerable amplification of higher frequency (below 30day mode) components; although the dominant northward propagating 30–70day mode of monsoon intraseasonal oscillations may not change appreciably in a warmer climate. It is speculated that the enhanced high frequency mode of monsoon ISOs due to increased GHG induced warming may notably modulate the ISM rainfall in future climate. Both extreme wet and dry episodes are likely to intensify and regionally extend in future climate with enhanced propensity of short active and long break spells. The SM (WM) could also be more wet (dry) in future due to the increment in longer active (break) spells. However, future changes in the spatial pattern during active/break phase of SM and WM are geographically inconsistent among the models. The results point out the growing climate-related vulnerability over Indian subcontinent, and further suggest the requisite of profound adaptation measures and better policy making in future.

An assessment of real‐time extended range forecast of 2013 Indian summer monsoon

AbstractSeveral aspects of real‐time forecast of Indian summer monsoon (ISM) in 3–4 pentad lead time (extended range) are discussed in this study to explore the operational capability of the Climate Forecast System model version 2 (CFSv2) developed by National Centre for Environmental Prediction (NCEP). 2013 summer monsoon was a near excess monsoon year in terms of seasonal mean and was a result of rich diversity of phenomena including strong intraseasonal variations and intense northward propagations over the Indian region. Eleven‐member forecasts were made at every 5‐day interval during the June–September monsoon season which included monsoon onset and withdrawal phases. The ensemble members were created by perturbing the initial conditions at each start time. In addition to the CFSv2 forecasts, we also carried out forecasts using the atmospheric‐only component (GFSv2) forced with CFSv2‐derived sea surface temperature (SST) subjected to a bias correction based on historical observations (GFSbc runs).Both the CFSv2 and GFSbc runs were able to predict the progression of ISM over the Indian region and the subsequent intraseasonal oscillations (active and break phases). The analysis for an extreme event (Uttarakhand flood) and monsoon revival (MR) towards the end of the season was also performed. Comparison between the two runs shows that active and break spells were predicted with good fidelity over the Indian region, though GFSbc outperforms CFSv2 on several occasions. Thus, improvement of the operational monsoon forecast over Indian region using NCEP CFSv2 requires better representation of air–sea interaction and mean states of ocean.

Assessment of the Indian summer monsoon in the WRF regional climate model

The performance of the regional climate model, Weather Research and Forecasting in simulating the three dimensional moist and thermodynamic structure of Indian summer monsoon (ISM) during 2001–2011 is examined in this study. The model could simulate monsoon elements and convective precipitation zones over ISM region with some overestimation. Statistical analysis of sub-regional precipitation indicates that model has better skill over the monsoon core region with correlation of 0.7 and root mean square error of 2.3 mm day−1 with respect to observations. The model simulated seasonal mean vertical structures of temperature and water vapour mixing ratio (WVMR) are consistent with the Atmospheric Infrared Sounder observations. However, the core of low level jet is shifted southward in the model due to unrealistic convective heating over the lower latitudes of Indian Ocean and southern peninsular India. The tropical easterly jet is confined to 15°N in the model, which is due to the midtropospheric cold bias over the Tibetan region. The meridional asymmetric bias of sea level pressure (SLP) in model leads to weaker vertical wind shear, limiting the northward migration of maximum rain band to south of 23°N. These discrepancies have marked effects on the proper simulation of monsoon climate. The large scale spatial patterns of SLP, precipitation and winds during active and break spells are well simulated by the model. The lead-lag evolution of vertical structure of model temperature shows baroclinic structure during the active phase. It is evident from the observations that enhanced (suppressed) convection is generally preceded by a low-level moist (dry) anomaly and followed by a low-level dry (moist) anomaly. The model is inadequately representing the temporal evolution of vertical moist and thermodynamic processes. The evolution of vertical structures of temperature and WVMR is better simulated in the break phase compared to that of active phase. The evolution of cyclonic vorticity in the model is different from the observations during the convective phase. In short the model has limitations in representing convectively unstable regimes. It is anticipated that these findings will significantly contribute to the regional climate model assessment programs.

