Downloaded 09/26/21 05:57 AM UTC 1242 JOURNAL of APPLIED METEOROLOGY VOLUME 40

JULY 2001 POTTY ET AL. 1241 Simulation of Boundary Layer Structure over the Indian Summer Monsoon Trough during the Passage of a Depression K. V. J. POTTY IBM India Research Laboratory, Indian Institute of Technology, New Delhi, India U. C. MOHANTY Centre for Atmospheric Sciences, Indian Institute of Technology, New Delhi, India S. RAMAN Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina (Manuscript received 14 January 2000, in ®nal form 11 October 2000) ABSTRACT The planetary boundary layer (PBL) structure over the Indian summer monsoon trough region has been simulated using a regional numerical model during the passage of a monsoon depression along the monsoon trough. Monin±Obukhov similarity theory for the surface layer coupled with turbulent kinetic energy closure scheme (1½-order closure) for the mixed layer is used for parameterizing the PBL in the model. The results show that the model is able to simulate the evolution and the structure of the monsoon trough boundary layer and its associated features. The main observed characteristic features of the monsoon trough, such as the southward tilt of the trough with height and warmer air mass toward the northern side of the trough line, are simulated well by the model. Numerical results also indicate that the dynamic and thermodynamic characteristics of the monsoon trough boundary layer are modi®ed by the passage of monsoon depressions. 1. Introduction formation of monsoon depressions over the north Bay of Bengal moving west to northwest across the country. The Indian summer monsoon trough is an elongated Average rainfall is larger toward the south of the axis low pressure system that extends southeastward from beause of the fact that the heavier rains occur in the Pakistan to Gangetic West Bengal, with southwesterly southwest quadrant of the depressions. Major differ- winds south of the trough line and easterlies to the north (Fig. 1). The presence of this semipermanent feature ences are noticed between the dynamic and thermal during the monsoon season (June±September) is con- characteristics of the eastern and western ends of the sidered as the equatorial trough of the summer period trough. The western end of the trough line is somewhat in the Indian longitudes. This trough extends aloft up diffused because of a weak wind ®eld being associated to about 4 km and slopes southward with height. No with the heat low. The moisture content of the air is other monsoon feature has such a control on monsoon relatively low, and the boundary layer is capped with a activity over northern India as the monsoon trough (Rao stable layer above 1.5 km. The eastern end is well de- 1976; Das 1986). The position of the trough changes ®ned with a deep moist layer, which is warm and un- day to day and has an important bearing on the summer stable and thermodynamically very active. Sikka and monsoon rainfall over the Indian subcontinent. Once the Narasimha (1995) have given a detailed description of trough line shifts toward the foothills of Himalayas, a the large-scale meteorological environment of the mon- ``break'' in the monsoon occurs. During the break in soon trough region. Because the monsoon rainfall is the monsoon period, rainfall decreases sharply over the mainly due to convection, and the roots of deep con- subcontinent except over the foothills of the Himalayas. vection lie near the surface, it will be of interest to study Marching of the trough toward the south invokes the the evolution and the structure of the boundary layer over this convective low pressure (monsoon trough) region. Corresponding author address: Dr. K. V. Jayaraman Potty, IBM There have not been many modeling studies of plan- India Research Laboratory, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India. etary boundary layer (PBL) over the monsoon trough E-mail: [email protected] region. Holt and Sethu Raman (1988) compared the q 2001 American Meteorological Society Unauthenticated | Downloaded 09/26/21 05:57 AM UTC 1242 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 TABLE 1. A brief description of the model. Domain 13.5±34.58N, 668±998E Independent variables l, F, s, t Prognostic variables U, y,T,q,Ps Diagnostic variables f, s Topography Envelope (mean 1 1 std dev) Vertical grid system 16 levels in s coordinates Vertical levels of the model 0.05, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, 0.82, 0.86, 0.90, 0.935, 0.960, 0.975, 0.987, and 0.997 Horizontal grid system Arakawa C grid Time integration scheme Split-explicit time integration Time step 150 s Horizontal resolution 0.58 lat±long Horizontal diffusion Linear second order Initialization Nonlinear normal mode Physical processes Dry convective adjustment; Cumulus