legacy/src/main/java/ucar/nc2/NetcdfFileWriteable.java
public void addVariableAttribute(String varName, Attribute att) {
    if (!defineMode)
      throw new UnsupportedOperationException("not in define mode");

    if (!N3iosp.isValidNetcdfObjectName(att.getShortName())) {
      String attName = N3iosp.makeValidNetcdfObjectName(att.getShortName());
      log.warn("illegal netCDF-3 attribute name= "+att.getShortName() + " change to "+ attName);
      att = new Attribute(attName, att.getValues());
    }

    Variable v = rootGroup.findVariable(varName);
    if (null == v)
      throw new IllegalArgumentException("addVariableAttribute variable name not found = <" + varName + ">");
    v.addAttribute(att);
  }
public void addVariableAttribute(String varName, Attribute att) {
    if (!defineMode)
      throw new UnsupportedOperationException("not in define mode");

    if (!N3iosp.isValidNetcdfObjectName(att.getShortName())) {
      String attName = N3iosp.makeValidNetcdfObjectName(att.getShortName());
      log.warn("illegal netCDF-3 attribute name= "+att.getShortName() + " change to "+ attName);
      att = new Attribute(attName, att.getValues());
    }

    Variable v = rootGroup.findVariable(varName);
    if (null == v)
      throw new IllegalArgumentException("addVariableAttribute variable name not found = <" + varName + ">");
    v.addAttribute(att);
  }
Add an attribute to the named Variable. Must be in define mode.

@param varName name of variable. must already have been added to the file.
@param att     Attribute to add.