For “一场”，you have two things to learn. Look at the example:
Although there was a heavy rain yesterday, I went to watch an excellent film showing. 虽然昨天下了一场大雨，但我还是去看了一场很精彩的电影。
“一场” is used as classifier to describe things: 1. rain, snow, storm, turmoil, disaster, disturbance, etc. In this case, it is pronounced yī cháng. 2. sports and recreation. In this case, it is pronounced yī chǎng.