convection Kuo scheme with convective precipitation Large-scale strati®ed precipitation with RH .95% PBL; M±O similarity scheme for the surface layer, TKE (E 2 e) for the mixed layer FIG. 1. Normal position of the monsoon trough (solid line), the track of the depression that formed during the period 3±5 Aug 1988 (dashed line), and the stations chosen for the simulation studies. structure and associated processes over the monsoon trough and adjoining oceanic region with the TKE closure scheme combined with the Monin±Obukhov (M± boundary layer structure over the oceanic region as sim- O) surface layer similarity theory for PBL parameteri- ulated by a set of ®rst-order and turbulent kinetic energy zation. In this paper, the simulation results up to 48 h (TKE) closure schemes using a dataset from the 1979 using a regional model pertaining to the evolution and Monsoon Experiment (MONEX-79). They used a one- structure of the PBL over the monsoon trough are pre- dimensional boundary layer model for the study, and it sented. Initial conditions for integrating the model have showed that a 1½-order closure scheme performs better been chosen in such a way that an already-formed mon- than ®rst-order closure schemes. Alapaty et al. (1994) soon depression was moving along the trough so that used a nested-grid regional model with a high vertical its impact on the PBL can be studied as well. resolution in the atmospheric boundary layer to simulate various atmospheric processes during an active mon- 2. Model description soon period. They found signi®cant variation in boundary layer structure and associated processes over land For the current study, a hydrostatic primitive equation as compared to oceans. Lykossov et al. (1991) simulated model with a terrain-following coordinate system is the temporal evolution of the boundary layer over the used. The horizontal resolution of the model is 0.58 eastern region of the monsoon trough with a one-di- latitude±longitude, and, in the vertical, the model has mensional TKE±dissipation closure model using the 16 levels (8 levels up to 850 hPa) in sigma coordinates. Monsoon Trough Boundary Layer Experiment (MONT- The ®nite-difference form of the governing equations BLEX) pilot experiment dataset at Calcutta during 1988. is of second-order accuracy in space. A split-explicit Kusuma et al. (1991) have conducted sensitivity studies scheme (Madala 1978) is used for time integration. The on atmospheric boundary layer for the monsoon trough details of the time-integration scheme employed in the region during the southwest monsoon season with one- model are described by Mohanty et al. (1990). The Ar- dimensional TKE closure model and simulated the lo- akawa-C grid (Arakawa and Lamb 1977) in the model cally observed boundary layer structure with reasonable has an energy-conserving staggered-grid system with accuracy. Potty et al. (1997) studied the effect of three uniform resolution in longitude and latitude. A sponge different boundary layer parameterization schemes in a boundary condition of Perkey and Kreitzberg (1976) is regional model on the simulation of summer monsoon employed for updating the lateral boundaries of the circulation and found that the Monin±Obukhov simi- model, and a second-order diffusion equation is used in larity theory for the surface layer and TKE closure (1½- the model to handle the computational instabilities, order closure) scheme for the mixed layer constitute an which arise because of the subgrid scale processes. The appropriate combination for the parameterization of physical processes included in the model are dry con- PBL over the Indian monsoon region. These results vective adjustment, large-scale precipitation, convective prompted us to simulate and examine the boundary layer precipitation, and internal diffusion. Details of the mod- Unauthenticated | Downloaded 09/26/21 05:57 AM UTC JULY 2001 POTTY ET AL. 1243 FIG. 2. Observed mean sea level pressure at 0300 UTC [0830 Indian standard time (IST)] for the dates (a) 2 Aug 1988, (b) 3 Aug 1988, (c) 4 Aug 1988, and (d) 5 Aug 1988 (source: India Meteorological Department). el equations and numerics can be found in Madala et u u*2 L 5 y , al. (1987). The horizontal domain of the model chosen kgu* for this study is from 13.58 to 34.58N in the north±south direction and 668 to 998E in the east±west direction so where g represents acceleration due to gravity, k is the that the entire monsoon trough region falls in this do- von KaÂrmaÂn constant, u is the wind speed, z is the main. An overview of the model is given in Table 1. reference height, and uy is the virtual potential temper- , and q are de®ned by u2 ature. The terms u*, u* * * 5 u w w , u q w q , respectively. 2 9 9, u*u* 52 9u9 * * 52 9 9 3.

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