Trend analysis of evapotranspiration and its response to droughts over India

Six temperature homogeneous regions of India were chosen to calculate evapotranspiration (ET) using Hargreaves and Samani method on a monthly basis. Break Trend Analysis has been applied to the annual and seasonal ETs of the respective homogeneous regions as well as for whole India and the factors contributing for the changes in ET have been analysed. By considering the data of Reconnaissance Drought Index (RDI), Rainfall, Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) for South West (SW) monsoon (June to September) and Rice yield (major Khariff crop yield) of All India, we investigated the variations in ET during drought years. The trend analysis of seasonal and annual ETover the test regions have shown significant (levels of 0.05 to 0.01) increase over the past 107 years of the study period (1901 to 2007). The enhancement in ET is found to be 2.9 mm/decade overIndia and with a maximum in West Coast India (6 mm/decade), followed by Northwest India (3.9 mm/ decade). The increasing trends in ET may be due to the increase in difference of maximum and minimum tempera- tures over these regions, which is of the order of 0.64 °C/ decade on all India scale. The analysis of ET in drought years consistently shows higher values in the years when India suffered under moderate and severe droughts. It is also ob- served that lower rainfall values in the drought years such as 1983, 1987 and 2002 were associated with the higher ETs accompanied by lower NDVI and Rice yields. The analysis has been extended to the comparison of ET with rainfall through wavelet spectrum to understand the interannual vari- ations which inferred the El Nino impact on ET. The correla- tionanalysis of ETover the Interior Peninsular regionwith the number of days falling in active and break spells of monsoon by which drought can be characterised, revealed the signifi- cant negative and positive values, respectively.

The dominant intraseasonal mode of intraseasonal South Asian summer monsoon

AbstractFrom June through September, the intraseasonal variability of the Asian summer monsoon is dominated by the so‐called “monsoon intraseasonal oscillation (MISO).” This paper provides a comprehensive description of the MISO based on outgoing longwave radiation (OLR) data. The MISO is characterized by alternating active periods, in which the primary rain area of the Asian summer monsoon that stretches from the northern Arabian Sea east southeastward almost all the way to the northwest Pacific Intertropical Convergence Zone is relatively intense, and break periods, in which the heaviest rainfall shifts from south Asia to the central and eastern equatorial Indian Ocean. The MISO is attended by well‐defined but weak sea surface temperature (SST) perturbations whose phase is indicative of a negative feedback upon the atmospheric perturbations. Meridional profile of variables on the various regression maps shown in this paper averaged along a set of tilted axes parallel to the west‐northwest to east‐southeast (WNW‐ESE) sloping lines in empirical orthogonal function 1 of OLR have been made, and it is found that the strongest westerly 850 hPa wind anomalies are located two grid points (5° of latitude) to the south of the reference latitude. At the 150 hPa level, the meridional profile of divergence is closely aligned with the OLR profile. SST profile is lowest at approximately 2.5° of latitude to the south of the minimum OLR and 2.5° to the north of the strongest westerly 850 hPa wind anomalies. The sea level pressure profiles and the midlower tropospheric geopotential height profiles are almost in phase. It is observed that in most years, there are two–three bands of intensified and suppressed rainfall that cross the reference line from south to north (northward propagating) at the interval of 30–60 days over South Asia. The degree of correspondence between the MISO and active and break spells of the Indian summer monsoon rainfall is also documented.

Does bias correction in the forecasted SST improve the extended range prediction skill of active‐break spells of Indian summer monsoon rainfall?

AbstractImpact of bias correction of sea surface temperature (SST) forecast on extended range (ER, ∼3–4 weeks) prediction skill is studied using the bias‐corrected forecasted SST from Climate Forecast System version 2 (CFSv2) as the boundary condition for running the Global Forecast System version 2 (GFSv2) model. Potential predictability limit is comparable (∼16 days) for both bias‐corrected GFSv2 (GFSv2bc) and CFSv2. Prediction skills of active and break spells and of low‐frequency monsoon intraseasonal oscillations is higher for GFSv2bc at all lead pentads. Although initially same, predictability error after 14 days grows slightly faster for GFSv2bc compared to CFSv2. Bias correction in SST has minimal impact in short‐to‐medium range, while substantial influence is felt in ER between 12–18 days.

