Search Results

You are looking at 1 - 6 of 6 items for

  • Author or Editor: Sandeep Sahany x
  • Refine by Access: All Content x
Clear All Modify Search
Sandeep Sahany
,
J. David Neelin
,
Katrina Hales
, and
Richard B. Neale

Abstract

Properties of the transition to strong deep convection, as previously observed in satellite precipitation statistics, are analyzed using parcel stability computations and a convective plume velocity equation. A set of alternative entrainment assumptions yields very different characteristics of the deep convection onset boundary (here measured by conditional instability and plume vertical velocity) in a bulk temperature–water vapor thermodynamic plane. In observations the threshold value of column water vapor above which there is a rapid increase in precipitation, referred to as the critical value, increases with temperature, but not as quickly as column saturation, and this can be matched only for cases with sufficiently strong entrainment. This corroborates the earlier hypothesis that entraining plumes can explain this feature seen in observations, and it places bounds on the lower-tropospheric entrainment. Examination of a simple interactive entrainment scheme in which a minimum turbulent entrainment is enhanced by a dynamic entrainment (associated with buoyancy-induced vertical acceleration) shows that the deep convection onset curve is governed by the prescribed minimum entrainment. Results from a 0.5° resolution version of the Community Climate System Model, whose convective parameterization includes substantial entrainment, yield a reasonable match to satellite observations in several respects. Temperature–water vapor dependence is seen to agree well with the plume calculations and with offline simulations performed using the convection scheme of the model. These findings suggest that the convective transition characteristics, including the onset curve in the temperature–water vapor plane, can provide a substantial constraint for entrainment assumptions used in climate model deep convective parameterizations.

Full access
Sandeep Sahany
,
J. David Neelin
,
Katrina Hales
, and
Richard B. Neale

Abstract

Tropical deep convective transition characteristics, including precipitation pickup, occurrence probability, and distribution tails related to extreme events, are analyzed using uncoupled and coupled versions of the Community Climate System Model (CCSM) under present-day and global warming conditions. Atmospheric Model Intercomparison Project–type simulations using a 0.5° version of the uncoupled model yield good matches to satellite retrievals for convective transition properties analyzed as a function of bulk measures of water vapor and tropospheric temperature. Present-day simulations with the 1.0° coupled model show transition behavior not very different from that seen in the higher-resolution uncoupled version. Frequency of occurrence of column water vapor (CWV) for precipitating points shows reasonable agreement with the retrievals, including the longer-than-Gaussian tails of the distributions. The probability density functions of precipitating grid points collapse toward similar form when normalized by the critical CWV for convective onset in both historical and global warming cases. Under global warming conditions, the following statements can be made regarding the precipitation statistics in the simulation: (i) as the rainfall pickup shifts to higher CWV with warmer temperatures, the critical CWV for the current climate is a good predictor for the same quantity under global warming with the shift given by straightforward conditional instability considerations; (ii) to a first approximation, the probability distributions shift accordingly, except that (iii) frequency of occurrence in the longer-than-Gaussian tail increases considerably, with implications for occurrences of extreme events; and, thus, (iv) precipitation conditional averages on CWV and tropospheric temperature tend to extend to higher values.

Full access
Shoobhangi Tyagi
,
Sandeep Sahany
,
Dharmendra Saraswat
,
Saroj Kanta Mishra
,
Amlendu Dubey
, and
Dev Niyogi

Abstract

The 2015 Paris Agreement outlined limiting global warming to 1.5°C relative to the pre-industrial levels, necessitating the development of regional climate adaptation strategies. This requires a comprehensive understanding of how the 1.5°C rise in global temperature would translate across different regions. However, it’s implications on critical agricultural components, particularly blue and green water, remains understudied. This study investigates these changes using a rice-growing semi-arid region in central India. The aim of this study is to initiate a discussion on the regional response of blue-green water at specific warming levels. Using different Global Climate Models (GCMs) and Shared Socioeconomic Pathways (SSPs), the study estimated the time frame for reaching the 1.5°C warming level and subsequently investigated changes in regional precipitation, temperature, surface runoff, and blue-green water. The results reveal projected reductions in precipitation and surface runoff by approximately 5-15% and 10-35% respectively, along with decrease in green and blue water by approximately 12-1% and 40-10% respectively across different GCMs and SSPs. These findings highlight 1) the susceptibility of blue-green water to 1.5°C global warming level, 2) the narrow timeframe available for the region to develop the adaptive strategies. 3) the influence of warm semi-arid climate on the blue-green water dynamics, 4) the uncertainty associated with regional assessment of a specific warming level. This study provides new insights for shaping food security strategies over highly vulnerable semi-arid regions and is expected to serve as a reference for other regional blue/green water assessment studies.

Restricted access
Sushil K. Dash
,
Saroj K. Mishra
,
Sandeep Sahany
,
V. Venugopal
,
Karumuri Ashok
, and
Akhilesh Gupta
Full access
Chen Chen
,
Sandeep Sahany
,
Aurel F. Moise
,
Xin Rong Chua
,
Muhammad E. Hassim
,
Gerald Lim
, and
Venkatraman Prasanna

Abstract

The Maritime Continent (MC), located in the heart of the Indo-Pacific warm pool, plays an important role in the global climate. However, the future MC climate is largely unknown, in particular the ENSO–rainfall teleconnection. ENSO induces a zonal dipole pattern of rainfall variability across the Indo-Pacific Ocean, that is, positive variability in the tropical Pacific and negative variability toward the MC. Here, new CMIP6 models robustly project that, for both land and sea rainfall, the negative ENSO teleconnection over the MC (drier during El Niño and wetter during La Niña) could intensify significantly under the Shared Socioeconomic Pathway 5–8.5 (SSP585) warming scenario. A strengthened teleconnection may cause enhanced droughts and flooding, leading to agricultural impacts and altering rainfall predictability over the region. Models also project that both the Indo-Pacific rainfall center and the zero crossing of dipole-like rainfall variability shift eastward; these adjustments are more notable during boreal summer than during winter. All these projections are robustly supported by the model agreement and scale up with the warming trend.

Open access
John l. Mcbride
,
Sandeep Sahany
,
Muhammad E. E. Hassim
,
Chi Mai Nguyen
,
See-Yee Lim
,
Raizan Rahmat
, and
Wee-Kiong Cheong
Full access