Elucidating intra-seasonal characteristics of Indian summer monsoon. Part-I: Viewed from remote sensing observations, reanalysis and model datasets

Abstract

In this study, we examine the transitions in the monsoon phases (onset, active, break and the withdrawal) during an entire monsoon season. This makes use of a host of observational tools that come from GPM (Global Precipitation Measurement) and TRMM (Tropical Rainfall Measuring Mission) satellites for precipitation estimates, the vertical structure of rain, hydrometeors and cloud types from TRMM and CloudSat datasets. During onset, the mean moisture convergence, especially over west and south-west coast of India is 2 × 10⁻⁴ kg m⁻¹ s⁻¹; however, it carries much higher value of >4 × 10⁻⁴ kg m⁻¹ s⁻¹ during the active phase over central eastern India. Much lesser moisture convergence (<1 × 10⁻⁴ kg m⁻¹ s⁻¹) is noted over Western Ghats area during the break phase. However, there are northeasterly moisture fluxes present over southern part of India during withdrawal phase. The tall cumulonimbus clouds that extend out to 16 km are illustrate during onset, the active phase is dominated by alto stratus and nimbostratus type clouds that are somewhat shallower. In general, we noted an absence of such clouds during the break and the withdrawal phases. Those structures were consistent in a number of derived fields such as the moisture convergence, moisture fluxes, the energy conversions between the rotational and the divergent kinetic energy and the corresponding phases of the intra-seasonal oscillations.

Appendix

The vertically integrated moisture transport vectors and convergence of flux of moisture (MFC) transport are estimated from the following formulations:

$$ \overrightarrow {{M_{x} }} = - \frac{1}{g}\int_{Psuf}^{Ptop} {\vec{U}q\;dp,} $$

is the total column zonal flux of moisture.

$$ \overrightarrow {{M_{y} }} = - \frac{1}{g}\int_{Psuf}^{Ptop} {\vec{V}q\;dp,} $$

is the total column meridional flux of moisture.

$$ \vec{M} = i \cdot \overrightarrow {{M_{x} }} + j \cdot \overrightarrow {{M_{y} }} , $$

representation of vertically integrated moisture transport vector.

$$ MFC = \left( { - 1} \right)*\vec{\nabla } \cdot \vec{M}, $$

convergence of vertically integrated moisture transport.

Here, $g$ is the acceleration due to gravity, $\vec{U}$ and $\vec{V}$ are the zonal and meridional wind components, $\vec{\nabla }$ is the del operator, and $q$, the moisture parameter.