5.1 Energy And Water Budgets

OBJECTIVE: Determine the time-space variability of the hydrological and energy budgets over the Mississippi Basin.

There are four near-term objectives for the period covered by this major activities plan:

1. Sustain and enhance the program for the routine production of monthly-averaged energy and water budgets for the Continental-Scale Area (CSA) and four large-scale (LSA) sub-basins of the Mississippi River Basin.

2. Develop and implement, in support of the studies of cold season hydrological processes a capability to produce multi-scale energy and water budgets over the LSA-NC from basic and derived data sets and variable fields generated by four dimensional data assimilation (4DDA) procedures.

3. Produce and evaluate multi-scale water and energy budgets for the LSA-SW and LSA-NC during the WY 1998 and perform comparisons with WY 1997.

4. Implement the methodology developed for the LSA-SW to the evaluation of multi-scale budgets over the LSA-NC in support of the WY 97 and WY 98 studies of cold season hydrological processes and adapt and develop new methodologies to the study of hydrological processes over complex terrain for the LSA-E in WY 98 and WY 99.

In order to meet these near-term objectives, diagnostic studies will be undertaken which (1) will obtain area-averaged variables from the available data and derived data products; (2) compare budget results obtained from model-generated 4DDA fields and MOLTS with results obtained from different sources of data and analyses in order to evaluate their relative quality and sources of error; and (3) critically compare budget residuals with limited measurements and empirically derived values of evaporation and soil water storage.

5.1.1 Budget Variables

5.1.2 Basic Strategy

5.1.4 WY 99 Activities:

1) LSA and CSA Budgets.

a) Continue the routine evaluation of mean monthly budget time series for all LSAs and the CSA.

b) Develop a description of the WY 99 annual cycle of the land surface and atmosphere hydrological and energy cycles over each LSA and the CSA and compare to the WY 97 and WY 98 analyses.

c) Continue ongoing intercomparisons between atmospheric budgets obtained directly from observations and those computed from operational analyses.

d) Continue investigation of long-lasting hydrologic regimes including those that may have developed in WY 98.

e) Initiate studies with the Pan-American Climate Studies (PACS) Program to investigate the relative roles of land surface boundary, i.e. Local conditions, and large scale boundary conditions, i.e. ocean sea surface temperatures, on the initiation, maintenance and demise of long-lasting hydrologic events.

2) ISA/SSA Budgets.

a) Continue compilation and analysis of area averaged surface and atmosphere energy and water budgets for the area within the LSA-NC ISS/SSA and appropriate data from the ARM/CART rawinsonde array.

b) Continue the routine computation of area-averaged estimates of soil moisture and surface meteorological parameters, including fluxes, over the Little Washita Watershed, and begin similar computations for the ARM/CART array. Compare these values with output from operational mesoscale models.

c) Compare soil moisture estimates from observational and model- derived moisture budgets with instrumental estimates over the ARM/CART array.

d) Continue evaluation of available surface/atmosphere budget information acquired during WY 98 in the ISA/SSA and initiate evaluation of ISA/SSA budget computations over LSA-E.

e) Complete planning and implementation of a program of ISA/SSA budget computations over the LSA-NW during WY 2000.

5.1.5 Outlook for WY 2000

1) Continue evaluation of CSA and LSA water and energy budgets. Mean monthly LSA budget time series will be extended into WY 99. The fourth year (WY 2000) annual cycle will be analyzed and compared with the earlier years.

2) Continue studies with the Pan-American Climate Studies (PACS) Program to investigate the relative roles of land surface boundary, i.e. Local conditions, and large scale boundary conditions, i.e. ocean sea surface temperatures, on the initiation, maintenance and demise of long-lasting hydrologic events.

3) ISA/SSA Budgets. Continue evaluation of ISA/LSA budgets within the LSA-SW and LSA-E. Began compilation of data for ISA/SSA budgets for specified areas in the LSA-NW.

5.2 Land-Surface Boundary Layer Coupling

OBJECTIVES:

1. Develop an improved documentation and understanding of the processes controlling the seasonal cycle of fluxes of water and energy across the land/atmosphere interface and within the planetary boundary layer.

2. Establish relationships between surface conditions and boundary layer processes, particularly as they relate to the partitioning of surface fluxes between latent and sensible heat.

Surface fluxes, including evaporation, are at the end of a long chain of processes and interactions involving cloudiness (which affects surface net radiation), soil water content (which is dependent on rainfall), and vegetative cover. The planetary boundary layer can act as a governor on the transfer process at the surface. In turn, the boundary layer response depends on the partitioning between surface latent and sensible heat fluxes.

