Chapter 7

Meteorological Analysis of the NE Monsoon

T.N. Krishnamurti, G. Zhang and P.J. Rasch

With the anticipation that INDOEX would cover a winter monsoon period between December and March, we propose to address the following issues:

Scientific Objectives

* Provide an improved analysis of a comprehensive research dataset for INDOEX investigators.

* Determine the boundary layer structure, its diurnal variability and interaction with various chemistry and aerosol components of INDOEX between 30šS-40šN and 50šE-120šE.

* Understand the dynamics of the Indian Ocean ITCZ, its spatio-temporal variability, and its role in cloud absorption of solar radiation and in chemistry transport.

* Study four-dimensional passive tracer transport from the Indian subcontinent, including diurnal changes and detailed physical and dynamical processes.

Specific Tasks

Preparation of a comprehensive meteorological data set for the INDOEX period.

We propose to perform data assimilation including physical initialization, using a global model at very high resolutions to incorporate satellite and surface based special data sets for the period from January to April for 1996, 1997 and 1998. The following data sets will be included:

* SSM/I based rain rates from F10, F11, F12 and other available DMSP satellites

* SSM/I based total precipitable water

* SSM/I/ERS2 based surface winds over the ocean

* OLR, INSAT and GMS plus NOAA polar orbiters

Along with these we will include the special INDOEX data sets such as profiler winds from atolls and islands, data from specially deployed moored buoys, flight level data from research aircraft such as the P-3 and the C-130, observations from specially deployed research ships (French and Indian), and possibly wind observations from the constant level balloons. If special efforts are made to collect data from available geostationary and polar orbiting satellites (geostationary operational environmental satellite (GOES), METEOSAT) and additional observations from commercial aircraft and ships of opportunity, these will be incorporated in the data assimilation.

Figure 25 (a,b,c): 24 hourly rainfall (mm/day) ending Dec. 1, 1992 at 1200 UTC. a) satellite/rain-gauge based observed estimates, b) based on physical initialization in a very high resolution global model, and c) based on a control run that does not include rainrate initialization.
Our approach incorporates rain rates, surface fluxes, OLR and surface winds in the high resolution global model. Details of this procedure are published in Krishnamurti et al. (1991, 1993, 1994). Figure 25 shows one example of how this procedure improves the analysis and the nowcasting skill of rainfall, resulting in a correlation of roughly 90% between the observed rain (averaged over 6 hours in time and a spectral transform grid square in space) and the predicted rain measured in the same manner. Such an initialization also provides a marked improvement in short range predictions of the rainfall [Krishnamurti et al., 1993; Treadon, 1995; Tsuyuki, 1995]. This analysis procedure further provides a mesoscale structure for the surface fluxes, especially that for moisture [Krishnamurti et al., 1995]. This data set will be archived for 6 hourly periods over the globe at a horizontal resolution of roughly 50 km, with 17 vertical levels between the earth surface and roughly 30 km above. The analysis would cover a 6 month period from December to May of INDOEX.

Analysis of the boundary layer and its diurnal variability over a large INDOEX domain between 30šS and 40šN and 50šE and 120šE.

An analysis of the diurnal variability of the planetary boundary layer would be an important issue for INDOEX. The winter monsoon north easterlies over south Asia and southeast Asia are very shallow, and the diurnal variability in the height of the monsoonal inversion is quite large. Thus, we can expect some problems in estimating transport between the continental air masses of the north and the ITCZ of the southern Indian Ocean. This problem arises because these shallow north easterlies turn back with height very rapidly in the lowest 1.5 km and become westerlies by about 2 km above the earth's surface. The height of the base of inversion varies diurnally. We have noted that this base often encounters west winds in the afternoon hours (12 UTC) and north easterly wind in the early morning hours (00 UTC). Thus, there remains the scientific issue as to what proportion of the boundary layer air would make the long traverse from the land areas near 20šN to the oceanic region of the ITCZ near 7šS. In order to answer this question, we propose to examine 6 hourly 4-D analysis for a selected period during INDOEX. The global model explicitly resolves the constant flux layer. With several levels of data below the 2 km level, it would be possible to study the thermodynamic transition of the diurnal variability of the inversion layer and to explore the issue of the north-south communication.

Analysis of spatial and temporal variability of ITCZ; correlation of the ITCZ cloud systems with estimates of cloud absorption of solar radiation.

For this analysis, variations of the ITCZ on weekly to interannual timescales will be examined using the assimilated datasets, conventional datasets, and datasets from the proposed deployment of observation systems over an island and a moored buoy. Together with the satellite data listed above, this analysis will help to understand how the ITCZ cloud systems enhance the absorption of solar radiation, and affect the Indian Ocean surface heat budget. It will also provide valuable information on the role of the ITCZ in chemistry transport.

Preparation of derived fields for INDOEX: cloud cover, precipitation, and surface fluxes.

We will prepare a processor history from the global model initialization. The results of these computations would include the ensemble of 6 hourly analyses for selected periods:

* Time history of surface fluxes (sensible, latent heat and momentum) at transform grid points.

* Time history of the components of diabatic heating.

* Time history of cloud fraction, related radiative heating and cooling distribution in three dimensions.

Although the model dependence is inevitable in any 4-dimensional assimilation, we believe that these products would provide very useful information because the physical initialization minimizes the errors in model based high resolution rainfall and OLR. Furthermore, we expect to incorporate some special platform data sets from INDOEX that are listed above.

The analysis of cloud cover also improves the nowcasting skill of cloud fractions. This was tested from the special Air Force data sets called the RTNEPH (Real Time neph Analysis) (based on pixel level cloud interrogation). A new improvement in physical initialization restructures the humidity variable such that the modeled and the RTNEPH based cloud fractions are in very close agreement. This feature would hopefully provide a useful cloud cover data set for INDOEX scientists. A preliminary study appears in Lee and Krishnamurti (1995).

Four dimensional passive tracer transport studies from the Indian subcontinent, including diurnal variation and detailed physical and dynamical processes.

In a recent paper (Krishnamurti et al., 1996), we have developed techniques for the construction of time dependent plumes from biomass burn areas in Brazil and South Africa to study passive tracer transport in global models. The Lagrangian technique is one part of the global model's history variables. Studies based on the physically analyzed products can be used to investigate the transport of air masses from land areas of winter monsoons to the ITCZ south of the equator. This information would be of value to the INDOEX scientists studying chemical transport processes.