“…The use of the theory of adjoint equations was an important step in this direction (LIONS, 1968;MARCHUK, 1995, MARCHUK et al 1996. Beginning with well-known works (PENENKO 1976;MARCHUK and PENENKO 1978;LE DIMET and TALA-GRAND, 1986), adjoint equations have been used extensively by many researchers to investigate and numerically solve data assimilation problems and to calculate the gradient of the functional (AGOSHKOV and MARCHUK, 1993;MARCHUK and ZALESNY, 1993;MARCHUK and SHUTYAEV, 1994;WENZEL et al 2001;COURTIER et al 1998;SHUTYAEV, 2001;BENNETT, 2002;AGOSHKOV et al, 2005, 2008, ZALESNY and RUSAKOV, 2007BLUM et al, 2008;ZALESNY and GUSEV, 2009;NAVON, 2009). Currently, growing interest is being aroused by four-dimensional data assimilation (4DVAR), which uses forward and adjoint models for observational data assimilation not at a concrete moment in time but rather in a specified time interval (BLUM et al, 2008).…”