Predictability during active break phases of Indian summer monsoon in an ensemble prediction system using climate forecast system

This study examines the phase dependant temporal and spatial error evolution and prediction of active break spells of Indian summer monsoon rainfall in an ensemble prediction system (EPS) on a pentad time scale using climate forecast system (CFS). The EPS system shows systematic wet bias (overestimation) over west coast over the Arabian Sea and Myanmar coast and dry bias (underestimation) over Indian land mass even at pentad 1 lead and these biases consistently increase up to 4 pentad lead and saturate thereafter. Irrespective of the phases of the monsoon, the lower bound of predictability is 2 pentads, while upper bound of predictability for initial conditions starting from active phase saturates at 3 pentads and for break and transition phases predictability error saturates at a later stage at about 5 pentad. Initial conditions started from transition phase shows higher potential predictability followed by break phase and then active phase.

Characteristics of monsoon breaks and intraseasonal oscillations over central India during the last half century

Using 1°×1° daily rainfall data set of 50years, prepared by the India Meteorological Department, a detailed analysis of active and break spells has been carried out along with the investigation of propagation characteristics and the temporal variation of the intraseasonal oscillations of Indian summer monsoon rainfall. Present analysis reveals that the frequency of the short break spells (3days) and moderate active spells (4–7days) have increased after 1977 over a statistically homogenous Central Indian region (16.5–26.5°N; 74.5–86.5°E). It appears that most of the break spells are spatially localized over a smaller region of central India and the frequency of breaks over this region has increased in the recent decades. The area that is prone to breaklike conditions is found to be increasing after 1977 as compared with before 1977. It is also illustrated that the behavior of the intraseasonal oscillations differs in terms of propagation during the two study periods. A new method is also proposed to identify breaks over central Indian region, which shows a good match with previous studies. The results presented here are statistical in nature.

Space–Time Scales of Northward Propagation of Convection during Boreal Summer

Abstract This study uses precipitation estimates from the Tropical Rainfall Measuring Mission to quantify the spatial and temporal scales of northward propagation of convection over the Indian monsoon region during boreal summer. Propagating modes of convective systems in the intraseasonal time scales such as the Madden–Julian oscillation can interact with the intertropical convergence zone and bring active and break spells of the Indian summer monsoon. Wavelet analysis was used to quantify the spatial extent (scale) and center of these propagating convective bands, as well as the time period associated with different spatial scales. Results presented here suggest that during a good monsoon year the spatial scale of this oscillation is about 30° centered around 10°N. During weak monsoon years, the scale of propagation decreases and the center shifts farther south closer to the equator. A strong linear relationship is obtained between the center/scale of convective wave bands and intensity of monsoon precipitation over Indian land on the interannual time scale. Moreover, the spatial scale and its center during the break monsoon were found to be similar to an overall weak monsoon year. Based on this analysis, a new index is proposed to quantify the spatial scales associated with propagating convective bands. This automated wavelet-based technique developed here can be used to study meridional propagation of convection in a large volume of datasets from observations and model simulations. The information so obtained can be related to the interannual and intraseasonal variation of Indian monsoon precipitation.

Validation of TMPA and GPCP 1DD against the ground truth rain‐gauge data for Indian region

