Make update query with append(concat) using sequelize in nodejs

I am using Sequelize ORM, i had a need a help with update query.
I have a model, user(id, name, mobile).

While update, I want a particular field to get append instead of getting replaced.
Eg. Mobile number should be added to existing value instead of replacing

User.update({mobile: 87477777888}, {id: userId})
.success(function(user){
console.log(user);
}).error(function(user){
console.log("Error in update");
});