The diurnal and annual cycles have a fundamental effect on the coupling of the surface and the Planetary Boundary Layer (PBL). The diurnal cycle itself has a pronounced annual cycle, with maximum amplitude during the warm months, when the land surface and atmosphere are most strongly coupled.

ARM-CART Setting

The observations from the ARM/CART array provides data required for process studies and more detailed intercomparisons and validation of both surface and atmospheric model subcomponents. Among the major enhancements to the operational data which are available from the ARM/CART area are the following:

1) Data for the evaluation of the surface radiation balance and surface fluxes. These data are provided from a number of different ARM instrument systems and sites. Emphasis is placed on instrumental calibration to assure that the measurements are consistent, compatible and reliable.

2) Soil moisture measurements. Continuous automated soil moisture measurements in the ARM/CART site were initiated in the spring of 1996 with the installation of instruments at sites. An additional 15 sites are scheduled to be instrumented prior to April, 1997, thus providing a large scale but sparse array of soil moisture monitoring sites over the ARM/CART site beginning in April 1997.

3) Aerosol concentration measurements from the ARM/CART central site. These data will provide important information on the effect of aerosols on the radiation balance.

4) PBL Structure. Detailed monitoring of the PBL structure will take place during the three-week intensive observational periods, when rawinsondes will be launched eight times daily from the ARM/CART central facility and four profiler sites. These data will provide the time/space sampling required to characterize the detailed structure of the PBL, and evaluate the heat and moisture budgets on this spatial scale during different seasons.

There will likely be several intense synoptic or mesoscale events which will pass across the ARM/CART site during the these intensive observing periods. These occurrences will be viewed as "targets of opportunity" and designated for special study.

Little Washita Watershed Setting

WY 99 Activities:

1) Continue diagnostic studies of the data collected over LSA-NC and subareas during ESOP 98.

2) Continue the analysis of the data collected over LSA-SW and the two sub-areas during ESOP 96 and ESOP 97.

3) Begin implementation of plans for land surface and diagnostic studies over the LSA-E in WY 99.

WY 2000 Outlook:

OBJECTIVE: Provide more complete descriptions and understanding than previously available of the initiation, evolution and decay of long lasting (months) continental-scale anomalous hydrologic regimes; particularly, as they relate to budget derived evapotranspiration and surface and subsurface storage.

The profound societal impacts of anomalous large-scale hydrological regimes is well illustrated by the series of major regional fluctuations which have occurred during the past quarter century. Of particular significance to GCIP are the upper Midwest drought of 1988 and the winter and spring wet spell which culminated in the catastrophic 1993 summer floods in the upper Mississippi River Basin. These two contrasting lengthy, continental-scale anomaly regimes will continue to be a focus of studies during WY 98 and WY 99.

We anticipate that these studies will serve as "benchmark cases" for use in subsequent simulation experiments and continental-scale validation of land- atmosphere hydrological subcomponents. The relevant questions can be addressed most effectively if the diagnostic studies are carried out in tandem with activities of the GEWEX Numerical Experimental Group (GNEG) and the Pan American Climate Studies (PACS) Program.

Because of the global component of these studies, steps are being initiated to carry out these studies as a joint effort between the GCIP and the PACS Program. The effort requires collaboration among the global and regional diagnostics studies communities, global and mesoscale modeling groups as well as scientists involved in land surface parameterization and data assimilation.

The development of large-scale anomaly patterns will be examined in the context of the annual cycle; e.g. the "cold season carry-over" contribution to anomalies during the growing season. Underlying these studies is the important question of the relative roles of regional surface anomalies, remote forcing, and internal dynamics in the perpetuation and intensity of the anomalous regimes. In addition there is the question of the extent to which positive feedback between anomalous land surface conditions and an anomaly-sustaining atmospheric circulation exist during these regimes. Is such feedback a significant factor in the evolution of land surface anomalies, or is it easily overpowered by other influences, e.g. a remote response to large-scale SST anomalies? Are changes in precipitation recycling over the continent an important factor?

WY 98 Activities:

GCIP-GNEG-PACS joint planning activities will continue. This will include specifying the required data sets. The feasibility of generatingEDAS reanalysis data sets for the appropriate periods will be examined. Diagnostic studies based on output from the NCEP global reanalysis project will be initiated.

The GCIP-PACS joint study of the North American Monsoon System will be initiated.

WY 99 Activities:

Diagnostic studies of the 1988 and 1993 anomaly regimes will extend into WY 99 and new studies of large-scale anomaly regimes which occurred during the 1995 to 1997 WYs will be initiated.

WY 2000 Outlook:

Continuation of studies of large-scale anomaly regimes which occurred during 1995-1999 with emphasis on the interactions among the large scale atmospheric circulation features and the ISA/SSA, LSA and CSA hydrology.