On the Maintenance of Large Anomaly Fields during a Record-Breaking Monsoon Drought Via Scale Interactions.

During the year 2002, record breaking drought over India was noted. In this period anomaly fields (with respect to climatology) were quite large. This study addresses the maintenance of these anomaly fields. The evolution of an atmospheric anomaly is usually defined in terms of the departure of a state variable from its climatological field. A growing anomaly must receive kinetic energy (KE) either on its own scale at the expense of some other form of energy or from other scales via nonlinear interactions. The latter mechanism is called as out-of-scale energy exchanges while the former is known as in-scale energetics. In a multi-scale framework, energy budget in the spectral space of the anomaly could not only shed light on the maintenance of the anomaly but could also isolate processes responsible for it. The generation of zonal and eddy available potential energy (APE) from all forms of diabatic heating (largely via invoking the covariance of heating components and temperature), conversions of zonally APE to zonal KE (the local Hadley type overturning), and eddy APE to eddy KE (the local vertical overturning, described largely by the co-variance of vertical velocity) fall under the in-scale category. The energy exchanges between zonal and eddy APE, among zonal KE and eddy KE (signifying barotropic exchanges for stable or unstable situations), and among eddy KE (APE) to eddy KE(APE) fall under the out-of-scale category. In spectral space, the co-spectra is interpreted as the spectrum of the covariance. Using this interpretation along with the terms in equations of motion related to aforementioned processes, we can derive spectral equations for the in-scale and out-of-scale energy exchanges related to the anomaly to study its maintenance quantitatively.

We used this approach to study a drought event over India in 2002 using reanalysis data sets in the wavenumber domain. Our study comprises of three stages: 1) recognition of all active scales (wave numbers) during this drought event using spectral analysis, 2) recognition of the scales related to the drought, 3) compute all scale interactions among drought scale and other active scales. Our preliminary results show that this anomaly (drought) was associated with the zonal wave numbers 5, 6 and 7. Wavenumbers 1 to 10 were found energetically active during this event. We computed energy exchanges using a daily reanalysis as well as with a daily climatology and compared both computations to point out the salient differences during the event. We present results of scale interactions among these scales and discuss the maintenance of this drought event in terms of energy exchanges that contribute to the maintenance of the anomalies that describe that period.