Adiabatic Westward Drift in Monsoon Depressions

Introduction and Methods

Boos et al. 2014

What are Monsoon Depressions (MDs)

• MDs are large low-level cyclonic circulations that possess the following characteristics• 2000 km outer diameter• Lower-level cold core (and upper-level warm core)• “Moderate winds” (between 8.5 – 13 m s-1 @ 850hPa)

Intermediate category of monsoon lowsAll three categories collectively are attributed to > 50% of all

monsoonal precipitation in the Indian Summer!

Propagation Mechanism of MDs• Northwest drift of low-level vortex despite being

embedded in low-level easterly flow

• Hypothesized role of QG ascent downshear leading to low-level vortex stretching (Rao and Rajamani 1970, Sanders 1984, Chen et al. 2005).– These studies neglect moist convection or use significant

approximation– Author wants to reexamine the propagation mechanisms

for MDs using a new framework

Thermal Wind Gradient of Absolute Vorticity

(Trenberth 1978; Sutcliffe 1947)

Potential Vorticity (PV) Framework

Useful characteristics of PV• Conserved in absence of

diabatic heating and friction• Can be inverted to obtain 3D

wind structure• While strongest vorticity of

MDs occurs at low levels, PV is maximized in the mid troposphere (Fig. 1)

Where: is the absolute vorticity vector; is density; is potential temperature

Fig. 1. PV (shaded, PVU), zonal wind (black contours [dotted is negative], every 5 m s-1), and climatological zonal wind (pink contour = 0 m s-1)MD from Sanders (1984)

• Low-levels embedded in easterly flow• PV maxima of MD in westerly flow

PV max @ 450-hPaW

E

(Rossby 1940; Ertel 1942)

Fundamental Questions• Is MD vortex movement dominated by adiabatic PV

advection or diabatically induced PV tendencies?• Are diabatic PV tendencies needed to prevent vortex

from being tilted by shear or can MD remain upright by adiabatic dynamics?

• Does non-linear or linear advection dominate the advective tendencies of PV

• Is the PV structure in Fig. 1 representative of a typical Indian Ocean MD?

Methodology

• Datasets– ERA-Interim (Dee et al. 2011)

• Gridded, Isobaric, 0.7o x 0.7o

– TRMM version 7 3B42

Identification of MDs

Algorithm Characteristics • Tracks 850-hPa vorticity maxima > 1x10-5 s-1

• Sea level pressure anomaly relative to a 21-day running mean must have amplitude between 4 – 10 hPa

• Max surface winds between 8.5 – 13 m s-1

• Pressure and wind criteria must be satisfied within 500 km of vorticity maxima.

• Algorithm run year round 1979-2012• Indian MD Domain: 0o – 30oN 50o-105oE

(171 MDs)– Composite domain used for this study:

10 – 27oN 75 – 95oE (117 MDs)

Fig. 2. Number of genesis points per square degree (shaded, ~ 12,000 km2) per summer season (Jun – Sept) after smoothing with a Gaussian filter. Vectors show average propagation speed of MDs and are coarsened to a 2x2o grid for clarity; vectors are shown only if the mean zonal or meridional propagation speed at each grid point is statistically significant at the 1% level by a two-tailed t-test.

Compositing Technique

• Composite over entire MD event, then composite over all MDs identified

• Storm-relative• Only use middle third of storm’s lifespan in composite• Averaged taken over integral 24 hour periods (to avoid

uneven sampling of diurnal cycle).• Data not used below 950-hPa (prevent using variables

below surface)

Case Study for PV tendencies

• September 2008 – MD exhibited classic northwest propagation in Bay of Bengal

• Dataset: ECMWF Year of Tropical Convection (YOTC)• Provides tendencies associated with model

parameterizations• Used to calculate diabatic PV tendencies for this MD

case study