ABSTRACTThe satellite datasets, namely, the Tropical Rainfall Measuring Mission (TRMM) MultiSatellite Precipitation Analysis (TMPA) product, 3B42 version 6 and Global Precipitation Climatology Project (GPCP) one‐degree‐daily (1DD) from NASA Goddard Space Flight Center, have been validated against the ‘ground truth’ rain‐gauge data from the India Meteorological Department for all‐India during the period 1998–2007. The comparisons reveal that both the satellite datasets are good enough in representing the spatial rainfall distribution over India. The categorical statistics provides an insight in evaluating the satellite's ability and its accuracy in estimating the precipitation. The average bias along the heavy‐rainfall regions (e.g. Western coast, Arunachal Pradesh, Sub‐Himalayan West Bengal, Sikkim, and east Uttaranchal) for the TMPA (GPCP 1DD) is −8.03 (−8.29) mm d−1, whereas the mean is 7.47 (6.97) mm d−1, which signifies that the satellite datasets underestimate the Indian summer monsoon rainfall in sharp rainfall regions. Albeit, the TMPA excels over GPCP 1DD where the rainfall is mainly attributed to orography, yet both fail in capturing the extreme rainfall year, 2005. The satellite datasets do capture the intraseasonal oscillations as is evident from spectral analysis, whereas the empirical orthogonal function analysis of standardized daily and seasonal anomalies exemplifies that the satellite datasets also capture the dominant modes. Additionally, the active and the break spells calculated for the months of July and August for the satellite datasets are in agreement with the ones obtained for the rain‐gauge.

A possible new mechanism for northward propagation of boreal summer intraseasonal oscillations based on TRMM and MERRA reanalysis

Boreal summer intraseasonal oscillations (BSISOs) manifest in the active and break spells and act as the primary building block of the Indian summer monsoon. Although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the BSISO, the role of different hydrometeors in modulating these processes remains poorly explored. In this study, TRMM-2A12 retrievals and Modern Era Retrospective-analysis for Research and Applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the BSISOs. The study reveals that the cloud liquid water leads the deep convection during the northward propagation of BSISOs in the lower troposphere, while the cloud ice slightly lags the convection. This distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer (PBL) convergence, followed by triggering of the deep convection. The analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and PBL convergence of moisture anomaly north of the convection center. A new internal dynamical-thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation. It is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub-grid scale eddies within the PBL induces lower level instability required to precondition the lower atmosphere for triggering the deep convection. Vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere. To check the robustness of the proposed hypothesis, similar analysis is performed for the weak northward propagating BSISO cases.

Aerosol indirect effect during the aberrant Indian Summer Monsoon breaks of 2009

The significant role of aerosol–cloud interaction during the large-scale drought producing breaks of 2009 Indian Summer Monsoon is investigated in the present paper. This mega drought had already been attributed to two long breaks, one in June and the other in July-August. While Central India (CI) and northern parts of the country experienced deficient rainfall, the rainfall over the southern Peninsular India (PI) remained close to normal. During the first break in June, which was associated with mid-latitude intrusion of dry air, the Twomey effect (positive aerosol indirect effect – AIE) was a dominant factor inhibiting efficient precipitation over CI region, as compared to that over PI. Moreover, the number of days that experienced significant (at 5% level of significance) positive AIE during the first break was more over CI compared to the same during the second break. The AIE on ice clouds was not as significant as that of the low-clouds. The resulting cloud properties during both break and active phases over CI differ significantly from that over PI for the corresponding periods. The positive AIE mentioned here is attributed to the large-scale deficit of moisture supply to the CI region due to dynamical reasons. However, it is shown that under ample availability of moisture, more aerosols could invigorate deep clouds over specific regions even during the break spells.

A study of vertical cloud structure of the Indian summer monsoon using CloudSat data

