An innovative method to achieve mass balance (see our other presentation abstract) gives us a new way to estimate land runoff. Mass balance corrections were applied to the weights in the divergence calculation, as is required to obtain reasonable estimates of dry static energy convergence.
An estimate of runoff that is constrained by net top-of-atmosphere radiation via the coupled water and energy balance of the atmosphere is presented for the Mississippi River Basin using long-term radiosonde and net top of atmosphere radiation data from the Earth Radiation Budget Experiment. The estimate is independent of river gauge measurements and water vapor convergence, and thus provides a third independent estimate. The coupled water and energy budget equation states that the vertically integrated divergence of total atmospheric energy transport is balanced by the net radiative flux at the top of the atmosphere. Therefore the long-term mean land surface runoff can be estimated, as a residual of the energy balance, from measurements of the net radiative flux and horizontal divergence of vertically integrated dry static energy.
The new, indirect estimate of atmospheric moisture convergence based on energy fluxes convergence and net radiation, is negative. The seasonal pattern of the new estimate is similar to the direct vapor convergence estimate, but slightly more divergent most of the year. The mean annual cycle of convergence of sensible heat (plus pressure-work) and potential energy approximately balance each other, reflecting near-adiabatic expansion and compression of air parcels, thus leaving a near balance between net top-of-atmosphere radiation and water vapor convergence. Our new estimation method implicitly assumes that the discrepancy between total energy flux convergence and the radiative flux at TOA is entirely due to undersampling (vertically, spatially, and temporally) and poor instrumental measurement of humidity.
The mass balance correction method is expanded to achieve energy balance as well as mass balance by dealing with the issue of capturing diurnal variability with twice-daily observations. The storage change of atmospheric energy is a large term in the energy balance equation on an hourly time scale. A major focus is to ascertain how much information in terms of diurnality of energy divergence can be inferred from the twice-daily observations and possibly hourly surface and radiation observations. We also examine the simulations from the second Atmospheric Model Intercomparison Project to find a robust relationship among the net TOA radiation, moisture divergence, and dry static energy divergence.
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