The Cryosphere Discuss., 7, C1625–C1630, 2013 www.the-cryosphere-discuss.net/7/C1625/2013/ © Author(s) 2013. This work is distributed under the Creative Commons Attribute 3.0 License. Open Access The Cryosphere Discussions Interactive comment on “Constraining GRACE-derived cryosphere-attributed signal to irregularly shaped ice-covered areas” by W. Colgan et al. W. Colgan et al. william.colgan@colorado.edu Received and published: 30 August 2013 We thank Anonymous Referee #2 for their interest in our work. We appreciate the keen eye with which they have reviewed our methodology. We would like to address their six major comments (numbered 1 through 6) while the discussion forum is still "open", in case Anonymous Referee #2 can provide further insight. My co-authors and I intend to address the remaining minor comments in final discussion. Re: #1 Clarification of precise inversion data – We apologize for being unclear in our discussion paper: We are indeed inverting the gridded ultimate rate of mass change field, not individual spherical harmonic coefficients. The inversion is therefore executed C1625 in the Cartesian (or "node") domain, rather than the spherical harmonic domain. We will explicitly state this clarification in a revised copy of the manuscript. We will similarly clarify that we regard the "characteristic scaling length" of a Gaussian filter as being equivalent to its "standard deviation". We note that spherical harmonics have only been available up to degree 60 since the inception of GRACE. Thus, it is not possible to trun- cate spherical harmonic solutions at higher orders (Tapley et al., 2004). We also note that rather than being prescribed a priori, the Gaussian filter length scale subsequently used by the inversion algorithm (200 km) was established through a sensitivity analysis to establish the optimal length scale at which the inversion minimized the root-mean- squared error when compared with the input GRACE data (discussion paper Figure 7). Thus, we contend that the combination of degree 60 spherical harmonic solution and Gaussian filter length scale of 200 km does indeed preserve maximum informa- tion of the magnitude and spatial distribution of mass changes throughout the inversion process, while honouring the fundamental spatial resolution of the GRACE satellites. Re: #2 Generating spherical harmonics from ground-level data – We completely agree that there is a pressing need for coarser resolution spherical harmonic solutions to be derived from higher resolution inverted ground-level mass change fields (i.e. discussion paper Figure 10; Barletta et al., 2012) to facilitate further inversion validation. This would certainly close the circle: generating higher resolution ground-level fields through inversion, and then forward modeling the corresponding coarser resolution spherical harmonic fields, and so on. We have given significant thought to forward modelling the spherical harmonic solution space that corresponds to the inverted field we present. As we are inverting/inferring a trend over a given period, rather than absolute mass anomalies at each monthly GRACE time point, we would have to carefully stitch this cryospheric trend into a forward model of global mass anomalies over the time period. This forward model would also have to incorporate the usual suite of processes that can substantially influence gravimetry observations (i.e. corrections for atmospheric, oceanic, isostatic rebound, etc.). We view this as a non-trivial task. We are currently working towards a framework to develop and implement such a complimentary forward C1626