Precise specification of the vertical distribution of cloud optical properties is important to reduce the uncertainty in quantifying the radiative impacts of clouds. The new global observations of vertical profiles of clouds from the CloudSat mission provide opportunities to describe cloud structures and to improve parameterization of clouds in the weather and climate prediction models. In this study, four years (2007–2010) of observations of vertical structure of clouds from the CloudSat cloud profiling radar have been used to document the mean vertical structure of clouds associated with the Indian summer monsoon (ISM) and its intra-seasonal variability. Active and break monsoon spells associated with the intra-seasonal variability of ISM have been identified by an objective criterion. For the present analysis, we considered CloudSat derived column integrated cloud liquid and ice water, and vertically profiles of cloud liquid and ice water content. Over the South Asian monsoon region, deep convective clouds with large vertical extent (up to 14 km) and large values of cloud water and ice content are observed over the north Bay of Bengal. Deep clouds with large ice water content are also observed over north Arabian Sea and adjoining northwest India, along the west coast of India and the south equatorial Indian Ocean. The active monsoon spells are characterized by enhanced deep convection over the Bay of Bengal, west coast of India and northeast Arabian Sea and suppressed convection over the equatorial Indian Ocean. Over the Bay of Bengal, cloud liquid water content and ice water content is enhanced by ~90 and ~200 % respectively during the active spells. An interesting feature associated with the active spell is the vertical tilting structure of positive CLWC and CIWC anomalies over the Arabian Sea and the Bay of Bengal, which suggests a pre-conditioning process for the northward propagation of the boreal summer intra-seasonal variability. It is also observed that during the break spells, clouds are not completely suppressed over central India. Instead, clouds with smaller vertical extent (3–5 km) are observed due to the presence of a heat low type of circulation. The present results will be useful for validating the vertical structure of clouds in weather and climate prediction models.

A comparative study of the Indian summer monsoon hydroclimate and its variations in three reanalyses

This study examines the Indian summer monsoon hydroclimate in the National Centers for Environmental Prediction (NCEP)-Department of Energy (DOE) Reanalysis (R2), the Climate Forecast System Reanalysis (CFSR), and the Modern Era Retrospective-Analysis for Research and Applications (MERRA). The three reanalyses show significant differences in the climatology of evaporation, low-level winds, and precipitable water fields over India. For example, the continental evaporation is significantly less in CFSR compared to R2 and MERRA. Likewise the mean boreal summer 925 hPa westerly winds in the northern Indian Ocean are stronger in R2. Similarly the continental precipitable water in R2 is much less while it is higher and comparable in MERRA and CFSR. Despite these climatological differences between the reanalyses, the climatological evaporative sources for rain events over central India show some qualitative similarities. Major differences however appear when interannual variations of the Indian summer monsoon are analyzed. The anomalous oceanic sources of moisture from the adjacent Bay of Bengal and Arabian Sea play a significant role in determining the wet or dry year of the Indian monsoon in CFSR. However in R2 the local evaporative sources from the continental region play a more significant role. We also find that the interannual variability of the evaporative sources in the break spells of the intraseasonal variations of the Indian monsoon is stronger than in the wet spells. We therefore claim that instead of rainfall, evaporative sources may be a more appropriate metric to observe the relationship between the seasonal monsoon strength and intraseasonal activity. These findings are consistent across the reanalyses and provide a basis to improve the predictability of intraseasonal variability of the Indian monsoon. This study also has a bearing on improving weather prediction for tropical cyclones in that we suggest targeting enhanced observations in the Bay of Bengal (where it is drawing the most moisture from) for improved analysis during active spells of the intraseasonal variability of the Indian monsoon. The analysis suggests that the land–atmosphere interactions contribute significant uncertainty to the Indian monsoon in the reanalyses, which is consistent with the fact that most of the global reanalyses do not assimilate any land-surface data because the data are not available. Therefore, the land–atmosphere interaction in the reanalyses is highly dependent on the land-surface model and it’s coupling with the atmospheric model.

Intriguing Aspects of the Monsoon Low-Level Jet over Peninsular India Revealed by High-Resolution GPS Radiosonde Observations

Abstract The strong cross-equatorial flow in the lower troposphere, widely known as the monsoon low-level jet (MLLJ), plays an important role in the Indian summer monsoon (ISM) rainfall during June–September. Using high-resolution GPS radiosonde observations over Gadanki (13.5°N, 79.2°E), some new aspects of MLLJ have been reported. In the present study it is found that, on average, the MLLJ exists at 710 hPa over southeastern peninsular India, rather than at 850 hPa as reported by earlier studies. It is observed that the ECMWF Re-Analysis (ERA)-Interim data provide better results on the spatial, temporal, and vertical variation of MLLJ. Further, the characteristics of the MLLJ during the active and break spells of ISM are also investigated; higher MLLJ core height and intensity are found during active phases of the Indian monsoon. This study emphasizes the use of high-resolution measurements for studying monsoon dynamics in detail.