The Frenchman ended up the slowest of the 11 drivers in Spain, his best lap over 12 seconds off the pace set by Mark Webber as the Lotus team focused on aerodynamic component testing rather than speed runs.
Webber tops first day of final F1 test
With Grosjean scheduled to have his final day in the car before Melbourne on Friday, when more rain is forecast, he is worried that he will not be prepared enough for the season-opener.
"We spent quite a lot of time today doing things that were good for the team and the car," Grosjean said.
"But usually the last test is about working with your engineers and getting a good baseline set-up for Melbourne. With the weather today, that was not possible.
"Hopefully it will be dry tomorrow and we can work on a few things that we wanted to test; things we cannot do them on a race weekend."
Grosjean's team-mate Kimi Raikkonen is scheduled to drive the Lotus on the final two days of the Barcelona test, for which dry conditions are expected.
All the photos from the Barcelona test